In recent years, an unshrouded impeller is being developed for rocket turbopumps to reduce production costs and disk friction losses. However, the internal flow structure in a diffuser influenced by an impeller has not been clarified yet. In this study, we focused on the investigation of unsteady flow in turbopumps with unshrouded impellers and vaned diffusers by experiments and CFD. Furthermore, we investigated the effect of changing the number of blades of the impeller and diffuser on the unsteady losses in the diffuser. First, we measured the static pressure at the shroud side in the impeller and the velocity at the diffuser inlet and outlet. Second, we confirmed the accuracy of the CFD by comparing CFD results with experimental results. Third, we investigated unsteady losses in diffusers by CFD. We could confirm increased entropy at the suction surface and shroud side because of the tip leakage and the vortex. Finally, we changed the number of impeller blades or diffuser blades. In conclusion, the loss region at the suction surface and the high-pressure region at pressure surface in the unshrouded impeller were mixed and caused unsteady losses through the diffuser. And diffuser efficiency decreased because channel width in diffuser became narrower by the number of diffuser blades increasing.

High efficiency, low-cost, and reliable hydraulic turbines are in high demand for hydroelectric power generation, especially in medium and small-scale power plants. The authors' laboratory has been developing unshrouded, i.e., band-opened runners of turbines to meet the demand. Those runners feature highly efficient performance and less expensive manufacturing costs produced by cutting, not by casting. For the sake of developing further improved unshrouded runners, it is necessary to understand their loss mechanism in detail. Therefore, in this study, the internal flow and the loss mechanism are investigated experimentally and computationally, with two different specific speed hydro turbine runners. First, Euler's theoretical head on each runner is discussed to identify if they are influenced by the distinct specific speed. It demonstrated that the low specific speed turbine consumes greater potential energy in centrifugal force. Secondary, the loss mechanism in the unshrouded runners is investigated. The result showed that the dominant loss is caused by a tip leakage vortex developed by the collision of a leakage flow at the tip clearance and a secondary flow.

An axial pump which is used to pump water out of a river often sucks in solid bodies such as leaves and branches. These foreign substances may clog the passage and lead to dangerous conditions such as a decline in the performance of the pump. In this investigation, we conducted the clogging experiments by putting polyester strings in an axial drain pump which has two types of impellers, a radial blade impeller and a swept back blade impeller. By visualizing behavior of the strings in the pump using a high-speed video camera, we compared the anti-clogging performance between the two impellers and investigated the mechanism of clogging. The results of these experiments showed that the pass rate of the strings that flowed into the swept back blade impeller was higher than that in the radial blade impeller. It is because strings that are caught in the leading edge of the swept back blade tend to move to the tip side and be pulled towards the suction surface. In addition to the experiments, we carried out the simulations of these experiments using a CFD-DEM (Discrete Element Method) coupling analysis. The results were in good agreement with the experiments qualitatively.

It is known that a compressor causes a flow instability phenomenon called surge when the flow rate becomes reduced. Previous studies performed a compressor performance test using an apparatus that simulated a turbocharger for cars. As a result, they have confirmed that surge frequencies differ depending on the capacity of the tank. In this study, performance tests of a compressor for a turbocharger were conducted to understand a mechanism of surge phenomena and to understand differences between them under the steady flow and the pulsating flow. The apparatus which was used at this time consists of a compressor, a pulsating flow generator, a tank, and a valve. Except for the compressor, they install on the downstream side of it in this apparatus. The pulsating flow generator and the tank are equivalent to an automobile engine. First, to understand the difference between the mild surge and the deep surge under steady flow, the flow rate and the pressure of the tank were measured by controlling the valve under five different rotational speeds. And the two types of surge phenomena (i.e. the mild surge and the deep surge) were determined. Compared to the mild surge and the deep surge, it was judged that the former has smaller fluctuations of the flow rate and weaker surge phenomena. And, Fast Fourier Transform (FFT) analysis was performed on the measured pressure. It was found that the higher the rotation speed became, the lower the surge frequencies of the deep surge were, and the lower the rotation speed became, the weaker the strength of the surge phenomenon became. Next, it was investigated the difference in surge frequencies between the mild surge and the deep surge under pulsating flow with the rotation speed fixed. It was found that in the mild surge, the effect of pulsating flow on the surge phenomenon was significant, and it was confirmed that the surge frequencies were drawn into the pulsating frequencies. On the other hand, in the deep surge, the pressure fluctuation of the surge phenomenon became stronger than that of the mild surge, and a pattern of drawing was changed. In addition, unlike the mild surge, in the deep surge, the frequency range of draw in the same magnification between the given pulsating frequency and the obtained surge frequency narrowed. And, it was confirmed that the surge frequency was drawn into the pulsating frequency at half times rate and two times rate.

This paper focuses on the generation mechanisms of synchronous pressure pulsations featuring a lower frequency than the rotating frequency of a vortex core which are observed in a simplified draft-tube cone through analyzing the results of experiments along with unsteady-state numerical simulations. A Venturi-tube is selected as a simplified geometry of hydro-turbine draft-tube; and a stationary-blades swirl generator is installed upstream to generate a uniform swirling flow in the Venturi cone (diffuser part). The experiments, including the dynamic pressure measurements, are conducted for several flowrates to determine the characteristics of pressure fluctuations in the Venturi cone. Two types of pressure fluctuations are identified, namely convective and synchronous. Both cover wide ranges of different frequencies, revealing that the phenomena responsible for generation of these pressure fluctuations feature a stochastic component. The synchronous pressure pulsations, which feature lower frequencies than the convective fluctuations, are identified as the product of self-excited oscillations of vortex breakdown. High-speed visualization of the vortex shape reveals that the vortex features three states: an axisymmetric vortex, a breakdown, and appearance of several helical vortices generated downstream of the breakdown location; while the location of the vortex breakdown fluctuates in time. By using the results of unsteady CFD simulation, it is confirmed that the location of the vortex breakdown fluctuates with the same frequency (Strouhal numbers 0 to 1) as the low-frequency synchronous pressure pulsations observed in the Venturi cone. Moreover, it is observed that the volume of the stagnation region produced due to the counterflow downstream of the breakdown location fluctuates with the same low frequency. Further analysis of the CFD results showed that the flow field at the location of the vortex breakdown experiences a severe shear layer formation around the center of the Venturi. This shear layer stretches the vortex center spatially on the plane perpendicular to the vortex axis. It is also observed that several intermittent helical vortices are generated downstream the breakdown location and rotate periodically in the Venturi while featuring a stochastic nature, as well.

This paper presents an experimental and numerical investigation of the internal flow in a Francis turbine draft tube previously designed for minimizing pressure fluctuations and energy losses in off-design conditions. The design of the draft tube geometry is based on an original approach combining Design of Experiments and steady/unsteady Computational Fluid Dynamics (CFD) simulations of the draft tube internal flow. The proposed method provides new insight on the influence of the draft tube geometry on the flow dynamic behaviour on one hand and enables the determination of a geometry promoting flow stability and hydraulic performance on another hand. CFD simulations of the internal flow in the final geometry showed promising results in terms of flow stability compared with the initial geometry designed by conventional CFD-aided methods. A reduced-scale model of the prototype machine featuring the final draft tube geometry is finally installed and tested in laboratory. Tests include performance and pressure fluctuations measurements over the complete operating range. The analysis of the results shows that the draft tube flow remains globally stable over the complete part-load range with pressure fluctuations amplitude lower than 1% of the net head. It is also shown that the dominant pressure component at the runner outlet in the draft tube cone is of synchronous nature. The physical mechanisms of excitation are finally highlighted by analysis of unsteady CFD simulation results.

Unsteady behaviors of free surface around a rotating vertical shaft in cylindrical stationary casing were investigated. Experiments were carried out with various rotating frequency of the shaft at two initial water levels. An axi-symmetrical free surface oscillation took place when the rotational speed of the shaft became larger than a certain value. The frequency of the free surface oscillation decreased as the rotating frequency increased. A theoretical model was developed, and the mechanisms of the free surface oscillation were clarified. The oscillation was found to be a sloshing mode excited by the change of fluid angular velocity, caused by the change of wetted areas on the inner rotating shaft and outer stationary casing, associated with the change in free surface height.

The disk friction loss is remarkably large in low specific speed centrifugal pumps, and an effective reduction method has not been established. Therefore, to develop the method, the loss mechanism was investigated. To grasp the internal flow structure in the narrow clearance, both experimental and computational approaches were used. An experimental apparatus that imitates clearance between a rotating impeller disk and a stationary casing disk was used and the static pressure distribution in the radial direction was measured. The internal flow where the disk friction loss occurs was investigated. In the case of outward flow, the static pressure decreased because the influence of the centrifugal force lessened toward the outer diameter of the disk, as the flow rate surged. For this reason, the pressure gradient became steep. According to the computational fluid dynamics (CFD) analysis, there was a vortex in the cross section of the clearance. This vortex encouraged flow recirculation and promoted the increase in the circumferential velocity in the potential core. When the flow rate grew, the vortex diminished. The circumferential velocity gradient and the shear stress intensified. As a result, the disk friction escalated. In the case of inward flow, the pressure gradient became steep as the flow rate increased. There was a vortex in the clearance, the size of which lessened when the flow rate surged. The disk friction had a minimum value at the flow rate was 6 X 10-4 m3/s. This research clarified that the vortex in the clearance has a remarkable effect on reducing the disk friction.

In this paper, the effect of the impeller-diffuser interaction on the unstable flow in a mixed-flow pump operating under one part-load condition is investigated based on the local energy loss analysis using a modified SST k-ω partially averaged Navier-Stokes (MSST PANS) model. The comparison on the pump performance shows a good agreement between the experiment and simulation. The characteristic curve of the test pump presents a positive slope region, where hydraulic loss increases 90.75% in the impeller and 16.75% in the diffuser. The internal flow analysis depicts that the rotating stall evolution captured in the impeller passages involves larger energy transfer while the separating flow in the diffuser passage involves lower energy transfer. As the impeller blade tailing edge (TE) begins to interact with the diffuser blade leading edge (LE), the vortex attached to the impeller blade TE is cut off into two parts, one of which keeps propagating circumferentially and forms the rotating stall in the impeller, and the other is regarded as the shedding vortex, moving into the diffuser. Further analysis by the Fast Fourier Transform and proper orthogonal decomposition reveals the frequency of rotating stall is 1.49fn, and the frequency of the shedding vortex is 0.53fn.

This paper focuses on the generation of twin vortex rope in the draft-tube elbow of a Francis turbine at deep part-load operation through analyzing the results of model tests along with numerical simulations. Model tests, including pressure fluctuations measurements, are conducted over ten speed factors. By considering the frequency of the pressure fluctuations with respect to the swirl intensity at the runner outlet, the part-load operating range is divided into three regimes, with two clear transitions between each occurring at swirl numbers 0.4 and 1.7. For operating conditions with a swirl number S > 0.4, a linear correlation between the frequency of the precessing vortex core and the swirl number is established. During the deep part-load regime (S > 1.7), low-frequency pressure fluctuations appear. Their frequency features another linear correlation with the swirl number. Unsteady computational fluid dynamics simulation of the full domain is performed to elucidate the generation mechanisms of the low-frequency fluctuations. By tracking the center of the vortical structures along the draft-tube, generation of three vortices in the elbow responsible for the pressure fluctuations at the lowest frequency is highlighted: the main precessing vortex core (PVC) hits the draft-tube wall in the elbow resulting in its break down into three vortices rotating with half the rotational speed of the PVC. Two of the vortices rotate with opposite angular position, constituting a structure of twin vortices. The periodic rotation of these three vortices in the elbow induces low-frequency pressure fluctuations.

Inconel pipes that transport cryogenic fluids in rocket engines manufactured by additive manufacturing (AM) were eroded by cavitation. The AM method selective laser melting (SLM) was used to manufacture at lower cost, but had seemingly lower erosion resistance. The cavitation erosion properties of Inconel 625 and 718 are studied as a function of hardness and surface roughness with the ASTM G134 cavitating jet. The samples were studied 3 surface conditions, as deposited/as used in applications, machined and polished, for both forged and AM manufacturing. Indentation reveals slight surface hardening for machined samples. X-ray diffraction (XRD) shows similar polycrystalline γ-Ni-based microstructure for all samples. Scanning electron microscopy (SEM) images of the cut cross-sections reveal the fractures and pits, as well as some porosity in the case of SLM samples. Images of the surfaces during erosion reveal some fracture mechanisms: machined samples erosion start quickly on pits and scratches. The SLM718 samples were found to have good cavitation erosion resistance if machined, while the SLM625 samples have comparatively poorer resistance. As-deposited samples have the lowest resistance, and surprisingly machined samples are more resistant than polished.

