| Cavitation is a complex hydrodynamic phenomenon,often accompanied by extremely complex physical and chemical reactions,which involves a wide range of fields,especially in fluid machinery,marine engineering and military engineering.For example sediment wear and cavitation in natural water flow will have an impact on hydraulic machinery components,the combined effect(abrasion)far exceeds the accumulation of the two separate effects.When the abrasion occurs near the free fluid boundary,it will not only cause damage to the components in the fluid,but damage the objects above it due to the huge impact force generated by the deformation.Thus,it is vital to research the influence of solid particle and free fluid boundary on the cavitation dynamics,such as cavitation initiation,development and collapse.In our paper,taking the bubble as the research object,the fluid volume method and the interface tracking method are used to analyze the influence of three different factors,namely the distance between the bubble and particle,the particle size and free fluid boundary,on the dynamic behavior of the cavitation,then the collapse mechanism of bubble near different boundary surfaces is studied.2Firstly,the effect of the distance between the bubble and particle on the cavitation dynamic is studied.When the bubble collapsed near fixed spherical particle,the cavitation-particle distance parameter γ has a great influence on the bubble shape,the movement of bubble-edg in different directions,the microjets and the pressure difference on particle surfaces.The results show that:(1)As the increase of γ,the bubble show three different shapes,mushroom-shaped(0≤γ≤0.6),pear-shaped(0.7≤γ≤1.4)and spherical(γ≥1.5),during the collapse process.(2)Compared with bubble changes in three different shapes,the variation of the distance Lx of bubble-edge in the X direction is very different,but Ly in the Y direction is very similar.(3)The microjets produced by different bubble shapes can be divided into contact jet,non-contact jet,and long-distance jet.(4)Within the range of γ researched in our article,as the increase of γ,the maximum jet velocity Vmax and difference of pressure on the particle ΔPmax both increase first and then decrease,and the peak value appears at γ=0.9,the maximum jet velocity Vmax is up to 180m/s,the maximum pressure difference ΔPmax is about 10.8MPa.Secondly,the effect of particle size on the cavitation dynamic is studied.When the bubble collapsed near spherical particle,the particle size has a great influence on bubble collapse time,the microjets and the shock wave on the particle surface.The results indicate that:(1)Under the conditions of three different distance parameters γselected in our paper,the bubble collapse time increases with the increase of particle size ratio δ.(2)Under the same distance parameter γ,the jet velocity Vjet,the maximum jet velocity Vmax,the surge pressure generated by microjets,and the shock wave all increased first and then decreased with the increase of δ,the maximum value appears at γ=0.9 and δ=1.2,the maximum jet velocity Vmax is up to 165.3m/s,the surge pressure is as high as 248.1MPa,and the shock wave is about 10.46 MPa.(3)In our research,the effect of surge pressure generated by microjets on the particle surface is much greater than that of the shock wave,but the impact of the shock wave cannot be ignored.Finally,the effect of free surface on the cavitation dynamics is studied.When the bubble collapse between the free surface and particle,the distance parameter ε has a great influence on the deformation of free surface,the microjets and the surface pressure of particle.The results indicate that:(1)In the range studied in this paper,when ε=0.3,the deformation of free surface is the largest,at this time the maximum liquid column height Hmax generated by the free surface is 12.5mm,and the maximum value of the rising velocity at the top of the liquid column VMAX is 98.8m/s.(2)During the collapse process,the jet velocity generated on the upper surface of the cavitation will produce three different changes,the maximum jet velocity Vmax occurs in the growth stage when ε=0.4,and Vmax is about 120m/s.(3)When the bubble is close to the free surface,there are three peaks of the pressure P on the upper surface of the particle with time,the maximum peak Pmax appears when ε=0.4,about 3.8MPa.When the distance gradually increases,there is only one peak value of the pressure P on the upper surface of the particle with time,and the maximum peak value Pmax appears when ε=1.5,which is up to 12.3MPa.The results of our study will help to deepen the understanding of the erosion phenomenon,and can provide theoretical and experimental basis for studying the damage of the erosion phenomenon in the fields of fluid machinery,marine engineering and military engineering. |
