Study On Hydrodynamic And Cavitation Characteristics Of Flat Hydrofoil

Cavitation generally exists on the surface of blades and other components of hydraulic machinery,and occurs in the area where the local pressure changes and drops below the liquid saturated vapor pressure.The occurrence and collapse of cavitation will not only lead to the loss of cavitation damage on the surface of fluid machinery components,but also affect the safe and stable operation of hydraulic machinery.Hydrofoil is an important component element of hydraulic machinery blades,so it is of great significance to study the cavitation of hydrofoil.Taking the flat hydrofoil as the research object,this paper makes an in-depth study on its hydrodynamic and cavitation characteristics through the combination of numerical simulation and practical test.The test was completed on the water tunnel test bench of Jiangsu Key Laboratory of hydraulic power engineering of Yangzhou University.The test data of cavitation characteristics of flat hydrofoil under different angles of attack and different pressure values were collected.The collection method was mainly to observe and record the cavitation image through high-speed photography,analyze the change of cavitation length,the falling off development and periodic evolution of cloud cavitation,and summarize the cavitation map of the hydrofoil;In terms of numerical simulation,ANSYS ICEM software is used for three-dimensional structured grid division,and ANSYS CFX software is used for numerical simulation of three-dimensional transient cavitation flow field of Hydrofoil Based on RNG k-epsilon FBM turbulence model and Zwart Gerber Belamri cavitation model.The main contents and results of the research are as follows:(1)The maximum cavitation length of flat hydrofoil under different working conditions is analyzed,and it is concluded that the relationship between the maximum cavitation length and the ratio of cavitation number to angle of attack conforms to the approximate inverse proportional function,and the relationship between the maximum cavitation length and the maximum cavitation length approximately conforms to the primary function when it is in the range of 4 to 10.The distribution of different angles roughly coincides on a trend line.(2)The shape and dynamic evolution characteristics of hydrofoil cavitation under different working conditions are analyzed and studied,and the cavitation map of flat hydrofoil is summarized.The distribution law of cavitation shape and cavitation length of hydrofoil under different attack angle and cavitation number is clarified.It is found that the longer the cavitation length is,the more unstable the cavitation shape is.When the maximum cavitation length is less than 1/3,the cavitation shape is easier to maintain a stable state of sheet cavitation.When the cavitation length exceeds 1/3 of the hydrofoil chord length,the cavitation shape will become unstable and prone to periodic large-scale cloud cavitation shedding;According to the analysis of the dynamic evolution characteristics of cavitation under typical working conditions,it is found that the lower the number of cavitation,the longer the maximum cavitation length,and the lower the periodic frequency of cavitation development.(3)The numerical simulation of the hydrofoil cavitation flow field is carried out,and the numerical simulation of the hydrofoil cavitation shape and periodic characteristics are verified with the experiment,which proves the reliability of the numerical simulation of the hydrofoil three-dimensional transient cavitation flow field based on RNG k-epsilon FBM turbulence model and ZGB cavitation model.(4)Through the unsteady analysis of the pressure distribution characteristics,flow field velocity vector characteristics and entropy generation characteristics under the typical working conditions of cloud cavitation of flat hydrofoil,it is found that the falling off of cloud cavitation leads to the backward movement and disappearance of the low-pressure area on the hydrofoil surface,and the return jet caused by the reverse pressure gradient causes the local high pressure in the low-pressure area covered by the cavitation on the hydrofoil surface and extends upstream.The entropy yield at the interface between sheet cavitation and mainstream is higher,and the entropy yield inside sheet cavitation is lower;Before the cloud cavitation falls off,the flow loss inside the bubble is intensified and the local entropy production at the tail of the bubble is increased.(5)The vortex identification of the flow field under the typical working conditions of cloud cavitation is carried out to capture the U-shaped vortex phenomenon in the process of cloud cavitation.At the same time,the advantages and disadvantages of the two commonly used vortex identification criteria are compared.The results show that the advantage of ?criterion is to eliminate the wrong influence caused by the shear pollution on the hydrofoil wall,and the advantage of Q criterion is that the simulation reproduction of vortex shape is relatively coherent,which can better show the structural evolution characteristics of U-shaped vortex.