Numerical Simulation Study Of Acceleration Mechanism Of Shock Wave In The Interaction Between Microbubble And Microsphere

The interaction between cavitation bubble and different types of boundary has been a hot topic in bubble dynamics.When the characteristic scale decreases,the collapsing bubble will drive the suspended microsphere forward significantly.So far,the mechanism of accelerated motion of microsphere is not very clear.In this paper,the interaction between cavitation bubble and microsphere with the same scale is studied,focusing on the impact of shock wave on particle caused by bubble collapse.The numerical simulation method based on VOF method is used to study the interaction between shock wave and microsphere.The numerical method based on the volume of fluid is employed for tracking liquid and gas phase,while compressibility effects is introduced for each phase.Firstly,the condition of bubble collapse is obtained by static analysis,and a twodimensional axisymmetric model is established.The results,is simulated by the VOF method for bubble collapse in free field,were validated by the Rayleigh-Plesset equation.It is found that the deviation between the R-P equation and the numerical simulation results is very small,which meets the requirement of followed research.Secondly,the morphological evolution of bubble collapse during the interaction between cavitation and suspended microsphere at the same scale is studied.The propagation law of shock wave generated by bubble collapse in the flow field is analyzed.The effects of different fluid physical parameters,bubble size and pressure difference inside and outside the bubble during bubble collapse are compared.The results show that the collapse strength is sensitive to the pressure difference and the bubble size,but not to the physical properties of fluids in collapse stage.The characteristics of fluid motion can be changed during shock wave propagation.Thirdly,the effects of different boundary types on the process of bubble collapse are studied.The influence of the spacing between bubble and different types of interface on the collapse characteristics and the intensity of shock wave produced by bubble collapse are analyzed.The results show that spacing to the interface has a significant effect during bubble collapse.The deformation of the interface can change the location of the bubble collapse point and affect the intensity of the collapse shock wave.The surface tension is the fundamental reason to weaken the collapse strength of cavitation bubble.At last,the stress of microsphere under shock wave is analyzed theoretically and numerically.The mechanism of primary and secondary propulsion of microsphere under shock wave is described.The maximum velocity of microsphere can be estimated by integrating the impulse of microsphere as a whole.The results show that the interaction between shock wave and microsphere is non-linear at micro-scale.Shock wave produced by bubble collapse can cause asymmetric pressure distribution around microsphere,which is the main reason of instantaneous acceleration of microsphere,and the acceleration time scale by shock wave is in the order of ~10?s.The simulation results are qualitatively consistent with the experimental results.The analysis shows that the efficiency of shock wave propulsion can be improved by shortening the distance between the center point of bubble collapse and microsphere.This special micro-flow problem can be used for reference in understanding the micro-scale cavitation problem and improving the efficiency of self-propulsion.