Analysis Of Cavitation Shock Wave Effect And Flow Field Characteristics Under Ultrasonic Excitation

With the development of science and technology,ultrasonic cavitation effect has been widely used.It is found that rational use of ultrasonic cavitation effect can bring great convenience to human production and life.The study of cavitation shock wave effect and the characteristics of the surrounding flow field is of great significance to promote the application of cavitation phenomenon.In this paper,the cavitation shock wave effect in ultrasonic field and the characteristics of the surrounding flow field in the process of cavitation are studied.Based on the assumption that the cavitation bubble changes spherically,the cavitation shock wave effect is studied theoretically.Through simulation analysis,the dynamic process of single cavitation bubble,double cavitation bubble and the flow field characteristics in the process of cavitation bubble movement are studied.Through observation and experiment,the motion of cavitation structure and its impact effect in the process of cavitation impact on solid wall surface are studied.The results of the research will provide technical reference for better utilization of ultrasonic cavitation effect and enhancement of cavitation effect.Firstly,the relationship between cavitation parameters and shock wave effect is studied.The model of ultrasonic cavitation shock wave pressure considering acoustic pressure is established based on the relationship between fluid and cavitation motion characteristics.The effects of acoustic pressure amplitude,ultrasonic frequency,initial bubble radius,liquid viscosity and surface tension on shock wave effect are analyzed numerically.On this basis,the shock wave caused by a spherically collapsed bubble on the solid wall is analyzed.The results show that the maximum pressure of shock wave increases with the increase of acoustic pressure amplitude,but decreases with the increase of ultrasonic frequency,initial radius,liquid viscosity or surface tension coefficient.Results demonstrate that the pressure on the solid surface can still reach more than one gigapascal after the shock wave decays.Secondly,finite element analysis method is used to simulate the dynamic process of the single cavitation bubble in the water under ultrasonic excitation,and the change of the radius of the single cavitation bubble over time was compared with the numerical analysis results of the theoretical equation.On this basis,the pressure,density and velocity characteristics of the flow field around the cavitation bubble in the process of single cavitation bubble movement were analyzed.The motion of the single cavitation bubble near a solid wall is studied based on the simulation of the single cavitation bubble.The results show that the changes of pressure and gas density in the bubble are inversely proportional to the changes in the volume of the single bubble.In the process of expansion,the pressure inside the bubble is greater than the pressure outside the bubble.When the single bubble radius is smaller,the high pressure layer will be formed around the bubble,and the density inside the bubble is the largest.During the subsequent small oscillations,the cavitation bubble will show non-spherical changes.The micro-jet velocity produced by cavitation collapse near the solid wall increases for a while and then decreases.Thirdly,the dynamic process of the double cavitation bubbles under ultrasonic excitation was studied by finite element analysis method considering the interaction between the double cavitation bubbles.The motion characteristics of the double cavitation bubbles and the pressure characteristics of the surrounding flow field in the double cavitation bubble model are obtained.On this basis,the movement of single cavitation bubble and double cavitation bubble and the characteristics of the surrounding flow field under the same conditions were compared and analyzed combined with the simulation results of the dynamic process of single cavitation bubble.The micro-jets produced by the collapse of the smaller cavitation bubble during the movement of double cavitation bubbles are analyzed.The results show that the pressure gradient is generated along the vertical direction of sound pressure between the double cavitation bubbles after the collapse of the smaller cavitation bubble,and the pressure decreases with the distance from the center of the smaller cavitation bubble.Compared with the movement of the single cavitation bubble,the time of the first expansion of the larger cavitation bubble in the double cavitation bubble is shorter,and the time of the first compression is longer.In the second expansion process,the smaller cavitation bubble will collapse and produce a micro-jet.In the process of double cavitation bubble movement,the micro-jet generated by the smaller cavitation bubble and the pressure gradient act on the larger cavitation bubble,resulting in non-spherical deformation of the larger bubble.Finally,the formation and development of cavitation structure in the process of cavitation structure impact on solid surface were observed by high-speed photography technology,and the movement of cavitation bubble group was analyzed through the establishment of cavitation impact observation and test platform.Then,multiple groups of cavitation impact tests were carried out under different parameters,and the influence of ultrasonic machining parameters on the effect of cavitation impact on solid surface was studied.The results show that the cavitation bubble group develops from hemispherical to columnar,and finally forms a stable inverted T-shaped cavitation structure in the region between the radiation end face and the solid surface during the process of cavitation impact.As the distance between the radiant end face and the material surface decreases,the cavitation pits on the material surface gradually increase and the surface morphology changes become more and more drastic.The increase of ultrasonic amplitude will lead to a stronger effect of cavitation structure on the material,and the more severe the cavitation impact.