This paper proposes an original approach to investigate the influence of the geometry of Francis turbines draft tube on pressure fluctuations and energy losses in off-design conditions. It is based on Design of Experiments (DOE) of the draft tube geometry and steady/unsteady Computational Fluid Dynamics (CFD) simulations of the draft tube internal flow. The test case is a Francis turbine unit of specific speed Ns = 120 m-kW which is required to operate continuously in off-design conditions, either with 45% (part-load) or 110% (full-load) of the design flow rate. Nine different draft tube geometries featuring a different set of geometrical parameters are first defined by an orthogonal array-based DOE approach. For each of them, unsteady and steady CFD simulations of the internal flow from guide vane to draft tube outlet are performed at part-load and full-load conditions, respectively. The influence of each geometrical parameter on both the flow instability and resulting pressure pulsations, as well as on energy losses in the draft tube, are investigated by applying an Analysis of Means (ANOM) to the numerical results. The whole methodology enables the identification of a set of geometrical parameters minimizing the pressure fluctuations occurring in part-load conditions as well as the energy losses in both full-load and part-load conditions while maintaining the requested pressure recovery. Finally, the results of the CFD simulations with the final draft tube geometry are compared with the results estimated by the ANOM, which demonstrates that the proposed methodology also enables a rough preliminary estimation of the draft tube losses and pressure fluctuations amplitude.

When a Francis turbine operates over an extended range of regimes far from the best efficiency point, the formation of a helical precessing vortex rope can lead to reduced efficiency, severe pressure fluctuations, and power swings. Because the existence of a vortex rope limits the operating range of the Francis turbine, it is necessary to adopt certain measures to mitigate the occurrence of vortex ropes and the associated pressure fluctuations, to improve the operating flexibility of the turbine. In the present study, a novel method to mitigate vortex ropes was proposed. This method involves using a modified draft tube with an inclined conical diffuser. Under the operating condition of a 16° Guide Vane Opening, four different inclination angles (from 0° to 24.4°) were investigated to determine the optimal inclination angle. Computational fluid dynamics results demonstrated that an inclination angle of 18.8° was the most effective for hindering the development of strong swirling flow and resulted in a decline in the pressure pulsation amplitude. This angle was later used under three other partial load operating points, and the results were compared with those of a traditional draft tube. The modified draft tube with an inclined conical diffuser exhibited satisfactory and stable performance in terms of reducing the flow instabilities within the draft tube. Based on an analysis of the mechanism for alleviation of the vortex rope, it was concluded that the inclined conical diffuser plays an effective role in reducing the swirling flow in the draft tube and thus destroying the development of the vortex rope. As a result, the proposed approach could be adopted to ameliorate the instability issue in Francis turbines.

Pressure fluctuations in the draft-tubes of hydraulic turbines are the most dominant issues that may cause damages to a hydraulic power plant. When the frequency of the fluctuations coincides with one natural frequencies of the plant, resonance with high amplitude vibrations and pressure pulsations occurs, dramatically impacting the integrity of the plant. This research focuses on the part-load behaviour of a medium specific-speed Francis turbine (with specific speed 184 m.kW), designed by the authors in the framework of an industrial project. This paper combines the results of model tests with CFD simulation to investigate the part-load behaviour of the turbine. To do so, performance of the turbine is numerically simulated at one operating condition by solving the RANS equations via the commercial code ANSYS CFX 19.0. The k-ω SST turbulence model is used to predict the turbine performance. In addition, model tests are conducted in the range of 20% to 110% guide vane opening. To capture pressure fluctuations in the draft-tube, 12 static pressure sensors are installed in 4 different sections of the draft-tube. The no-swirl and other iso-swirl lines of the draft tube flow are first determined by using CFD results. Furthermore, pressure signals from model tests are analysed by performing cross spectral density analysis. Two transitions in the behaviour of the precessing vortex under part-load conditions occurring at given values of swirl number are observed. Finally, it is observed that linear correlations between the Strouhal number of the vortex rope frequency and the swirl number can be established within the 2nd and 3rd part-load regimes, independently of the values of the speed coefficient.

The operating range of Francis turbines is limited at full-load conditions by the formation of a cavitation vortex rope that may enter self-oscillations under certain conditions. This induces severe pressure pulsations in the entire system, as well as output power swings putting at risk the integrity of the electro-mechanical components. The understanding of the underlying physical mechanisms and the prediction of the stability of hydropower units at full-load conditions are therefore crucial to ensure a safe extension of their operating range. In the present paper, the dynamic behaviour of a stable cavitation vortex rope at full load is investigated by high-speed visualizations while the test rig is excited at its first and second hydroacoustic eigenfrequencies. It is first demonstrated that the cavitation volume and the pressure in the draft tube are more likely to oscillate at the first eigenfrequency, in agreement with the observations of self-excited oscillations at the first eigenfrequency of the cavitation vortex rope during unstable full-load conditions. In addition, it is observed that the amplitude of both the cavitation volume and pressure fluctuations in the draft tube reach a limit value when the amplitude of the excitation is further increased. Further investigations will determine if this behaviour can be generalized to any full-load conditions and will focus on the determination of the hydro-acoustic parameters of the draft tube cavitation flow based on the behaviour of the vortex rope during forced oscillations.

With the goal of broadening the operation range of hydraulic turbines, we propose a new-type hydraulic turbine in our research. The new turbine has no draft tube, but instead is equipped with a parallel diffuser and a volute, i.e. a radial vaneless diffuser and a spiral discharge collector, respectively. In this research, we carried out a CFD investigation on specific speed 77 [m-kW] prototype turbine to predict its performance. The simulation results showed that under large discharge there is a high risk of cavitation or pressure fluctuation. In this condition, the amplitudes of the pressure fluctuation are 0.6 percent and 1.3 percent of the effective head, on the volute wall and the parallel diffuser wall, respectively. The possibility of flow instabilities such as a vortex rope precession and a rotating stall to occur is small enough to keep operating the turbine at both deep part-load and full load. The final design of the turbine has been decided based on this investigation. The turbine verification tests are going to start in early 2021.

At part-load conditions, Francis turbines experience the development of a precessing vortex rope in their draft tube (DT), rotating with a frequency between 0.2 and 0.5 times the runner frequency. It induces pressure pulsations at the precession frequency in the whole hydraulic system, undesirable vibrations and noise putting at risk the stability of the system and the lifetime of the machine components. In specific machines, a dramatic amplification of the noise and vibrations can be observed between 70% and 85% of the design conditions. In these cases, synchronous pressure pulsations with a high frequency, typically between 2 and 4 times the runner frequency, are observed, referred to as the so-called Upper-Part-Load (UPL) pulsations. These pulsations are induced by a self-excitation of the entire hydraulic system including the cavitation vortex rope at one of its high order eigenmodes. Besides, the cavitation vortex rope features an elliptical cross-section rotating around the vortex axis at a high frequency. In this manuscript, unsteady two-phase flow simulations using a Scaled-Adaptive-Simulation (SAS) turbulence model and ZGB cavitation model of a Francis turbine draft tube at 80% of the design condition are performed to clarify the mechanisms responsible for the formation of the elliptical shape of the vortex and the UPL pulsations. However, the numerical simulation with a constant inlet boundary condition cannot reproduce the UPL pulsation phenomenon since it is a self-excited oscillation at one eigenmode of the complete system, which is not considered. Therefore, the UPL pulsations component is extracted from the measured pressure fluctuations data by applying a band-pass filter and is then set as the inlet boundary condition in the numerical simulation. The flow field is therefore artificially excited which aims to confirm whether the same phenomenon can be observed and to compare with the flow field obtained with constant inlet boundary condition. The numerical simulations are validated by experimental results, and the wave propagation in the DT is clarified.

A mixed flow pump is regarded as an important power facility in the hydropower field for renewable energy. Pump impeller design shows a great attempt to alleviate the hump characteristic and suppress the severe pressure fluctuations as the mixed flow pumps operate under part-load conditions. In this respect, an advanced turbulence model, i.e., modified SST k-ω partially averaged Navier-Stokes (MSST PANS) model, was adopted to numerically investigate the effect of forward skew angle blade on the hump characteristic using different impeller blades. Both experimental and numerical results indicate that the pump with forward skew angle blade shifts the hump region to the deeper part-load region. In the pump impeller with the forward skew angle blades, a synchronous stall cell is observed near the blade leading edge and near the hub side, while typical rotating stall cell evolution is observed in the conventional pump impeller. Analysis on the blade loading also indicates that under the unstable condition, the forward skew angle blade could switch the mid-loaded distribution to the fore-loaded distribution. Finally, the low frequency pressure fluctuations induced by the rotating stall cell evolution could be also eliminated successfully as the forward skew angle impeller blades are adopted.

In general, Francis turbines are known to last for 40 to 50 years of use. However, exchanging an old turbine for a new is expensive because of modernization costs. Therefore, many modernization projects will take place through new runner changes. The Francis turbine in this paper has a high specific speed of about 330m-kW and a runner diameter of about 4.3m. The new runner can be expected to have a more stable flow and improved performance by flow analysis. In this paper, the performance analysis of the runner was performed using the CFD by Design Of Experiments (DOE). The sensitivity of variables was examined using L18 method. Numerical analysis was performed via BladeGen, Turbogrid, ICEM CFD, and CFX, which are commercial CFD codes. The design goal is to enable not only the best efficiency point but also a more stable operation than before even at partial load conditions. Find out how to find the shape of a runner blade by searching for design points that maximize annual power generation according to newly selected operating conditions.

In current study, a partially averaged Navier-stokes model based on a modified SST k-? model is proposed (MSST PANS) to predict the turbulent flows with flow separations, recirculation and reattachment. A benchmark case, turbulence flows over the backward facing step (Re=50000), is treated to evaluate the capacity of the MSST PANS model. Several other turbulence models, i.e., the standard k-? model, SST k-? model, the standard k-? PANS (SKE PANS) model and SST k-? PANS (SST PANS) model are also implemented for comparisons with some available experimental data. Results show that the MSST PANS model performs the closest results in predicting the reattachment length as well as the corner vortex. Furthermore, the MSST PANS model yields improved statistics on the skin frictions, pressures, velocity profiles together with Reynolds stresses. In contrast to the SST PANS models, the modifications on the eddy viscosity and ? equation with consideration of streamline curvature show promising in capturing the small-scale flow separations, recirculation and reattachments in some industrial applications.

Turbocharger compressors operate under pulsating flow due to continuous opening and closing of engine valves placed downstream of the compressor but exhibit a hysteresis loop deviating from steady characteristics. As the flow in a compressor is throttled, both pressure and flow discharge unstably oscillate. The instability is known as surge and it affects not only the compressor itself but also its whole engine-turbocharger system. For estimating the stable operation range, partial flow tests of a turbocharger compressor under pulsating flow were conducted. Using a compressor test apparatus with a pulsating flow generator and a tank whose capacity can be changed, the flow rate was controlled by a valve. Under steady flow, surge hysteresis loops caused by oscillating pressure and flow rate were obtained in each rotational speed. By Fast Fourier Transform (FFT) of pressure signals, the occurrence of surge with different frequencies was confirmed depending on the tank capacity. Next in pulsating flow tests, a compressor rotational speed is fixed, and pulsating frequencies given by the pulsating flow generator are changed. The obtained surge frequencies were different from those in steady flow tests. Especially, it is likely to be drawn to the frequency that is equal, two times and a half rate of the pulsating frequency. Numerical computation with one-dimensional calculation is conducted to investigate such a phenomenon.

Due to increasing usage of Feed-in Tariff (FIT), which is a subsidy system to encourage expansion of renewable energy and to reduce costs associated with introduction of renewable energy, demand for hydraulic turbine designers has increased solely for the purpose of improving turbine performance. With these requirements, CFD simulation is a powerful tool for designers to improve the internal flow and pressure distribution in turbines. Traditionally, most designers use a trial-and-error process based on previous experience to design new turbines. Usually, achieving the goal is a lengthy and painstaking process. This paper proposes a novel method which combines both CFD and optimization methods to automatically optimize a Francis turbine. This procedure includes four components: a design program, a CFD solver, a scheduler, and an optimization algorithm. This paper describes an attempt to automatically optimize a Francis turbine runner by using ANSYS Vista TF as the 2D CFD solver based on through-flow theory, and ANSYS CFX 19.1 as the 3D CFD solver based on Finite Volume Method (FVM). The runner geometry is parameterized by Bezier curves in the design program. CFD analysis is performed to assess the efficiency of the runner, and also the output power is calculated by runner torque. These two parameters are used as the objective function of the optimization algorithm. This method is used to optimize the runner geometry, which consists of two parts: the meridional plane and profile camber lines. We confirmed that the new turbine, which was optimized by the above-mentioned method, has better efficiency than the existing one.

