Research On The Effect Of Mesoscopic Rectangular Texture On Cavitation And Microbubble Collapse

Cavitation erosion commonly occurs on the surface of flow parts of hydraulic.machinery,which seriously affects reliability and service lifetime of hydraulic machinery.The surface morphology of flow parts is one of key factors affecting cavitation erosion.It has an important influence on microbubble collapse and cavitation of flow field in proximity to the surface of parts.In this paper,a distributed rectangular surface texture in meso scale is selected as the studied wall surface,and the collapse process of microbubble and flow cavitation near to the texture surface are numerically simulated based on the Computational Fluid Dynamics(CFD)method.The shape evolution and dynamics characteristics of microbubble collapse are explored.The physical quantities of flow field including pressure,vapor volume fraction(VVF)and velocity,and details in development of cavitation flow field are obtained.The effects of rectangular surface texture on microbubble collapse and flow cavitation are discussed.Considering bubble surface tension,liquid viscosity and gas compressibility,the collapse processes of a micrometer-sized vapour bubble near to a plane wall and a distributed rectangular groove wall are numerically simulated by the Volume of Fluid(VOF)methodd,respectively.For a plane wall,the tendency of bubble movement towards to the wall becomes more obvious and the high pressure above bubble prior to formation of jet decreases as the non-dimensional stand-off parameter ? decreases.Moreover,the collapse time is prolonged and the maximum jet velocity decreases with decreasing ?.Under the condition of ?=1.0,the cavitation erosion mechanism of the plane wall is the micro jet mechanism.For 1.5>?>1,both micro jet and shock wave are responsible for cavitation damage of the plane wall.For?>1.5,the action of micro jet is faint and cavitation erosion is mainly caused by shock wave.Cavitation erosion on the plane wall can be avoided for y>3.For a distributed rectangular texture wall,as the microbubble is inside the groove,a high pressure zone is not formed above the microbubble prior to occurence of indentation,the motion of bubble wall is relatively slow and the jet velocity is small.Under the condition of smaller stand-off parameter ?2 for the groove texture with the larger depth and smaller width,the effect to impete bubble contraction is enhanced so that the collapse time is further prolonged.As the microbubble is above the groove,its collapse characteristics are similar to ones for the plane wall,when the stand-off parameter ?1 for the grove txture is equal to y for the plane wall.The stand-off parameter ?1 is the main factor and the stand-off parameters ?2 and ?3 are minor ones affecting the collapse characteristics of the bubble above the groove texture wall.Cavitation flow near to the rectangular texture wall is also numerically simulated by applying the mixture two-phase flow model with Singhal full cavitation model.The effects of the geometric parameters of rectangular texture unit,the inlet velocity and the mass fraction of non-condensable gas on flow cavition are discussed.The simulated results show that the disturbance of the groove texture on the main flow is weak and the effects on pressure and velocity distribution of flow field is faint.The rib texture has a significant influence on the cavitation characteristics of the flow field near to the texture wall.Among the geometric parameters of rectangular rib texture,the rib height d is a key parameter affecting flow cavitation.For a constant inlet velocity,as the rib height d is increased,the minimum pressure coefficient decreases,and the low pressure zone and the cavitation zone are enlarged.The incipient cavitation number is rasied with increasing the rib height d,meaning that cavitation easily occurs.Moreover,increasing the mass fraction of non-condensable gas fg in flow field can lead to raise the incipient cavitation number.