Influence Of Water Quality On The Tip Vortex Cavitation Inception

Tip vortex cavitation is the earlist type of cavitation on marine propeller,which can lead to significant noise,vibration and erosion.Therefore,there is an urgent need to accurately predict the occurrence of tip vortex cavitation.Water quality has a great influence on the inception of tip vortex cavitation,however,there is a lack of suitable method to quantitatively assess the effect of water quality since none existing macro cavitation models had taken into account this effect so far.Therefore it is desiderated to develop a new theory and model to predict tip vortex cavitation.In the present work,Eulerian-Lagrangian approach is applied to simulate the tip vortex cavitation inception,i.e.tip vortex flow as the Eulerian flow field,migration and growth of nuclei traced in Lagrangian frame using Newton's second law and bubble dynamics.This will explain how water quality with different nuclei population affect the inception of tip vortex cavitation.Based on the numerical simulation as well as experimental results,analytical solutions are employed to establish a new model quantitatively considering influence of water quality on tip vortex cavitation inception.Firstly,numerical simulation of tip vortex flow is carried out for elliptic hydrofoil with a mod-ified NACA 662-415 airfoil section using DDES turbulence model on the OpenFOAM platform.The accuracy of DES turbulence model is verified by comparing the simulation data with exper-imental results.Secondly,for the nuclei,theoretical solution of motion equation in the core of Rankine vortex is solved to illustrate the accuracy of simulating motion of nuclei on the platform,and analytical solution of motion equation in the Rankine vortex is solved so that capture time of nuclei can be acquired fast.Finally,on the one hand,nuclei according to nuclei population will be released into the tip vortex flow obtained in the simulation,then the nucleus which appeared first explosive growth can be picked out according to the grajectory and growth of nuclei.On the other hand,nuclei according to the same nuclei population will be released into an ideal Rankine vortex flow which is set up based on the radius of vortex core and circulation of tip vortex obtained in the simulation.For this case,analytical solutions are used to get the capture time of nuclei quickly.The nucleus with biggest size and capture time shorter than critical time will be choosed as the one occurring first explosive growth.This size of nucleus will be compared with the one selected in the simulation,which is intended to verify the ability of the analytical solution in selecting the nucleus growing explosively first accurately and quickly.The tension of this nucleus is assumed as the tensile strength of the liquid,i.e.the quantitative effect of water quality on tip vortex cavitaion inception.It can be employed to correct the inception cavitation number without the effect of water quality which is obtained by interpolation of cavitation number with different water quality in the experiment.This assumption will be testified by comparing the corrected cavitation number to the one from the experiment results with corresponding water quality.In summary,a quantitative study of the effect of water quality on tip vortex caviation inception has been carried out,and a new model suitable for tip vortex cavitation has been proposed in this paper.