Multi-stage pumps used for boiler water supply of thermal power plants have large capacity and pressure. Fluctuations may cause vibration and noise in pumps, piping systems, and surrounding structures. Lots of studies have been conducted on the pressure fluctuation phenomenon of turbopump piping systems. Some of the factors are the rotor-stator interaction, a resonance between the fluid in the pipe and, excitation source and resonance between structure and pump operation. Besides, studies on mitigation method for pressure fluctuations have been conducted. Inserting orifice to add damping, adjusting the length of pipe to change resonance frequency and, installing resonator to mitigate a specific frequency. However, these studies have not quantified the solution to the pressure fluctuations, so further research is needed to elucidate the causes and establish countermeasures quantitatively. In this study, pressure fluctuations due to acoustic resonance are generated by using a speaker. An orifice, a bent tube, and a branch pipe are installed at the node and antinode of the secondary pressure standing wave in the pipe, and the effects on the standing wave are observed. Moreover, the mitigation method for the pressure standing wave is established by performing a sweep test. Numerical analysis is conducted by AMESim to validate the results of experiment. It is clarified that by inserting the orifice at the appropriate position of a standing wave, pressure fluctuations are mitigated. In the case of branch pipe, pressure fluctuations are effectively mitigated by inserting it having appropriate pipe length and boundary condition.

Under the part-load conditions, the flow instability in a mixed-flow pump could lead to strong pressure fluctuations and vibrations, posing a great threat to the safety of the system. The current paper investigates effect of the forward skew blade angle on the positive slope characteristic in a high specific speed mixed flow pump using Reynolds averaged Navier-Stokes (RANS) equations coupled with the SST k-? model. The simulation results are compared with the experimental data and show good accordance. In the present study, five types of the mixed-flow pumps with different skew blade angles are prepared based on the conventional pump. It is found that with the increasing of the skew angle, the positive slope region becomes wider and moves towards a deeper part-load region. Further investigations are carried out by calculating the impeller head. Results depict that compared with the dynamic pressure head H d, the static pressure head H st plays more significant role in the formulation of the positive slope characteristic (PSC). Besides, the relative static pressure head H rel drops dramatically while the pump operates under the positive slope region, which matches with the pump performance well. Finally, analysis on the spanwise velocity profiles are further investigated in this paper, and results indicate that the flow accumulated near the impeller hub is one of the reasons to alleviate the head drop and shift the positive slope region to a deeper part-load condition.

An unshrouded impeller is being developed for high head pumps to reduce costs and disk friction losses. On the other hand, research of internal flow in a diffuser did not clearly reveal flow structure. In this experiment, we measured the velocity distribution at the diffuser inlet and outlet plane using laser doppler velocimetry (LDV) to capture the flow from the unshrouded impeller. The relation between the total pressure and velocity distribution was evaluated. The circumferential velocity and meridian velocity were measured by short-focus LDV (Diode Laser, 74mW) about the circumferential and the height direction of the vane direction. Operating conditions in this steady measurement are at the design point flow rate. The result was compared with computational fluid dynamics (CFD) simulations carried out in steady conditions at the previously defined operation points. In this experiment, a phenomenon that the streamline moved toward the shroud side was confirmed. There was also a region where the static pressure increased on the shroud side at the diffuser inlet. This phenomenon was caused by the influence of the tip leakage flow of the unshrouded impeller downstream and the gap of the impeller's main plate. Furthermore, two high velocity regions on the hub and shroud side at the diffuser outlet were observed because of the secondary flow in the diffuser. From the above studies, it was clarified that the ununiform flow in the diffuser was caused by the influence of the secondary flow in the unshrouded impeller.

As the use of renewable energy is promoted, much researches on propeller-type turbines are carried out. To improve reliability, it is important to evaluate the fluid exciting force due to blade row interaction that may cause vibration or fatigue fracture. In this paper, CFD(URANS) simulations and experimental studies were conducted to understand the characteristics of the fluid exciting force due to blade row interaction of a propeller turbine. To evaluate the fluid exciting force, strain gauges and pressure sensors were used in the closed-loop water channel. The fluid exciting force acting on the rotor due to the stator was measured by changing stator load type and the rotor-stator distance. Based on the difference in the attenuation tendency of potential interaction and wake interaction due to changes in the rotor-stator distance, the influences of these interactions could be distinguished. As results of experiments and computation, wake interaction affecting pressure fluctuation was hardly attenuated due to the swirling flow and changing the rotor-stator distance has little effect on the attenuation of blade row interaction.

A fire pump mounted on the fire engine is driven by a car engine. The engine has plurality of combustion chambers and converts the reciprocating motion of the pistons into rotational motion while shifting the combustion timing of each combustion chambers. When the force generated by the combustion is converted into torque, periodic torque fluctuation is generated. Torque fluctuation finally generates the rotation fluctuation. We analyzed pump performance by the rotation with the sinusoidal fluctuation for the impeller. From the viewpoint of torque and total head, each variable fluctuated in the same phase, but it was confirmed that there was a difference in fluctuation range. With engine rotation fluctuation given to the pump, we verify the pressure characteristics and efficiency on the outlet side change by CFD. In this paper, therefore, we report the results of CFD on pump output characteristics when periodic rotational fluctuations occur due to unsteady torque characteristics of the engine.

In recent years, automobile exhaust gas regulations have become stricter due to environmental problems such as global warming. A project by the Cabinet Office called the Strategic Innovation Promotion Program (SIP) began in 2014. SIP has 11 themes in total. One of them, innovative combustion technology, aimed to improve the thermal efficiency of automobiles from the existing 40% to 50%. To improve the thermal efficiency of the automobile, it was essential to improve the efficiency of the turbocharger. In this study, we developed a turbocharger for gasoline and diesel engines. First, to confirm the efficiency of the conventional turbocharger, experiments and CFD analysis of a commercial turbocharger were performed. ANSYS-CFX was used as a numerical code. To confirm the accuracy of the CFD, the CFD results were compared with the experimental results, and it had good agreement with the experimental results. From the analysis results, the loss region of the conventional turbocharger was clarified. The designed turbocharger compressor was tested as a prototype compressor. The results of the compressor had good agreement with CFD results, so it was confirmed that the accuracy of CFD and design method was valid. Finally, A one-dimensional simulation using GT-Power which is a system analysis software for automobiles was performed to evaluate the developed turbocharger on the engine. In the fifth year of the project, the target efficiency of 50% was achieved.

In recent years, because of the energy crisis, improving the thermal efficiency of automobiles has become a very important issue. It is known that a pulsating flow generated by engine valves affects the performance and efficiency of the turbocharger adversely [1]. However, the turbocharger characteristics under pulsating flow have not been clarified to a satisfying degree yet, and no prediction method of turbocharger performance has been established yet. In this study, an unsteady one-dimensional flow model of a turbocharger is constructed by modeling the physics of aerodynamic loss and unsteady characteristics based on experiments and 3D-CFD. Furthermore, the characteristics of the turbocharger under pulsating flow were predicted using the built one-dimensional unsteady flow model. Moreover, this one-dimensional model was implemented in GT-Power which is a one-dimensional commercial code used by many automobile companies.

This paper investigates the effect of an open pipe exit on the dynamic behaviour of the helical precessing vortex core (PVC) occurring in a confined swirling flow after vortex breakdown. This is achieved by carrying out extensive particle image velocimetry and dynamic pressure measurements downstream of an air swirling flow generator in two pipes configurations at two values of swirl number S = 0.7 and S = 1.1. The first pipe configuration features a diffuser followed by a straight pipe and an open exit. In the second configuration, the open exit is placed closer to the outlet of the swirl generator by removing the straight part. In both configurations, the PVC structural parameters are properly determined by phase-averaging of the instantaneous velocity fields. In the first configuration, a sudden change in the pitch of the helical vortex is observed at both swirl number values downstream of the connection between the diffuser and the straight part. Beyond this transition, the vortex trajectory starts widening as it is getting closer to the open exit of the pipe. The results in the second configuration confirm this enhancing effect of the open exit on the PVC: at both swirl numbers, the vortex trajectory is strongly widened and its radius continuously increases from the swirl generator outlet to the pipe exit, resulting in an increase in the Strouhal number and pressure fluctuations amplitude by 20% and a factor up to 9, respectively, compared with the first configuration. Finally, based on a dimensional analysis, a linear relation between two dimensionless parameters including the vortex parameters and the pressure fluctuations amplitude is derived and supported by the data obtained in both pipe configurations.[GRAPHICS].

This paper investigated the performance of a radial flow turbine by coupling metaheuristic algorithms with Computational Fluid Dynamics. We performed the optimization of the casing and wheel of the turbine simultaneously. The computer codes of four metaheuristic algorithms, namely the Genetic Algorithm, Flower Pollination Algorithm, Grey Wolf Optimizer, and the Grasshopper Optimization Algorithm were developed using MATLAB. The optimization results indicated that Grey Wolf Optimizer is the most powerful algorithm to achieve a higher temperature drop in comparison with other algorithms. Revealed by the study, choosing the best angle at the blade inlet is the most influential factor for efficiency improvement. Besides, casing optimization has a positive effect on the pressure recovery of the turbine by eliminating swirling flow. A comparison of the physics of fluid in the optimized and base wheel showed that the flow is more attached to the optimized blade since the backward-facing step produces a favorable pressure gradient mainly in the recirculating bubble. Employing pulsating flow confirmed that the efficiency of the optimized turbine has a significant increase in comparison to the base turbine by more than 2% in high mass flow rates. (C) 2020 Elsevier Masson SAS. All rights reserved.

Under the circumstances of rapid expansion of diverse forms of volatile and intermittent renewable energy sources, hydropower stations have become increasingly indispensable for improving the quality of energy conversion processes. As a consequence, Francis turbines, one of the most popular options, need to operate under off-design conditions, particularly for partial load operation. In this paper, a prototype Francis turbine was used to investigate the pressure fluctuations and hydraulic axial thrust pulsation under four partial load conditions. The analyses of pressure fluctuations in the vaneless space, runner, and draft tube are discussed in detail. The observed precession frequency of the vortex rope is 0.24 times that of the runner rotational frequency, which is able to travel upstream (from the draft tube to the vaneless space). Frequencies of both 24.0 and 15.0 times that of the runner rotational frequency are detected in the recording points of the runner surface, while the main dominant frequency recorded in the vaneless zone is 15.0 times that of the runner rotational frequency. Apart from unsteady pressure fluctuations, the pulsating property of hydraulic axial thrust is discussed in depth. In conclusion, the pulsation of hydraulic axial thrust is derived from the pressure fluctuations of the runner surface and is more complicated than the pressure fluctuations.

Insufficient understanding of flow instability interaction between hump characteristic and the rotating stall is a major problem in the application of mixed flow pumps. In this paper, the unsteady flow characteristics of a mixed flow pump are studied numerically to understand the hump characteristic generating mechanism using a modified SST k -u partially averaged Navier-Stokes (MSST PANS) model. The predicted pump characteristic curves show good agreement with experimental data. Fundamental results on time-averaged flow pattern and energy loss distributions depict that the substantial rise of energy loss in the pump impeller is the main reason for the hump characteristic. Detailed flow structures show the peak regions of circumferential vorticity (Uq) and boundary vortex flux (BVF) are located at the blade tip near the trailing edge (TE) and leading edge (LE). Further analysis on the flow angle and blade loading are carried out. The results indicate that the peak region located near the LE of spanwise = 0.8 and streamwise = 0.3 is the source of the rotating stall evolution. In this process, the substantial reduction in the blade loading occurs near the LE, eventually induces the head drop. Finally, pressure fluctuations analysis depicts that low-frequency signal is observed induced by the rotating stall evolution. (C) 2020 Published by Elsevier Ltd.

This article presents preliminary results of an experimental study of the upper-part load instability and the associated elliptical form of the cavitation vortex rope in a Francis turbine draft tube. The influence of the operating parameters on the onset and the development of the instability is first briefly studied by pressure measurements in the draft tube. Visualizations of the cavitation vortex rope and its associated elliptical form are performed by using two synchronized high-speed cameras spaced by an angle of 90°. This allows to reconstruct the instantaneous position of the vortex center along the draft tube. It is confirmed that using a single camera leads to biased estimations of the cavitation volume fluctuations. The breathing behaviour of the vortex rope, responsible for the pressure pulsations according to several authors, cannot be definitely demonstrated without a proper reconstruction of the vortex shape based on high-speed videos from several positions. Finally, unique intermittent collapses of cavitation in the vortex center, giving rise to two distinct cavities connecting on both sides the vortex rope, are highlighted.

