Study On Mechanism Of Tip Leakage Vortex Cavitation In Hydrofoil

Tip leakage flow is a common phenomenon in hydraulic machinery.Because of the existence of shear flow in the clearance,vortices are prone to appear in the flow field near and downstream of the clearance.Cavitation caused by gap leakage often occurs under conditions of high flow velocity and low pressure.Such phenomena will adversely affect the efficiency of hydraulic machinery,causing vibration,erosion and spalling of local equipment materials.This kind of flow is accompanied by strong unsteady characteristics and various secondary flows,and has very complex flow mechanism.The mass transport between vapor and liquid caused by cavitation makes the mechanism of leakage vortex cavitation more complex.Therefore,the flow characteristics of this kind of flow are studied experimentally and numerically.In this paper,three-dimensional hydrofoils with different clearance sizes are taken as the research object.The pressure fluctuation and vorticity in the flow field of clearance airfoils are analyzed under different conditions.The relationship between these characteristics is studied,and the influence of different factors on clearance cavitation is analyzed.The main contents are as follows:Firstly,the experimental and numerical calculations of airfoil flow with clearance for cavitation are carried out.The pressure fluctuation at the monitoring point was measured in the experiment.The pressure fluctuation data under different conditions are analyzed in time domain and frequency domain.It is found that the pressure fluctuation amplitude of the sensor located at the leading edge of the airfoil clearance side is larger than that of the sensor located at the rear edge,and the amplitude increases obviously with the increase of the angle of attack of the airfoil.The frequency domain characteristics of pressure fluctuation at each sensor monitoring point in airfoil flow field with clearance are given by using FFT.The results show that the main frequency of pressure fluctuation at each sensor is relatively stable for different flow rate and angle of attack,but larger angle of attack and flow will lead to some higher frequency of pressure fluctuation.In addition,the pressure fluctuation near the rear edge of the clearance side airfoil is obviously affected by the clearance size of the airfoil.The vorticity downstream of airfoil flow without cavitation is analyzed by numerical calculation.The specific influence of different flow field parameters on the flow is explained by the strength and relative position of vorticity region.Subsequently,the numerical simulation of airfoil flow model with clearance is carried out,and the accuracy of different RANS models for calculating flow characteristics is compared.It is found that the RANS model is inadequate for predicting the length of the cavitation vortex band caused by the leakage vortices.When the gap is large,the prediction of the velocity of the leakage vortices is too low,and the prediction of the initial point of the leakage vortices by the various models has greater errors.Finally,the relationship between cavitation and turbulent kinetic energy is seldom analyzed in the existing literature.The influence of turbulent kinetic energy on cavitation is used to analyze the interstitial cavitation.The relationship between turbulent kinetic energy and vorticity distribution in the flow field is obtained,and the temporal variation of turbulent kinetic energy is analyzed.The results show that the distribution correlation between them is very high.The influence of pressure surface roundness on leakage flow is compared and analyzed by means of turbulent kinetic energy and vector contour,and its different flow characteristics are revealed.