The waterjet pump is widely applied in the high-speed marine vessels to exploit various kinds of resources in the vast ocean. The transient cavitating flows in a waterjet pump are numerically ...investigated under a non-uniform inflow for the purpose of revealing the correlation mechanism between the cavitation and the vorticity diffusion as well as the exited pressure fluctuations. The unsteady numerical simulation is conducted by using the Reynold-Averaged Navier-Stokes (RANS) method coupled with a homogenous cavitation model. Both the hydrodynamic performance and the cavitation performance are well predicted by the present numerical approach when compared with the available experimental data. The cavitation occurrence would cause larger pulsations to the hydrodynamic characteristics and the nonuniformity together with perpendicularity at the impeller inlet plane. As the blade passes through the non-uniform inflow, the instantaneous cavitation dynamics behaviors include the cavity generation, development and extinction, and the dominant frequency corresponds to the impeller rotating frequency. Based on analyses of the boundary vorticity flux, the cavitation is an important mechanism for vorticity diffusion from the blade into the mainstream with the major contributor of the variable density due to cavitation. Furthermore, combined computational and theoretical analysis illustrates that the cavity volume variations would cause the flow-rate fluctuations and the cavity volume acceleration is the major source for the pressure fluctuations inside the mixed-flow waterjet pump.
•Cavitation causes large pulsations to hydrodynamic characteristics.•The dominant frequency of cavity variations is the impeller rotating frequency.•Cavitation is responsible for the boundary vorticity diffusion.•Cavity volume variations would cause the flow-rate fluctuations.•Cavity volume acceleration is the major source for the pressure fluctuations.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The objective of this paper is to investigate the space-time frequency spectra for cavitating flows in a mixed-flow pump by using both fast Fourier transform and wavelet transform. Unsteady ...cavitating flows in a mixed-flow pump are numerically investigated by using the Reynolds-averaged Navier-Stokes method, which is closured with SST k-ω turbulence model and Zwart cavitation model. The cavitation performance is fairly predicted when compared with available experimental data. There are two stages for unsteady cavitation evolution during one impeller rotating cycle, including the cavity growth stage and diminution stage. The cavitation in the impeller is characterized by the spatial non-uniform distribution since a high-pressure region presents at the impeller inlet plane. The pressure amplitude decreases when the cavitation becomes severer at a smaller operating velocity. Besides, the dominant frequency in the impeller is the impeller rotating frequency (fn), i.e. the cavity evolution frequency. Due to the rotor-stator interaction from the six-blade impeller, there is a dominant long-term frequency of 6fn in the intake duct and the diffuser inlet. Furthermore, a broadband low-frequency around 1.5fn exhibits near the diffuser exit, and the 1.5fn amplitude varies over time corresponding to different corner-vortex dynamics. Therefore, wavelet analysis is a more favorable and practical method to obtain time-dependent frequency information for unsteady cavitating flows.
•Spatial non-uniform cavity distribution due to a high-pressure region at the inlet.•The amplitude of pressure fluctuations decreases when the cavitation becomes severer.•The 1.5fn amplitude varies over time with different corner-vortex dynamics.•Wavelet transform can obtain time-varying frequency information for cavitating flows.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
This paper investigates the ventilation elimination mechanisms during the deceleration process of a surface-piercing hydrofoil using the unsteady Reynolds-averaged Navier-Stokes (RANS) method ...together with a Volume of Fluid (VOF) model. The numerical results are in good agreement with the experimental data. The ventilation elimination mechanism of the surface-piercing hydrofoil is analyzed from the perspectives of the hydrofoil hydrodynamic performance, the ventilated cavity evolution, vortex structures, and re-entrant jets. The results indicate that the ventilation elimination includes three stages, i.e. a decrease in the ventilated cavity, washout, and reattachment. The decrease in the ventilated cavity is due to the hydrofoil speed decrease in the FV flow. Washout is the transition from fully ventilated to partially ventilated flow, and reattachment is the transition from partially ventilated to fully wetted flow. The underwater vortex structures around the surface-piercing hydrofoil are composed of a tip vortex, an unstable vortex induced by the shear layer, and a Karman vortex caused by the vortex shedding from the trailing edge of the hydrofoil. Ventilation stability strongly depends on the re-entrant jet. When Φ (the angle between the flow direction and the closure line of the ventilated cavity) is greater than 45°, the re-entrant jet impinges on the ventilated cavity's leading edge and destabilizes the ventilated cavity.
•Ventilation elimination mechanisms are revealed during the hydrofoil deceleration.•Ventilation elimination includes three stages, i.e., cavity reduction, washout and reattachment.•Vortices include a tip vortex, a vortex induced by the shear layer and a Kaman vortex.•Ventilation stability strongly depends on the re-entrant jet.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Abstract Design optimization for widely used axial flow pumps presents a formidable challenge due to the significant impact of numerous parameters associated with impeller geometry on hydraulic ...performance. The expansive design space raises concerns about the cost and time implications of the optimization process. This paper introduces a machine learning-based algorithm with a dynamic sampling approach to enhance the hydraulic performance of axial flow pumps. The focus is on an axial flow pump designed for China’s South-to-North Water Diversion Project. Optimization involves selecting 15 design variables governing impeller geometry, considering meridional shape and mean blade profiles. The optimization process predicts hydraulic performance using CFD methods, with a primary objective of maximizing efficiency at the axial flow pump’s design point while maintaining pump head around the design value. The results indicate that the proposed machine learning-based algorithm exhibits commendable convergence, delivering a notable improvement in performance. For instance, the optimized axial flow pump displays 2% efficiency increase compared to the initial design. Further analysis employing concepts like entropy generation rate and boundary vorticity flux reveals that the optimized pump has more uniform flow near the pressure side of the impeller blade. Additionally, design optimization effectively suppresses flow separation at the blade trailing edge near the impeller hub. This study offers valuable insights and a practical tool for the design optimization of axial flow pumps.
Unsteady turbulent flows in a waterjet propulsion system are investigated at various cruising speeds with the emphasis on pressure fluctuations. The numerical methodology is based on the ...Reynolds-Averaged Navier-Stokes (RANS) equation with the SST k-ω turbulence model and a sliding mesh technique. The head and efficiency of the waterjet pump are predicted fairly well compared with the available experimental data. The pressure fluctuates intensively in the impeller and the dominant frequency is the impeller rotating frequency with the largest amplitude near the impeller inlet. Besides, two dominant frequency components exist in the intake duct and the diffuser. A high-frequency component is caused by the rotor-stator interaction, and another component is generated by the unsteady vortex evolution in the diffuser passage and would propagate upstream to the impeller and the intake duct. Analyses based on the vorticity transport equation demonstrate the great contribution of the vortex stretching term to the vorticity distribution and evolution in the diffuser. Finally, at the cruising speed of 45 knot, the flows inside the duct are strongly affected by the impeller rotation and present a periodic prewhirl motion with the dominant frequency of the impeller rotating frequency.
•Two dominant frequency components of pressure pulsations are illustrated at various cruising speeds.•Pressure pulsations are caused by the rotor-stator interaction and the unsteady vortex evolution in the diffuser passage.•At 45 knot, the flow inside the duct presents a periodic prewhirl and the dominant frequency is fn.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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