In many of the mixed flow pumps that have been studied in the past, the impeller has a sweepback wing. Especially, the positive slope characteristics have been studied to operate pump in a stable regime. The cause of the positive slope in the characteristic curve of the mixed flow pump is the reverse flow, with the turning becomes stronger, at the leading edge of the blade tip side in the flow rate range. Also, at the flow rate at which the slope of the characteristic curve becomes positive, the angular momentum becomes negative at the leading edge of the impeller, and the work amount sharply decreases at the leading edge more than the work amount increase at the trailing edge so the angular momentum decreases. However, the influence of the forward rake and skew blade on the performance, the positive slope characteristics, and the internal flow have not been studied much. In this study, we designed and analysed a forward rake and skew blade mixed flow pump. About the forward rake and skew blade, we found the cause of the positive slope characteristics is the same reason as the sweepback wing, but the flow rate range of occurrence of the positive slope characteristics was lower in the forward rake and skew pump than in the sweepback pump.

Hydraulic turbines should keep a high efficiency in the widest range of operation conditions. However, conventional turbine has been designed to reduce swirl at a runner outlet at its design point to maximize efficiency, and is unsuitable for partial load operation due to the growth of swirl at the outlet. The swirl of the runner outflow causes loss and unstable flow at the downstream draft tube. To prevent those problems, we developed a new type of hydraulic turbine capable of a wide range of operation conditions, specifically flow rate and effective head, with a high efficiency. The designed new turbine is equipped with a parallel diffuser and a volute diffuser downstream of the runner, instead of a draft tube. The outflow swirl is arranged to have forward turning constantly against runner rotation. It is possible to decrease downstream loss, and to mitigate the vortex core behaviour this way.

PAT (Pump as Turbine) has gradually been a popular solution for the small-scale hydropower generation. Because of the rich assortment and the low initial investment. However, PAT is designed as a pump but not a turbine. Therefore, a lot of research was conducted about the performance prediction and the loss distribution of pump and turbine mode. In this paper, we focused on the internal flow and loss mechanisms of the best efficient point of both modes. The characteristic and internal flow were investigated by experimental and computational approaches. The reasons of efficiency reduction in turbine mode and loss mechanisms were clarified by the computational results. The separation flow region occurs on the suction side or pressure side in non-BEP, and the swirling flow counter-rotates the PAT occurring at the outlet in BEP might be the reasons of the efficiency reduction. Moreover, because of the camber line shape of PAT, the separation flow occurs near the inlet could not reattach soon which induces the loss region. Furthermore, the secondary flow near the blade surface and the flow from the pressure side to suction side interfere with each other which induced the loss region.

In recent years, the size and the speed of axial flow type hydraulic turbines have been continuously increased, leading to an increase of the fluid exciting forces due to rotor-stator interactions in hydraulic machines featuring both rotor and stator. In addition, the use of composite material for the blades of large hydraulic machines is increasingly investigated. Such flexible and lightweight hydrofoils can however easily experience self-excited vibration such as flutter effect. Fluid exciting forces generated in hydraulic machines might cause resonance, fatigue of the blade and finally damage. This paper aims to evaluate fluid exciting forces produced by rotor-stator interactions in axial flow turbines and the hydro-elastic response to the flutter of a flexible hydrofoil. To evaluate fluid exciting forces due to rotor-stator interactions, experiments are carried out using a closed-loop water channel featuring an axial flow turbine. The pressure distribution on the blade surface and the influence of axial distance between rotor and stator on the pressure fluctuations amplitude on the blade surface are investigated. A good agreement between experimental and numerical values is found. Regarding the flutter effect, FSI (Fluid Structure Interaction) simulation of one hydrofoil coupling RANS (Reynolds-averaged Navier-Stokes equations) and FEM (Finite Element Method) simulation is carried out to study the three-dimensional behaviour of the flutter.

In present, many undeveloped locations of small hydropower generation exist in Japan. To promote the use of small hydropower generation it is required to reduce costs and increase efficiency. Unshrouded runners are proposed to simplify the manufacturing process and also to reduce the manufacturing costs. In this paper, the computation results of optimized unshrouded runners are presented. This research was conducted to minimize the shroud tip leakage loss, which is the main loss of unshrouded runners. In the author's laboratory, medium specific speed (160 [m-kW]) Francis turbines was developed. For this 160 [m-kW] Francis turbines, investigation of internal flow and loss mechanism were done by computation. Using this loss mechanism which was revealed by rothalpy, the design of the same specific speed Francis turbines with unshrouded runners was optimized. For the new runners, shroud tip clearance and the pressure distribution of vane surfaces was changed. Due to this, shroud tip leakage flow has weakened. In the simulation of whole domain, 2.45% of improvement was confirmed on the design point.

A method of analysis of cavitation peaks (impact events) using copulas is developed. Impact events, otherwise known as peaks, are defined as maximum in the pressure amplitude applied to a material surface. These impact events were measured using a high speed pressure sensor in a cavitation apparatus based on the ASTM G32 standard. A total of 46180 impacts were measured over 100 realizations of 4ms long recording. First, the impact duration and amplitude's joint marginals are modeled as gamma distribution (part of the exponential family), determined by a Kolmogorov-Smirnov test (KS test). Then, copulas enable the study of the dependence structure of the measured impacts characteristics. The measured parameters are shown to not be independent but instead have a complex, asymmetric dependence structure. There are almost no impacts that have a combination of a high amplitude (>12MPa) and low duration (<5μs). The Tawn copula best fitted the data, as determined by a maximum likelihood method. An extension of the KS test to two dimensions demonstrated that the copula is a better fit compared with a joint distribution of independent marginals.

Cavitation erosion tests were performed using a cavitating jet apparatus inspired by the ASTM G134 standard on a high-strength fiber reinforced composite material named Vectran. These tests were also performed on metallic materials, namely Al, SUS304 stainless steel and AlBC, to determine the effect of their elastic modulus and acoustic impedance on the erosion. The effects of the jet parameters, the standoff distance and cavitation number were also observed. Using a high speed-pressure sensor, a measure of accumulated impact energy was made: the number of high amplitude force counts decreased with the standoff distance, while the optimal erosion distance was negatively proportional to the cavitation number. The higher the intensity, the higher was the maximal mean depth of erosion rate (MDER) for all materials. The erosion rate decreased with the young modulus, but it was observed to be linearly dependent on the material's acoustic impedance.

Rocket engines require centrifugal pu mps used in its propellant feeding system to be smaller and lighter to increase the final speed of payloads, and be produced with lo wer costs. Reducing the number of stages and removing shroud rotating wall make pu mps lighter and easier to manufacture. Therefore, these pumps should be designed with higher head to reduce the number o f stages and have an unshrouded impeller. In this study, various shapes of shrouded and unshrouded impellers with the same meridional plane, that have various blade angle distributions, and splitter b lade shapes, have been analysed by a co mputational flu id dynamics(CFD) approach. Based on this survey, the impeller blade shape which demonstrates the highest head and efficiency has been designed considering the internal flow of the pump. As a result, it was observed that unshrouded impellers had the loss generation structure due to tip leakage flow wh ich was different fro m shrouded impellers had. Based on the above, a design method considering the tip leakage flow of unshrouded impeller was suggested.

Facing a growing demand for low initial cost and high-efficiency small scale hydropower plants, a new type of Francis turbine was developed for Kazunogawa small hydropower plant with cooperation from industrial, governmental, and academic sectors. To reduce the cost of the turbine while increasing its efficiency, a prototype turbine with an unshrouded runner and a cylindrical casing was developed. The unshrouded runner was designed by Waseda University. Typically, the turbine runner for small scale hydropower is manufactured via a casting method; however, the unshrouded runner can be milled with a 5 axis CNC machine, thereby reducing cost and manufacturing lead time. A cylindrical casing has advantages of low fabrication cost and compact footprint. However, it had lower efficiency compared to a spiral casing with conventional design. Therefore, in this study, topology optimization based on CFD analysis was conducted on the casing and double circular cascade to improve efficiency. In addition, the axial thrust was estimated from CFD analysis to design the bearings and to determine the runner gap. Finally, a performance test was conducted on-site and peak efficiency of 91% was confirmed. This paper also reports the follow-up observation of the prototype turbine after commercial operation started.

In this study, unsteady RANS simulation is attempted for a three-stage centrifugal pump, a target pump for the workshop "Single- & Multi-Stage Pump Flow Prediction" which is being held in 29th IAHR Symposium on Hydraulic Machinery and Systems. A commercial code SCRYU/Tetra developed by Software Cradle Co. Ltd. is adopted for the unsteady RANS simulation. Our primary interest is to understand the effect of the gap between impeller trailing edge and diffuser leading edge on the axial thrust characteristics of the multi-stage centrifugal pumps. To do so, in addition to the simulation for the original pump model, the impeller-diameter cut model is also simulated. The diffuser inlet to the impeller outlet diameter ratio is 1.05 for the cut model against 1.02 for the original model. The good agreement is obtained for the hydraulic performance of the original model at the design flow rate, but only the fair agreement is obtained for the axial thrust force. From the pressure distributions inside the front and back side gap of the impeller, it is found that the discrepancy is due to that of the pressure distribution inside the back side gap of the second (and also perhaps the first) impeller. The effect of impeller-diffuser gap on the hydraulic performance and axial thrust force is predicted to be small at the design flow rate through the present computations. At the low flow rate, the balancing flow rate is significantly over-predicted by the present simulation. The reason for this remains unclear and will be hopefully made clear in our future study.

Tidal power turbines take advantage of tidal energy to generate renewable hydropower. Since the tidal turbines are fixed in the ocean, it is common to paint the blade and the structure of tidal energy generator with antifouling coating to prevent marine organisms from attaching to them. In this research, hydrophilic and hydrophobic coatings which are thought to be useful as countermeasures to prevent marine organisms' adhesion are studied. We focused on the influence of the (hydrophilic and hydrophobic) coatings on the cavitation and flow field characteristics. The hydrophilic coated foil restrained the cavitation inception and growth compared to the hydrophobic coated foil from our experiment. And then, FFT was carried out on the pressure fluctuation measured in each coating foil, and the absolute value of the pressure fluctuation amount and the difference in the fluctuation period were clarified. And as another characteristic of the coated foils, the flow field near the coating surface was investigated. The velocity distribution near the foil's surface was measured using Laser Doppler Velocimetry (LDV). In this experiment, a flat plate with or without the hydrophilic and hydrophobic coatings were used. As a result, differences in the boundary layer thickness and the velocity near the wall were revealed.

Turbo pumpsfor rocket engines often equipped balance piston (BP) systems at the back-shroud of the impellers for cancelling their axial thrust. The BP system is self-balancing and stable under quasi-static conditions, but it is known that the BP systems can be unstable under certain dynamic conditions. The performance characteristics of turbo pumps equipped with unshrouded impellers might be affected by the axial position of the rotor. Thus it is necessary to consider this effect when calculating the balance of axial thrust. Few experiments have determined the characteristics of unshrouded impellers equipped with BP systems yet.In this research, an experimental study of a model turbo pump for rocket engines was carried out. This pump had an unshrouded impeller a BP system, a vaned diffuser, and a volute. Axial forced oscillations were applied on the rotor of the pump by an active magnetic bearing (AMB) test facility. This setup can oscillate with freely-selected amplitude and frequency applying thrust to the rotor. During the oscillations, the fluctuation of axial thrust under the operating conditions was monitored using strain gauges. The axial thrust compensation ability and the response of the BP system were evaluated by analyzing the magnitude, amplitude and phase delay of the axial position of the rotor. Moreover, 3D simulations and ID simulations were carried out for the model pump. In the 3D simulations, computational fluid dynamics (CFD) was used to calculate the internal flow of the model pumps. The BP system was equipped with an impeller on which were applied forced oscillations. The impeller movement was modeled using a mesh morphing method. The 1D simulation predicted the axial thrust by calculating the mass flow balance using the geometry of the model pump.The phase lag between the axial position and the thrust was dominated by the pressure fluctuation at the BP chamber caused by the mass flow balance. The 3D simulations well predicted the fluctuation, but the characteristics of the BP system estimated by the 3D simulations were more stable than those determined by the experiments. On the other hand, the characteristics estimated by the ID simulation was less stable than those by the experiments. However, these simulations grasped the tendency of the BP system to become unstable as the oscillation frequency increases, and are effective in predicting the characteristics.

