Study On Control Mechanism And Influence Evaluation Of Cavity Multiphase Flow Around Curved Surface

When the local pressure in the flow field decreased to the saturated vapor pressure of water,a phase transition from water to vapor occurs,the phenomenon is called cavitation.Cavitation will increase the drag of propeller,hydrofoil and other devices,reduce hydrodynamic efficiency,and cause cavitation erosion on their surfaces.In order to suppress the negative effects of cavitation and improve equipment performance,it is of great significance to study the cavitation control methods.In this paper,several control mechanisms are proposed for the cavity multiphase flow field around different surface structures by using numerical method,and the effects of different mechanisms are evaluated.The main research contents and conclusions are as follows:Firstly,the theory of numerical model used in this paper is introduced.Based on the assumption of incompressible and homogeneous flow,volume of fraction(VOF)multiphase flow model was used to describe different Euler phases in the flow field,and Schnerr-Sauer cavitation model was used to solve the cavitation process.The large eddy simulation(LES)method is used to calculate the turbulence process,and the prediction ability of cavitating flow field is improved by mesh local encryption and adaptive mesh refinement(AMR)technology.Secondly,in the launching process of underwater vehicle,the surface bulge structure makes its back flow surface dwell and hang bubbles.In order to investigate the influence of surface structure on the evolution of multiphase flow in non-cavitating flow field,the coupling relationship between bulge structure and the bubbles was studied from the aspects of bubble morphology,vorticity distribution and pressure variation.It is found that the existence of bulge structure makes the low-pressure reflux area of a certain scale formed downstream.With the increase of time,the range of low-pressure area gradually expands with the flow direction and eventually becomes stable,which makes the bubble exist stably.Then,using the NACA 0015 hydrofoil as a base hydrofoil,the effect of vortex generator on the evolution of cavitating flow field around the hydrofoil is studied from the following aspects: cavity morphology,cavity volume,vortex structure,pressure evolution,Lagrangian coherent structure,dynamic mode and turbulence characteristics.It is found that the vortex generator can suppress cavitation,change the distribution of vortex structure,reduce the Lagrangian coherent structure between trailing edge and trailing vortex of hydrofoil,reduce the energy of low frequency mode,increase the energy of high frequency mode,intensify the turbulence intensity of wake field,and improve the large-scale vortex structure.Finally,based on the NACA 63 A 018 airfoil,a bump structure was designed on its leading edge.The effects of the front bump on the cavitation evolution characteristics,pressure pulsation,vorticity field evolution,lifting resistance,dynamic modes and turbulence characteristics were studied.It is found that the bump structure can reduce the cavity volume,improve the flow separation,enhance the intensity of the tip vortex near the trailing edge,but promote its dissipation.When the bump peak is large,the pressure on the hydrofoil surface decreases and the mechanical properties are improved.In addition,the bump structure can restrain the flow along the span direction and increase the size of the wake vortex.