Investigation On Scale-Resolving Simulation Of Unsteady Cavitation Flow Around Blades And Its Bionic Control Mechanism

Fluid machinery is a general piece of equipment widely used in industry,and high power-density design is an essential method of energy saving in fluid machinery systems.However,the high speed and compact space generated by high power-density may easily cause low local pressure on the blade surface,resulting in cavitation,which has adverse effects such as vibration and noise,material erosion,and performance deterioration.Therefore,deeply investigating the cavitation dynamics of typical fluid machinery,revealing pulsation characteristics and the evolution law of various physical quantities in the cavitating flow around blades,then effectively controlling the cavitation generation and unsteady shedding not only has important scientific significance but also guides the anti-cavitation design and safe operation of fluid machinery.The hydrofoil is the basic shape of the cross-section of fluid machinery.Therefore,firstly,taking the stationary Clark-Y hydrofoil as the research object,a numerical simulation method of the unsteady cavitating flow is proposed in this thesis.Then,the control mechanism of bionic leading-edge protuberances on the cavitating flow field around the hydrofoil is investigated.Finally,the engineering application of cavitation control theory and method is performed on a hydraulic torque converter,turbomachinery with the typical rotor-stator interaction,and the effects of protuberance parameters on its hydrodynamic performance and cavitation performance are further summarized.The detailed research work and conclusions include:(1)To explain the unsteady turbulent pulsation behaviors in the quasi-periodic evolution of cloud cavitation,numerical simulations of the cavitating flow around twodimensional and three-dimensional hydrofoils are performed by turbulent viscosity correction and scale-resolving simulation methods.The results show that the reasonable prediction of turbulent viscosity is the premise to capture the complete evolution of cloud cavitation,and the re-entrant jet formed at the trailing edge is the source of cavity break-off and shedding.It has been demonstrated that the SBES(Stress-blended eddy simulation)turbulence model has advanced nature in predicting the multi-scale structure and compressible effect of the cavitating flow.It provides a new perspective on the switching mechanism between the RANS equation and the LES equation from the constitutive equation,namely that increasing the RANS portion ensures the stable growth of the sheet cavity,whereas expanding the LES coverage promotes the breakup of the gas-liquid phase interface as well as the shedding of complex large-scale vapor structures.The mapping relationship between gas-liquid phase transition,cavity collapse and turbulent velocity fluctuation has been revealed.The vorticity transport equation is used to investigate the cavitation–vortex interaction.The vortex stretching term caused by the change in velocity gradient,as well as the vortex dilatation term induced by the fluid compressibility,are found to dominate the generation and evolution of vortices in the unsteady cavitating flow.(2)A vortex control method based on leading-edge bionic protuberances is proposed,and the influence law of counter-rotating vortices induced by bionic protuberances on cavitating flow characteristics is clarified.The streamwise vortices can contribute energy to the boundary layer and effectively inhibit flow separation,thereby cutting off the spanwise sheet cavity and binding the cavitation inception inside the troughs.They also disturb the downstream flow,which considerably reduces the oscillation and shedding scale of the cavity at the hydrofoil's trailing edge.Cavitation,in turn,affects secondary streamwise vortices that are broken and mixed due to the disordered terms of the vorticity decomposition.A simplified one-dimensional model of the cavitating flow field is applied to further clarify the relationship between pressure pulsation and cavity evolution.The bionic hydrofoil effectively suppresses the acceleration of cavity volume by reducing pressure pulsation,revealing the control mechanism of cavity morphology.(3)The hydraulic torque converter with typical rotor-stator interaction effect is selected as the engineering application object,and the dynamic evolution law of cavitation is analyzed by combining the test results of hydraulic performance under different operating conditions and the accurate capture of the vortex morphology.The results show that cavitation mainly affects the transmission power of the torque converter under low-speed ratio conditions.Cavitation bubbles block the circular flow,resulting in a reduction in mass flow rate and a decrease in blade load.Under high-speed ratio conditions,the rotation speed difference between the pump and the turbine is reduced,and the turbulence pulsation in the impeller is lessened,so that the pressure in the whole fluid channel rises without cavitation,preserving maximum efficiency.The evolution of cavitation under different operating conditions is demonstrated to essentially correspond to the change in incident angle.The oil impacts the stator blade with the highest incident angle and flow rate under the stall condition,resulting in the worst-case-scenario for cavitation.Application research of bionic protuberances in cavitating flow control of the torque converter is carried out.Combined with the Optimal Latin hypercube experimental design method and the multi-objective optimization algorithm,the effects of amplitude and wavelength on the hydrodynamic characteristics and the cavitation performance are obtained.It has been confirmed that the amplitude has a more significant control effect on cavitation.The optimized model can weaken the development of cavitation,suppress the separation vortex,and reduce the flow loss,consequently improving the overall performance of the torque converter.