The unsteady internal flow in a low specific speed centrifugal pump was experimentally and numerically investigated. Unshrouded impellers enable high head designs but on the other hand, they exhibit complicated internal flow and an efficiency decline compared to shrouded impellers. Furthermore, the complicated impeller outlet flow induces unsteady internal flow in the vaned diffuser. Therefore, a detailed investigation of the internal flow is required in order to increase the efficiency of these low specific speed centrifugal pumps. The aims of this paper are to clarify the loss mechanisms in the impeller and to investigate the effect of impeller outlet flow to the diffuser internal flow at the design point. The detailed pump internal flow is investigated using unsteady computation, which enables the computation of the 3D flow pattern. The reversed flow and the secondary flow are induced by the tip leakage flow, and this creates a high loss region in the blade-to-blade region. On the other hand, the mixing effect is remarkable at the impeller outlet, and this affects the creation of the wake. This flow behavior makes the internal flow of the diffuser unsteady and the diffuser performance fluctuates due to the impeller wake at the design point.

The unsteady behavior of a radial fan in a pulsating flow field was studied experimentally. In a pulsating flow, compressor and turbine operating points do not match their steady performance curve, but exhibit a hysteresis loop instead. The shape of the hysteresis loop depends on the pulsating frequency. Measurements to calculate the Bode diagram of the fan and the phase lag of pressure and flow rate indicate that the phase lag controls the hysteresis loop. Moreover, to clarify the influence of air column resonance, the diffuser pump test was performed as a basic study. Computational fluid dynamics (CFD) analysis of the radial fan was carried out, and its results were compared with the experiment results. The unsteady curve calculated by CFD formed a hysteresis loop as observed in the experiment results.

In centrifugal pumps, axial thrust balancing is an important factor for stable operation. In particular, in rocket pumps, the stability of rotor assembly is very important to realize reliable high-pressure fluid delivery. Although balance piston, which is one of the self-compensating axial thrust balancing systems, was often used, there are sometimes axial vibration problems on the rotor. In this study, we examine static and dynamic characteristics of balance piston by performing experiments, computational fluid dynamics (CFD), and one-dimensional (1D) simulations, and we calculate parameters such as the static pressure in balance piston chamber and flowrate, orifice flow coefficient, phase difference from the rotor axial displacement to the static pressure fluctuation in the balance piston chamber. The results confirm that the experimental results can be predicted to some degree by performing simulations. Especially, it is found that the CFD simulation can be used to effectively predict the stability of balance piston..

A counter-rotating type tidal stream power unit, composed of tandem propellers and a peculiar generator with double rotatable armatures, was provided for verification tests at offshore. The front propeller with diameter of 1 m has three blades, and the rear propeller with diameter of 0.95 m has five blades. The blade profiles were optimized with the blade element-momentum theory and CFD, and then covered with CFRP. The front and the rear propellers connect to inner and outer armatures in a synchronous type generator with net output 1.5 kW, whose efficiency was improved by heat pipes and the modification of the armature profiles. The shafts are equipped with mechanical seals to protect electric circuits from seawater. A rotational speed control system was also prepared not only to adjust the output but also to overcome the static friction torque due to bearings, slip rings and mechanical seals while starting-up. The power unit takes the maximum output with excellent efficiency at the relative tip speed ratio specified in the unit design. The unit can start-up after maintenance works and the output can be adjusted smoothly in the stream.

In Francis hydro turbines, a small clearance between the guide vane blade and the facing plate is crucial in pivoting the guide vane blade and controlling the flow rate of the turbine. This clearance-to-blade height ratio is inversely proportional to the scale of the hydro turbine. Smaller hydro turbines have higher clearance-to-blade height ratio than larger ones. Most of the time, this clearance is not included in the simulation model for it can cause inconsistencies between the computation and model turbine. This paper is focused on the various guide vane clearance and its influences on the flow. Firstly, steady numerical calculation of the turbine model (casing, stay vanes, guide vanes and draft) with and without guide vane reference clearance was carried out using the commercial code ANSYS CFX. Experiments were conducted on model turbine of specific speed 180 [m-kW] with guide vane reference clearance. The inter-blade pressure near the leading edge (LE), mid-chord and the trailing edge (TE) of the guide vane were obtained using the pressure sensor located along the circumference of the turbine casing. The result shows a good agreement between numerical computation and experiment. Furthermore, guide vane models with different clearance heights were simulated and the impact on runner inlet energy loss was investigated. In conclusion, it can be clarified that the roll-up flow from the guide vane clearance interacted with the main flow upstream of the runner which then caused loss to the turbine performance.

Generally, in industrial applications, multi-stage pumps are used to obtain high working pressure. However, utilizing these pumps requires larger body size and longer shafts which will lead to substantial costs. As a result, a low specific speed machine is needed to minimize costs. In such case, energy can be saved and manufacturing costs can be reduced by increasing the efficiency of these low specific speed machines disk friction loss is one important cause of friction in low specific speed machines. It is caused by fluid friction occurring in the clearances between the shroud and back-shroud of the impeller and the casing. This loss is not directly related to the energy conversion in turbo machines, however it is related to the main characteristics of a machine. It will be increased proportionally with the impeller diameter and decreased with its specific speed. In this research, a test apparatus for measuring friction torque on a rotating disk was established, including a pressure resistant tank and boost pump connected in a closed loop using water as the working fluid. The sample rotating disk simulated the leakage flow path between the back-shroud and the casing of a centrifugal pump. Disks with different clearances were prepared, and fluctuations of torque were studied as a function of flow rate and Reynolds number. As a result, an optimum value for disk clearance with the minimum torque was observed to exist under each flow condition. CFD analysis was performed using a model simulating the disk clearance to understand the internal flow structure. For purpose of validation, the radial distribution of static pressure on the stationary wall was simulated by CFD and compared to experimental results. The CFD analysis agreed with the experimental distribution of wall static pressure in an acceptable manner, therefore it is proved that CFD model can reliably be used in evaluation of disk friction loss. Moreover, disk friction loss variation as a function of flow rate was investigated with inward and outward flow directions. It is seen that for both directions, disk friction loss decreases with flow rate. However, a vortex structure was observed at the inlet of the disk clearance for inward flow direction. Size of the vortex structure changes as a function of flow rate and has an influence on disk friction loss. The occurrence of this vortex structure may increase the efficiency of actual low specific speed machinery.

High efficiency and low-cost hydro turbines are becoming more popular for medium and small-scale hydropower generation in underdeveloped locations. Because of this, a shroudless hydro turbine of medium specific speed 160 [m-kW] was developed based on a low specific speed 80 [m-kW] design concept previously studied in the author's laboratory. We are aiming to develop an optimal design method and further improve performances of the medium- specific speed 160 [m-kW] shroudless hydro turbine. The most important factors are the internal flow and the loss mechanism of each flow path component. In this study, the internal flow and loss mechanisms were investigated and clarified by experimental and computational approaches. The main loss mechanisms in the double circular cascade is the high swirling strength vortices extending from the guide vane inlet to the outlet near the end wall of guide vanes. Furthermore, it has been found that the interference of the secondary flow and leakage flow, and the tip leakage vortex are the predominant loss mechanisms of the shroudless runner.

Rocket turbo pumps and industrial pumps such as water feed pumps are required to work under high pressure conditions, therefore low specific speed pumps are needed in spite of high rotational speed. In recent years, unshrouded impellers were used because of easy manufacturing and cost reduction. However, when low specific speed unshrouded impellers are used in such conditions, complex tip leakage flow occur and decrease impeller performance. In addition, splitter blades are often used, the internal flow becomes even more complicated. Therefore, such the internal flow of the unshrouded impeller must be clarified. In this research, we have studied such a centrifugal pump, and we have analyzed the internal flow using experiments and CFD (Computational Fluid Dynamics) simulations.The experimental verification was carried out by measuring the total pressure distribution on the outlet of the impeller and the diffuser. The unsteady static pressure distribution at the shroud side of the impeller was measured to confirm pump performance. We used two types of CFD simulation to evaluate the internal flow in detail. In the first CFD simulation, the unsteady internal flow of an impeller was evaluated by carrying out DES (Detached Eddy Simulation) with a periodic boundary condition that does not contain the diffuser. In the second CFD simulation, interaction between the impeller leakage flow and the diffuser internal flow was evaluated by DES with the whole impeller and diffuser.From the experimental verification and CFD simulation, it was confirmed that a large-scale vortex structure caused by the tip leakage flow and the secondary flow was observed in the impeller blade-to-blade. And the influence of the impeller leakage flow on the diffuser internal flow and the diffuser performance was evaluated.From the above studies, it was confirmed that the tip leakage flow has a large influence on the impeller internal flow and the diffuser performance.

Since wastewater pumps used in sewage systems have a trade-off relationship between non-clogging ability and efficiency, many internal flow and performance estimation studies so far have focused on optimizing both parameters. Here we focus on vortex pumps with a large space that allows foreign matters to pass through inside the casing that are used as sewage pumps.In this paper, using experimental and numerical methods, we clarified clogging mechanism in the pumps using CFD (Computational Fluid Dynamics), Motion Capture and PIV (Particle Image Velocimetry) method.Firstly, since it was difficult to observe the motion of foreign matter in the pumps, a simple experimental apparatus was built to quantify it. Five cylindrical rods were set in a rectangular water passage made of acrylic that has an 80mm*80mm cross section. The motion of strings in the passage was studied with experiments and computations. Strings were chosen as foreign matter because they are comparatively easy to compute and experiment with compared to fabric for example. In the experiments, the motion of strings could be recorded with a high-speed video camera and their motions were quantified with two-dimensional motion capture system.In the computations, the motion of strings was simulated using DEM (Discrete Element Method) in the CFD software STAR-CCM+ by connecting particles with straight lines and coupling CFD with DEM. It is a numerical value method in which the motion and interaction of a large number of disintegrating objects is simulated. Its characteristic is that the point of contact between the particles is included in the equation of motion. Particles are connected by massless rods. These rods transmit force and momentum to each particle. Furthermore, the force between each particle is computed of the interaction between soft-particles and their bonding strength. In order to formulate the contact force between the particles, the springdashpot system is used. In addition, the optimal parameters of the DEM particles were obtained by conducting a parameter study.We confirmed that the motion of the strings in the flow direction coincided with at a high precision in the passage. Furthermore, by applying the computational method with the above results, we were able to simulate the motion of strings in a vortex pump. We found that strings were pulled back into the pump by the backflow in the tongue of the pump. This backflow was observed using PIV results, providing experimental confirmation of this mechanism. Moreover, we were able to simulate the motions of fibrous materials in the passage and vortex pump.

Recently, the downsizing of engine using turbocharger attracts more and more attention. Generally speaking, a turbocharger is usually designed based on its steady performance curve. However, the operating point of a turbocharger turbine does not match the steady operating point: instead it shows hysteresis behavior because of the pulsating flow generated by the engine valves. Unfortunately, turbine efficiency drops under pulsating flow conditions, but the loss mechanisms of the turbine under these conditions are not understood. Internal flow measurements under pulsating flow are actually very difficult. In this study, the internal flow under pulsating conditions was measured using a high speed PIV (Particle Image Velocimetry) system. The loss mechanisms were investigated by experimental investigation and computational fluid dynamics (CFD). The instantaneous pressure, velocity and torque were measured using a turbine experimental apparatus at WASEDA University. To generate the pulsating flow, a pulse generator was placed upstream of the turbine: a rotational disk with holes that only lets the flow through periodically. The pulsating frequency could be changed freely by changing the, rotational speed of the disk. The visualization using PIV was performed at a frequency of 1 kHz at the turbine outlet.Many fine vortices which rotate in various directions were observed under pulsating flow. Such vortices mix in the exhaust diffuser and under low frequency flow, mixing of vortices took a long time. It was observed that one loss mechanism under unsteady conditions is the mixing of vortices at the turbine outlet.CFD was performed using ANSYS-CFX, with approximately 10 million nodes. Turbulent flows were treated by using the Reynolds-averaged Navier-Stokes (RANS) and Detached Eddy Simulation (DES) with the SST k-a) turbulence model.It was confirmed that the wheel and exhaust diffuser total pressure loss under pulsating flow was higher under steady flow conditions. In addition, the total pressure loss is proportional to the flow pulsation frequency. The analysis with DES agreed with the PIV results qualitatively. On the other hand, the analysis with RANS could not simulate the flow pattern at the turbine outlet.

