Numerical Simulation For The Evolution Of Co-rotating Lamb-Oseen Cavitating Vortices

Tip-vortex cavitation exists widely in engineering fields,such as hydrofoils,propellers,water turbines etc.Tip-vortex cavitation can cause unfavorable oscillation,noise and erosion of blades.Therefore,it is significant to study the mechanism of tip-vortex cavitating flow,and find out how to eliminate the damaging effects caused by tip-vortex cavitation.It is shown that tip-vortex is formed by two similar vortices.Cavitation will happen if the inner pressure of a vortex is low enough,which will form a special cavitating vortex.Two cavitating vortices will interact and merge in the three dimensional flow.Different kinds of foils and flow conditions can have significant effects on the structure of vortices.And the interaction of different kinds of vortex can produced different cavitating flows.This paper aim at exploring mechanism of the interaction and merging of two corotating vortices to better understand how the cavitating flow was affected by different vortex structure.A pair of Lamb-Oseen vortices were used in this paper.Numerical simulation was undertaken in Gerris,a software which use Volume of Fluid(VOF)method in tracing the gas-water interface and adaptive octree mesh to disperse the space.The simulation work involved two parts: studying the mechanism of co-rotating vortices merging in single phase flow(water)in order to simulation the flow before cavitation incepted,studying the mechanism of co-rotating vortices merging in two-phase flow in order to simulate the flow after cavitation incepted.During study,velocity and vorticity distribution,as well as the topology of gas-water interface was analyzed.During the study of single phase flow,the merging process of co-rotating vortices in three dimensional flow was studied,and the existing research method in studying two dimensional vortices merging was referenced.The merging process was divided into three stages according to the vortex distance,vortex radius and angular velocity of the rotating vortex pair.In the study of two-phase flow,the evolution of the gas-water interface and the development of its topological structure were particularly focused.Besides,a method in predicting the angular velocity of the gas-water interface was put forward.And the merging criterion of the gas-water interface was studied.The results of this paper were intended to provide references for the future research of tip-vortex cavitation.