Since wastewater pumps used in sewage systems have a trade-off relationship between non-clogging ability and efficiency, many internal flow and performance estimation studies so far have focused on optimizing both parameters. Here we focus on vortex pumps with a large space that allows foreign matters to pass through inside the casing that are used as sewage pumps. In this paper, using experimental and numerical methods, we clarified clogging mechanism in the pumps using CFD (Computational Fluid Dynamics), Motion Capture and PIV (Particle Image Velocimetry) method. Firstly, since it was difficult to observe the motion of foreign matter in the pumps, a simple experimental apparatus was built to quantify it. Five cylindrical rods were set in a rectangular water passage made of acrylic that has an 80mm*80mm cross section. The motion of strings in the passage was studied with experiments and computations. Strings were chosen as foreign matter because they are comparatively easy to compute and experiment with compared to fabric for example. In the experiments, the motion of strings could be recorded with a high-speed video camera and their motions were quantified with two-dimensional motion capture system. In the computations, the motion of strings was simulated using DEM (Discrete Element Method) in the CFD software STAR-CCM+ by connecting particles with straight lines and coupling CFD with DEM. It is a numerical value method in which the motion and interaction of a large number of disintegrating objects is simulated. Its characteristic is that the point of contact between the particles is included in the equation of motion. Particles are connected by massless rods. These rods transmit force and momentum to each particle. Furthermore, the force between each particle is computed of the interaction between soft-particles and their bonding strength. In order to formulate the contact force between the particles, the spring-dashpot system is used. In addition, the optimal parameters of the DEM particles were obtained by conducting a parameter study. We confirmed that the motion of the strings in the flow direction coincided with at a high precision in the passage. Furthermore, by applying the computational method with the above results, we were able to simulate the motion of strings in a vortex pump. We found that strings were pulled back into the pump by the backflow in the tongue of the pump. This backflow was observed using PIV results, providing experimental confirmation of this mechanism. Moreover, we were able to simulate the motions of fibrous materials in the passage and vortex pump.

Rotating stall phenomenon limits the operation range of turbomacnines, therefore it is important to understand the crucial parameters of this phenomenon. In the present study, the diffuser rotating stall in a three-stage centrifugal pump was experimentally studied. Examined main parameter was an axial offset of rotor against the stationary part, which might be unavoidable due to accumulation of geometrical tolerances and assembling errors. The effect of leakage flow rate at the balance drum section employed as a thrust balancing device, which increases the thru-flow rate at the first and second stages diffusers. was also studied. The effect of rotor axial offset was clearly observed and, with the rotor axial offset to the suction side, the rotating stall appeared only at the third stage diffuser. By setting the balance flow rate set to zero, the onset range of rotating stall became wider m the first and second stage diffusers, which was well explained by the decrease of the thru-flow rate.

Rocket turbo pumps and industrial pumps such as water feed pumps are required to work under high pressure conditions, therefore low specific speed pumps are needed in spite of high rotational speed. In recent years, unshrouded impellers were used because of easy manufacturing and cost reduction. However, when low specific speed unshrouded impellers are used in such conditions, complex tip leakage flow occur and decrease impeller performance. In addition, splitter blades are often used, the internal flow becomes even more complicated. Therefore, such the internal flow of the unshrouded impeller must be clarified. In this research, we have studied such a centrifugal pump, and we have analyzed the internal flow using experiments and CFD (Computational Fluid Dynamics) simulations. The experimental verification was carried out by measuring the total pressure distribution on the outlet of the impeller and the diffuser. The unsteady static pressure distribution at the shroud side of the impeller was measured to confirm pump performance. We used two types of CFD simulation to evaluate the internal flow in detail. In the first CFD simulation, the unsteady internal flow of an impeller was evaluated by carrying out DES (Detached Eddy Simulation) with a periodic boundary condition that does not contain the diffuser. In the second CFD simulation, interaction between the impeller leakage flow and the diffuser internal flow was evaluated by DES with the whole impeller and diffuser. From the experimental verification and CFD simulation, it was confirmed that a large-scale vortex structure caused by the tip leakage flow and the secondary flow was observed in the impeller blade-to-blade. And the influence of the impeller leakage flow on the diffuser internal flow and the diffuser performance was evaluated. From the above studies, it was confirmed that the tip leakage flow has a large influence on the impeller internal flow and the diffuser performance.

Recently, the downsizing of engine using turbocharger attracts more and more attention. Generally speaking, a turbocharger is usually designed based on its steady performance curve. However, the operating point of a turbocharger turbine does not match the steady operating point: instead it shows hysteresis behavior because of the pulsating flow generated by the engine valves. Unfortunately, turbine efficiency drops under pulsating flow conditions, but the loss mechanisms of the turbine under these conditions are not understood. Internal flow measurements under pulsating flow are actually very difficult. In this study, the internal flow under pulsating conditions was measured using a high speed PIV (Particle Image Velocimetry) system. The loss mechanisms were investigated by experimental investigation and computational fluid dynamics (CFD). The instantaneous pressure, velocity and torque were measured using a turbine experimental apparatus at WASEDA University. To generate the pulsating flow, a pulse generator was placed upstream of the turbine: a rotational disk with holes that only lets the flow through periodically. The pulsating frequency could be changed freely by changing the rotational speed of the disk. The visualization using PIV was performed at a frequency of 1 kHz at the turbine outlet. Many fine vortices which rotate in various directions were observed under pulsating flow. Such vortices mix in the exhaust diffuser and under low frequency flow, mixing of vortices took a long time. It was observed that one loss mechanism under unsteady conditions is the mixing of vortices at the turbine outlet. CFD was performed using ANSYS-CFX, with approximately 10 million nodes. Turbulent flows were treated by using the Reynolds-averaged Navier-Stokes (RANS) and Detached Eddy Simulation (DES) with the SST k-ω turbulence model. It was confirmed that the wheel and exhaust diffuser total pressure loss under pulsating flow was higher under steady flow conditions. In addition, the total pressure loss is proportional to the flow pulsation frequency. The analysis with DES agreed with the PIV results qualitatively. On the other hand, the analysis with RANS could not simulate the flow pattern at the turbine outlet.

Journal bearings are one of the most important mechanical elements for the stable operation of rotors. Rotational machinery is becoming larger and more complicated in recent years, so proper design of bearings is necessary to offer stable support. However, bearings with shapes and operating conditions that can not be predicted by the conventional Reynolds equation method exist. In this paper,therefore, a prediction method of vibration characteristics of journal bearings using CFD was constructed. To calculate the dynamic characteristics, mesh morphing methods simulating two oscillating states were studied. In order to compare with the calculation results, an active oscillating rotor using magnetic bearings was constructed. As a result, the calculation results were roughly consistent with the experimental results, and a prediction method by CFD was constructed.

Tidal power turbines take advantage of tidal energy to generate renewable hydropower. Since the tidal turbines are fixed in the ocean, it is common to paint the blade and the structure of tidal energy generator with antifouling coating to prevent marine organisms from attaching to them. Therefore, it is important to predict the influence of the coatings on the tidal turbine’s performance. In this paper, hydrophilic and hydrophobic coatings which are known to be useful in antifouling were studied from the perspective of flow field and cavitation. Cavitation was visualized with a high-speed video camera and the cavitation characteristics of blades painted with hydrophilic or hydrophobic coatings were compared. With this visualization, it was possible to observe that the hydrophilic foil and hydrophobic foil had distinctive characteristics in cavitation inception and growth. Moreover, the reliability of both coatings was evaluated in order to discuss whether these coatings were useful for long. Immersion tests were carried out to evaluate the deterioration of the coatings in pure water. In addition, magnetostriction vibratory tests were carried out to evaluate the resistance to cavitation erosion of both coatings. From these investigations, a chemical transformation of the hydrophilic coating was observed. Moreover, both coatings were easily removed when they were exposed to strong cavitation impacts.

Rotating stall phenomenon frequently causes the troubles such as vibrations acting on the shaft system and reduces the reliability of turbomachines. In the present study, the diffuser rotating stall in a. three-stage centrifugal pump was experimentally studied. Special emphasises were placed on the geometrical conditions; an axial offset of rotor against the stationary part, which might be unavoidable due to accumulation of geometrical tolerances and assembling errors., and the radial clearances of annular leakage paths which increases the thru-flow rate at the impellers and the first and second stage diffusers. As a result, with the rotor axial offset to the suction side, the rotating stall appeared only at the third stage diffuser, while with that to the discharge side, the rotating stall was more significant. By enlarging the leakage flow passages at the inter-stage bush and the balancing flow channel, the onset range of rotating stall became narrower in the first and second stage diffusers, which was well explained by the increase of the thru-flow rate. On the other hand, with the enlarged leakage passage at the liner ring, the onset range became slightly wider.

A swirling flow in a diffuser such as a draft tube of a hydro turbine may induce the flow instabilities accompanied by pressure fluctuations known as vortex rope behaviour and cavitation surge. Cavitation surge is the self-excited oscillation, which induces the large flow rate fluctuation that results from the change of the cavity volume. In this research, the investigation of the effect of the pipe length and the swirl intensity on the flow instabilities in a diffuser was performed by experiments and numerical analyses using the draft tube component experimental facility. The length of the pipe was modified by up to about 25 times as long as the diameter of the throat in order to validate the one-dimensional analyses. In addition, the swirl intensity was changed by replacing another swirl generator. The frequency of cavitation surge was changed with regard to the swirl intensity as the one-dimensional analyses in the previous study has predicted it. Unsteady numerical simulations of the swirling flow with cavitation in the diffuser was performed. The results of experiments and numerical analyses correspond qualitatively with the result of the one-dimensional analyses, which suggested that the coupling with the experiments, CFD analyses and the one-dimensional analyses is the more effective way in order to predict the flow instabilities in the diffuser.

The strong swirling flow at the exit of the runner of a Francis turbine at part load causes flow instabilities and cavitation surges in the draft tube, deteriorating the performance of the hydraulic power system. The unsteady cavitating turbulent flow in the draft tube is simplified and modeled by a diffuser with swirling flow using the Scale-adaptive simulation method. Unsteady characteristics of the vortex rope structure and the underlying mechanisms for the interactions between the cavitation and the vortices are both revealed. The generation and evolution of the vortex rope structures are demonstrated with the help of the iso-surfaces of the vapor volume fraction and the Qcriterion. Analysis based on the vorticity transport equation suggests that the vortex dilatation term is much larger along the cavity interface in the diffuser inlet and modifies the vorticity field in regions with high density and pressure gradients. The present work is validated by comparing two types of cavitation surges observed experimentally in the literature with further interpretations based on simulations.

Many industrial applications and experiments have shown that sliding bearings often experience fluid film whip due to nonlinear fluid film forces which can cause rotor-stator rub-impact failures. The oil-film whips have attracted many studies while the water-film whips in the water lubricated sliding bearing have been little researched with the mechanism still an open problem. The dynamic fluid film forces in a water sliding bearing are investigated numerically with rotational, whirling and squeezing motions of the journal using a nonlinear model to identify the relationships between the three motions. Rotor speed-up and slow-down experiments are then conducted with the rotor system supported by a water lubricated sliding bearing to induce the water-film whirl/whip and verify the relationship. The experimental results show that the vibrations of the journal alternated between increasing and decreasing rather than continuously increasing as the rotational speed increased to twice the first critical speed, which can be explained well by the nonlinear model. The radial growth rate of the whirl motion greatly affects the whirl frequency of the journal and is responsible for the frequency lock in the water-film whip. Further analysis shows that increasing the lubricating water flow rate changes the water-film whirl/whip characteristics, reduces the first critical speed, advances the time when significant water-film whirling motion occurs, and also increases the vibration amplitude at the bearing center which may lead to the rotor-stator rub-impact. The study gives the insight into the water-film whirl and whip in the water lubricated sliding bearing.

In recent years, attempts have been made to make multistage centrifugal pumps smaller in size and more efficient. However, such designs are known to cause positive-slope phenomena in the Q-H curve, especially under low-flow conditions. These phenomena, which have thus far been studied experimentally and numerically, stem from flow instability in the pump. However, their mechanisms have not yet been clarified because it depends on various parameters. In this study, we focused on diffuser rotating stall, observed in positive Q-H characteristics. This study elucidates the mechanism of positive-slope generation through experimental results and two-dimensional numerical analysis.

In this study, the performance and the internal flow of one stage model centrifugal pump with both vaned and vaneless diffuser were investigated. To measure the internal flow of the diffuser and the impeller easily, air was used in this pump test. As a result of measuring pressure fluctuation, the rotating stall was observed in the vaned and vaneless diffuser. We clarified the generating mechanism and characteristics of the rotating stall in the diffuser and the difference between the unsteady flow fields in both diffusers. In case of the vaned diffuser, the number of rotating stall cells were 4 in the diffuser and the cell propagation speed ratio was about 5 percent of the impeller rotating speed. On the other hand, in case of the vaneless diffuser, the cell number was 2, and the propagation speed ratio was about 10 percent of that. These phenomena in both diffuser pumps were simulated by unsteady 2D and 3D CFD computations. By using these computations, the vortex at the trailing edge of the diffuser vanes blocked the flow and induced separate flow at the leading edge resulted in the rotating stall. This research indicated that these vortices induced the total pressure loss increasing. Also, the rotating stall was found not only in the diffuser but also in the impeller by the flow simulation of the vaneless diffuser. And it was confirmed that the vortices at the impeller trailing edge and leading edge in the stall cells cause the total pressure loss.

In this study, radial and axial thrust forces working on the whole rotor in a three-stages centrifugal pump are measured in a wide range of flow rate. The forces are measured at two floating journal bearings and one floating ball bearing, which are supported by the individual load cells. The effects of the offset of rotor position in the axial direction on the thrust forces are investigated. It is found that the effect of the axial offset is significant for the axial thrust force in the low flow rate range, whereas it has little influence on the head and efficiency performances in the whole flow rate range.

In this study, new design concepts were structured by DOE based on internal flow evaluation by CFD to realize the efficiency improvement, reliability improvement and cost reduction of a medium or small capacity hydro turbine. As a part of new concepts, shroudless type and shroud liner type runner shape were adopted. In shroud liner type, shroud line of meridional plane shape inclines at 45 degrees to rotational axis. By adopting shroud less type, runner can be made not by casting but by cutting work. For medium or small hydroelectric power plant, cost reduction is strongly required in comparison with larger scale hydro one. By adopting shroud liner type, efficiency was improved because of mitigating secondary flow. In addition, to improve reliability, shroud partial band was bonded at inlet of runner. This plays a role to prevent runner blade from breaking by tip rubbing.By using high-speed video camera and CFD analysis, it was clarified that runner outlet cavitation is caused by jet which is leakage in the tip clearance region and that leakage flow of outlet is larger than leakage flow of inlet.Moreover, by using the three-hole Pitot tube, the runner outlet flow distribution was measured and the validity of the design point was verified. Finally, by measurement of pressure on the wall of the stationary parts such as the guide vane, it was clarified that the total pressure loss of guide vane increases in stream-wise direction in low mass flow rate.In this report, the development of the shroudless Francis turbine based on the loss mechanism and flow investigation was described.

The size of axial flow pumps used in drainage pump stations has recently decreased, and their rotation speeds have increased, causing an increase in the risk of cavitation. Therefore, to provide highly reliable pumps, it is important to understand the internal flow of pumps under cavitating conditions. In this study, high-speed camera measurements and computational fluid dynamics analysis were performed to understand the cavitation performance of an axial flow pump. The mechanism that causes the head to change as a result of cavitation under low net positive suction head values is shown to be the balance between the increasing angular momentum and the loss indicated by the changing streamlines.

Hydropower generation is now attracting attention because of the world's energy problems. With cooperation from the industrial, governmental, and academic sectors, a new type of Francis turbine runner was developed for the Kazunogawa small hydropower generation plant. To validate its performance, a model turbine test facility was built at Waseda University, and a performance test was performed. This experiment confirmed that the maximum hydraulic efficiency was about 88%, and the operational point of maximum efficiency agreed with the design point. Then, using a high-speed video camera, three-hole pitot tube, and computational fluid dynamics (CFD) with cavitation analysis, the loss generation mechanism of the turbine was clarified. In addition, a vibratory cavitation erosion test was carried out to select the material for a prototype runner. It was found that the volume loss of ALBC was lower than that of any other material. At the first stage of prototype turbine design, a design point that could generate the maximum output for a year was calculated using the flow duration curve at the installation location. Then, the loss in the casing and double circular cascade was improved using CFD. Performance and load rejection tests were performed before the prototype turbine operation. It was confirmed that the maximum efficiency was 91%, and the runaway speed was lower than the value predicted by the model turbine test.

The swirling flow in the draft tube of a Francis turbine can cause the flow instability and the cavitation surge and has a larger influence on hydraulic power operating system. In this paper, the cavitating flow with swirling flow in the diffuser was studied by the draft tube component experiment, the model Francis turbine experiment and the numerical simulation. In the component experiment, several types of fluctuations were observed, including the cavitation surge and the vortex rope behaviour by the swirling flow. While the cavitation surge and the vortex rope behaviour were suppressed by the aeration into the diffuser, the loss coefficient in the diffuser increased by the aeration. In the model turbine test the aeration decreased the efficiency of the model turbine by several percent. In the numerical simulation, the cavitating flow was studied using Scale-Adaptive Simulation (SAS) with particular emphasis on understanding the unsteady characteristics of the vortex rope structure. The generation and evolution of the vortex rope structures have been investigated throughout the diffuser using the iso-surface of vapor volume fraction. The pressure fluctuation in the diffuser by numerical simulation confirmed the cavitation surge observed in the experiment. Finally, this pressure fluctuation of the cavitation surge was examined and interpreted by CFD.

Unsteady cavitating turbulent flow around a twisted hydrofoil was analyzed to illustrate the physical mechanism of the cavitygenerated pressure fluctuations. The numerical simulations of cavitating flow were based on the Partially-Averaged Navier-Stokes (PANS) method and a mass transfer cavitation model. The validity of PANS model has been evaluated and confirmed in cavitation simulations by present authors using three different cases, 2D hydrofoil (Ji et al. 2012 [37]), 3D hydrofoil (Ji et al. 2013 [31]) and marine propeller (Ji et al. 2012 [38]), which shows that the PANS model with f (k) = 0.2 and f (epsilon) = 1 can obtain more accurate estimates of unsteady cavitating flows with large-scale fluctuations at a reasonable cost. In present paper we intended to shed light on the physical process responsible for the pressure fluctuations excited by cavitation. The cavity volume was analyzed to illustrate the relationship between the cavitation evolution and the pressure fluctuations. The results show that the cavity volumetric acceleration curve tracks remarkably well with the main features of the time-dependent pressure fluctuations except for the high frequency component. Thus, the cavity volumetric acceleration is the main source of the excited pressure fluctuations by cavitation. It is noted that the cavitation induced pressure fluctuations are transmitted along the suction surface of the hydrofoil and are synchronized with those on the pressure surface at the midplane of the twisted hydrofoil. Further, the pressure fluctuations on the pressure surface decrease towards the center from both the leading and trailing edges of the hydrofoil, with a minimum at 60% chord length from the leading edge.

Clogging trouble in a sewage pump is a serious problem caused by foreign substances like strings, towels, and cloths. However, mechanism of pump clogging is significantly complicated and it is difficult to clarify factors affecting pump clogging. In this paper, the simplified air duct test and axial fan test are introduced and some parameter studies based on design of experiments (DOE) were conducted. From the DOE results, the sensitivity of each parameter to the clogging was predicted. As the parameters related to the clogging, length and rigidity of the strings, leading shape and lean angle of obstacles, flow velocity in a measuring section were selected. Behavior of string in the pump was also observed in the actual pump apparatus and was compared to that in the air duct. Computational simulation with simplified flow fields and MBD (Multi Body Dynamics) was developed and applied to predict behavior of a string in the pump. These experiments and computation are effective to detect and classify the clogging mechanism. These approaches aided to establish one of the evaluation methods on anti-clogging performance against fibrous materials.

The swirling flow in the draft tube of a Francis turbine can cause the flow instability and the cavitation surge and has a larger influence on hydraulic power operating system. In this paper, the cavitating flow with swirling flow in the diffuser was studied by the draft tube component experiment, the model Francis turbine experiment and the numerical simulation. In the component experiment, several types of fluctuations were observed, including the cavitation surge and the vortex rope behaviour by the swirling flow. While the cavitation surge and the vortex rope behaviour were suppressed by the aeration into the diffuser, the loss coefficient in the diffuser increased by the aeration. In the model turbine test the aeration decreased the efficiency of the model turbine by several percent. In the numerical simulation, the cavitating flow was studied using Scale-Adaptive Simulation (SAS) with particular emphasis on understanding the unsteady characteristics of the vortex rope structure. The generation and evolution of the vortex rope structures have been investigated throughout the diffuser using the iso-surface of vapor volume fraction. The pressure fluctuation in the diffuser by numerical simulation confirmed the cavitation surge observed in the experiment. Finally, this pressure fluctuation of the cavitation surge was examined and interpreted by CFD.

For sewage pump that various foreign substance is flowed into, anti-clogging performance is a factor as important as pump efficiency in order to avoid clogging trouble by foreign substance. Many investigations about pump inner flow and pump efficiency estimation have been carried out conventionally in order to realize coexistence with anti-clogging performance and pump performance. And these results have been reflected in construction of the running water section design method. As a index of anti-clogging performance, "impeller passage diameter" which is diameter of spherical solid that can pass through the pump is used widely. And there are various type of the sewage pump which have large impeller passage diameter. However real cause of clog is not a solid, and it is fibrous material such as towel and clothes, vinyl and paper diaper. In most case these material accumulate in the pump, so that clog is occurred. In this study, for the purpose of quantification of anti-clogging performance against fibrous materials, the factor that affect to clogging of pump was investigated by pump model test using a string. The test is done based on Taguchi method. In this test, type of the pump model, diameter of the string, material of the string, length of the string and flow rate are selected for the factor, and the effect that they have on the clogging of the pump was investigated. As a result of this test, it was made clear that length of the string has a strong influence on the clogging of the pump. And from the result of this test, evaluation method of anti-clogging performance of the pump against fibrous material by using string was considered. According to the result of above test based on Taguchi method, it was assumed that quantification of anti-clogging performance against fibrous materials is possible by flowing plural strings into the pump and calculating the probability of passing. Plurality sewage pumps of different types were evaluated based on this assumption. And It was confirmed that it is possible to compare anti clogging performance of the pump against fibrous materials quantitatively. And the specification of the string was selected according to the result of the test based on Taguchi method. By this method, one of the evaluation method on anti-clogging performance against fibrous materials was established.

The dynamic characteristics of the sliding bearing have great influences on the vibration and stability of the rotorbearing coupled system. This study focus on a water lubricated bearing which supports the vertical rotor in atest facility. This rotor system is designed with maximum rotational speed of 5000 rpm and used to study the influence of water bearing on the rotor dynamics. Based on the unsteady computational fluid dynamics, a 3D model for the water film in the bearing with 3 grooves was built. The pressure oscillation in the water film was discussed. The dynamic characteristics including damping, stiffness and added mass of the bearing were calculated considering both the rotational effect and squeeze effect. The influences of the rotating speed of the shaft and prewhirl speed at water film inlet on the dynamic performance were also investigated. The results show that the instability of the film force on the journal is induced by the high-frequency vortexes in the grooves on the pad. At each rotational speed, the film force on the journal first decreases and then increase as the frequency ratio, while it increases with the increase of the rotational speed at each frequency. The rotational speed has great effects on the dynamic characteristics of bearing. Increasing the rotational speed contributes to strengthen the stability of the rotor system. The prewhirl velocity at the inlet has little impact on the dynamic characteristics and stability of the bearing. This study demonstrates the mechanism of lubrication dynamic characteristics of the siding bearing and shows the bearing nonlinear characteristics with the rotational speed. Using the CFD method to solve the water flow field is effective to predict the dynamic characteristics and can provide boundary conditions for the rotor dynamic analysis in the future study.

Turbine operation range has had to be extended to meet economic requirements in recent years. Specifically, efficiency under partial flow conditions has been improved by the development of low loss runners. The most effective runner design achieves suitable flow distribution from the runner exit, because the loss mechanism in the partial flow is due to swirl loss in the draft tube. In this study, a new design concept for improving turbine efficiency under low flow rate conditions is established and confirmed by both computation and experiment. The performance predicted by the CFD is compared with the experimental results and good agreement is observed. This report describes some runner shape improvement techniques to mitigate the effects of the flow conditions in a partial flow scenario, and validates their results through model performance testing.

Large-Eddy simulation (LES) coupling with mass transfer cavitation model was used to resolve the turbulent flow structure with cavitation. The results will focus on the cavitation shedding dynamics around NACA66 hydrofoil. The predicted results compare well with the experimental measurements for steady/unsteady partial cavitating flows. Numerical visualizations of cloud cavity evolution and surface pressure signals show relatively good agreement with the experimental data. © Published under licence by IOP Publishing Ltd.

High amplitude of pressure fluctuation is observed in a draft tube of a hydraulic turbine and a pump-turbine, for the case of partial load operation. Several methods had been reported to mitigate the amplitude so far, such as, air or water injection to the draft tube, fins on the draft tube surface, or runner replacement with optimized velocity profile at runner exit. However, several problems for each method can be considered, such as, negative influence on efficiency, high cost, technical difficulties for installation, and so on. To solve these problems and satisfy the demand for mitigating the amplitude of pressure fluctuation simultaneously, a new runner cone with spiral grooves on the surface was developed. It was developed with unsteady draft tube calculation based on Design of Experiment (DOE) method, and the effect was confirmed by model tests. Finally, developed runner cone was installed to the prototype pump turbine, and predicted performance was confirmed by on-site tests. However, the reason why the grooved runner cone can mitigate the amplitude of pressure fluctuation in draft tube was not clarified. Therefore, numerical investigation focusing around runner cone was carried out. As a result, it was clarified that the velocity profile at runner outlet was modified by the grooved runner cone, such as, reverse flow downstream of runner cone and tangential velocity was reduced. It means the shear stress between main stream and dead water core region was weakened, therefore, it can be estimated that the amplitude of draft pressure fluctuation was reduced. © Published under licence by IOP Publishing Ltd.

Model tests and CFD were carried out to find out the cause of cavitation surge in hydraulic power plants. In experiments the cavitation surge was observed at flow rates higher and lower than the swirl free flow rate, both with and without a surge tank placed just upstream of the inlet volute. The surge frequency at smaller flow rate was much smaller than the swirl mode frequency caused by the whirl of vortex rope. An unsteady CFD was carried out with two boundary conditions: (1) the flow rate is fixed to be constant at the volute inlet, (2) the total pressure is kept constant at the volute inlet, corresponding to the experiments without/with the surge tank. The surge was observed with both boundary conditions at both higher and lower flow rates. Discussions as to the cause of the surge are made based on additional tests with an orifice at the diffuser exit, and with the diffuser replaced with a straight pipe.

This paper illuminates the status of research and development on the integrated IHX/Pump concept. The integrated IHX/Pump is the incorporated component of the intermediate heat exchanger (IHX) and the primary pump. Among the innovative technologies of the Japan Sodium-Cooled Fast Reactor (JSFR) in the Fast Reactor Cycle Technology Development (FaCT) project, the integrated IHX/Pump concept is one of the major innovative ideas for plant economy by reducing the amount of material in the primary cooling system and the building volume. This report summarizes the view of the integrated IHX/Pump, a development plan, evaluation methods, and the present test results with the 1/4-scale IHX/Pump test device.

By a theoretical study with an one-dimensional model and an experimental investigation with conical diffusers, it has been found that the diffuser effect of the draft tube cause cavitation surge. The present study focuses on effects of the swirl inflow and its velocity distribution at the inlet of the diffuser. Experimental and numerical investigations are carried out with a conical diffuser and a two-stage swirler at the inlet. Results show that the inlet circumferential velocity distribution affects the wall pressure distribution, and finally occurrence of the cavitation surge.

High amplitude of pressure fluctuation is observed in a draft tube, for the case of partial load operation. Several methods had been reported to decrease the amplitude so far, such as, air or water injection to the draft tube, fins on the draft tube surface, or runner replacement with optimized velocity profile at runner exit. However, several problems for each method can be considered, such as, negative influence on efficiency, high cost, technical difficulties for installation, and so on. To solve these problems and satisfy the demand for decreasing the amplitude of pressure fluctuation simultaneously, a new runner cone with grooves on the surface was developed. As for the case with reversible pump turbine, grooved runner cone was developed with unsteady draft tube calculation based on Design of Experiment (DOE) method, and confirmed by model tests. Finally, developed runner cone was installed to the prototype pump turbine, and predicted performance was confirmed by on-site tests. Using this result, development of a new runner cone for hydraulic turbine had been started. For this case, meridian shape of the runner cone was also selected as a design parameter. Finally, we obtained the optimized shape, "diverged runner cone with spiral groove", and the performance is confirmed by the model test. In this paper, details of development, especially for the case of hydraulic turbine and considerations of the mechanism are treated. Copyright © 2011 by ASME.

Dynamic characteristics of the clearance flow between an axially oscillating rotational disk and a stationary disk were examined by experiments and computations based on a bulk flow model. In the case without pressure fluctuations at the inlet and outlet of the clearance, parallel and contracting flow paths had an effect to stabilize the axial oscillation of the rotating disk. The enlarged flow path had an effect to destabilize the axial oscillation due to the negative damping and stiffness for outward and inward flows, respectively. It was shown that the fluid force can be decomposed into the component caused by the inlet or outlet pressure fluctuation without the axial oscillation and that due to the axial oscillation without the inlet or outlet pressure fluctuation. A method to predict the stiffness and damping coefficients is proposed for general cases when the device is combined with an arbitrary flow system.

Dynamic characteristics of the clearance flow between a rotating and axially oscillating disk and a static disk were examined by experiments and numerical simulations based on a bulk flow model. In the case without pressure fluctuations at the inlet and outlet of the clearance, parallel and contracting flow paths had an effect of stabilizing the axial oscillation of the rotating disk. The enlarged flow path had an effect of destabilizing the axial oscillation due to the negative damping and stiffness for outward and inward flows, respectively. It was shown that the fluid force can be decomposed into the component caused by the inlet or outlet pressure fluctuation without the axial oscillation and that due to the axial oscillation without the inlet or outlet pressure fluctuation. A method to predict the stiffness and damping coefficients is proposed for general cases when the clearance is combined with an arbitrary flow system.

&nbsp;&nbsp;&nbsp;The flow field in the draft tube of a Francis turbine model was calculated by using Large-Eddy Simulation (LES). At first, the non-cavitating flow in the draft tube was computed and the results using two different computational models (draft tube without runner and draft tube with runner) are compared. Based on the above comparison, the computational model of draft tube with runner was used to compute cavitating flows at two cavitation numbers: one is at the plant cavitation number; the other is lower than the plant cavitation number. The cavitation phenomena in the draft tube can be predicted qualitatively. [This abstract is not included in the PDF]

In order to develop a high performance turbo-pump which consists of many components such as pump, turbine, bearings, shaft seals, etc., it is necessary not only to improve the performance of such component but also to reach good balance in performance and strength as a system, in other words, integration of performance and reliability is important. Efficiency of turbo-pump for rocket engines is generally around 30%, and its performance does not usually become a problem for the engine system. An important thing is to reduce an unstable dynamic load by cavitation of a pump. In addition, it is also important that there is an enough operation margin for components such as shaft seals and bearings. Mitsubishi Heavy Industries (MHI) has developed an oxidizer turbo-pump in the in-house research program. Through this development, new approach was tried for these problems. In this paper, the technical feature of this oxidizer turbo-pump and the results of component tests and turbo-pump tests are presented. The development approach for the oxidizer turbo-pump enabled achievement of the stable operation of the oxidizer turbo-pump in a short development period and few tests, and its effectiveness has demonstrated.

Eight hundred MWe class PWR turbine-generators operated smoothly and continuously at sudden load reduction, from full load to house load, maintaining NPSH of the feedwater booster pumps through transition even though the available NPSH never exceeded zero. NPSH of the pump at 151.3 degrees C, at which the available NPSH was minimal during transition, was evaluated as a slightly negative value, in spite of a positive value at room temperature, applying both the Ruggeri-Moore method extended to the negative area on condition of gas venting out of the system, using data of both room temperature and 95 degrees C of a model pump facility, and another restriction by gas presence at the impeller inlet. The above evaluated high temperature NPSH demonstrated successful operation of the units equipped with a low static suction head yielding zero available NPSH during transition, resulting in alteration of the design criteria of the feedwater system and thereby contributing to possible cost reduction.

The objectives of the present study were to investigate influences of fins, set in a suction part, on the positive slope and hysteresis loop in head/discharge characteristic curves of centrifugal pumps in the experiment as well as in the numerical prediction. The fins were located in upstream side of a pump impeller to suppress swirl flow occurring before the impeller inlet at partial load operation. We had two kinds of centrifugal pump with/without the fins, the number of which is 16. These two centrifugal pumps had a shrouded impeller with 7 blades and a diffuser with 20 guide vanes with the same configuration. In the experiment, the pump with them had a large hysteresis loop at partial load operation in the head/discharge characteristic curve, although the pump without them had no hysteresis loop. In the numerical simulation based on periodic flow, the incompressible turbulent flow field was calculated for partial blade-passages with periodic boundary conditions. As a result, the simulated characteristics had the same tendencies as the experimental results. Furthermore, the causes of the discontinuous head/discharge characteristics depending on the direction of partial load operation were clarified through calculating and comparing the internal flow fields in the cases with/without the fins. The pumps had the large backflow and recirculation areas in two places, one of which was near the shroud at the impeller inlet including the fins area and another near the central part of the diffuser. The difference in the hysteresis loop between with and without the fins was caused by the existence of the fins, which suppressed or promoted the backflow at the impeller.

The Japan Defense Agency is conducting a project to develop a cavitation tunnel called the FNS (Flow Noise Simulator project)(1). The FNS was designed as a large cavitation tunnel with low background noise level to measure the noise of an object. In order to satisfy the low noise level for the FNS, it is important to develop a low noise pump. In the present study, several new design concepts were developed using CFD (Computational Fluid Dynamics) and these concepts and the pump performance were verified by model test. In developing low noise pump, it is important to avoid cavitation generation in all operating conditions. It is also important to reduce blade-passing influence due to blade row interaction between impeller and diffuser. To control unsteadiness by interaction, the axial gap between two blade rows was widened and viscous wake from the impeller was decreased. Many extensive parametric studies (e.g. blade sweep, number of blades) were conducted using three-dimensional CFD computations. The impeller developed for the FNS pump has seven blades, 4.3 m diameter and the stator downstream has nine blades. Several model tests were carried out to verify the design concept of the pump. I was confirmed that the noise level of the new design pump was decreased compared to a conventional industrial pump and efficiency was also improved.

Static and dynamic characteristics of a mixed flow pump were measured at low flow rate, where the steady characteristic curves show a positive slope. Unsteady flow was measured at upstream and downstream of impeller, as well as on the casing wall of diffuser passage by using a fast response five-hole pitot tube and semi-conductor type pressure transducers. Dynamic response of the pump to the sinusoidal change in flow rate was measured under constant rotational speed. As a result of the unsteady flow measurements, the positive slope of steady characteristic was found to be caused by the impeller tip separation, inlet backflow, and pre-rotation. Moreover, the dynamic characteristics of the pump were found to become unstable with the increased frequency of flow rate.

The final target of this research is to establish a method for optimizing the design of a draft tube that is installed to recover the static pressure of water flow discharged from a hydraulic turbine runner. For this purpose, detailed measurements of the draft tube internal flows were implemented by using a draft tube element model to make it easy to control the inlet swirl flows, and the measured data was compared with the result of the flow analysis. Since the static internal fl ow in the draft tube was accurately simulated with three-dimensional viscosity analysis using a turbulence model, it proved useful as a designing tool for clarifying flow phenomena. And it becomes clear that the internal flows and the characteristics are largely influenced by the inlet flow velocity distribution, so this fact must be taken into consideration when designing a runner.

In the present study, the head/discharge characteristics of a centrifugal pump depending on the direction of partial load operation have been investigated through calculating internal flow. Based on the hypothesis of axisymmetric flow, the numerical simulation on the incompressible turbulent flow has been carried out for partial blade-passages with periodic boundary conditions. In the numerical calculation, the data previously calculated at a little more or less discharge are given as the initial condition. In the numerical results at partial flow operations the pump has the large backflow and recirculation regions in three locations, one of which is near the casing at the impeller inlet, another of which around the fins and the other of which near the central part of the diffuser. The scale of these regions varies steeply under two partial flow operation points as the discharge increases or decreases. As a result, the hysteresis loop appears in the head/discharge characteristics curve depending on the direction of partial load operation.

This paper focuses on development and validation of a viscous solver for the computation of unsteady flows in turbomachinery blade rows and stages consisting of rotors and stators. The code has been evolved from steady-state single flow solvers developed by Wiedermann based on time-marching finite difference schemes. A two-equation eddy viscosity model is applied, and the wall boundary conditions are determined by the y+-distance of the first grid line away from the wall. For the solution of transient flow fields the original time-stepping algorithm is replaced by a time-accurate scheme. The emphasis of the code validation will be on blade-row interaction in a complete turbomachinery stage. A 3-D example will be discussed and those parameters evaluated which are important for actual blading design. © 1999 OPA (Overseas Publishers Association) N.V.