Coupling Of Bubbles In A Bubble Group Under The Action Of A Sound Field

Bubbles have important applications in many fields,such as the erosion of propeller in marine engineering,the improvement of chemical reaction rate in chemical engineering,medical ultrasound coating and so on.Among them,acoustic cavitation is an important topic to study the bubble problem.Since the phenomenon of acoustic cavitation was found,it has attracted the attention of researchers and become a hot topic in hydrodynamics,ultrasound and physics.In practical engineering applications,cavitation bubbles do not always appear alone,but in the form of bubble groups.There are tens of thousands of bubbles in the bubble group.At this time,there is a very complex coupling between bubbles,which makes the vibration form of each bubble completely different.Therefore,it is of great academic significance and engineering value to reveal the coupling mechanism of multiple bubbles and to explore the law of interaction between bubbles.In view of this,this paper will mainly study the influence of the interaction between bubbles in different shapes on the resonant frequency of bubbles,simulate the growth and collapse of bubbles in the bubble group under the action of sound field through software,and modify the resonant frequency of bubbles in the bubble group.The main research contents are as follows:The dynamic characteristics of cavitation bubble group is the dynamic behavior under the action of sound field,which is influenced by many factors,such as the initial radius of cavitation bubble,the amplitude of driving sound pressure,the frequency of driving sound pressure,the number of bubbles in the bubble group and so on.First of all,starting from the dynamic equation of single cavitation bubble and bubble in bubble group,the equation is solved by the fourth-order Runge-Kutta algorithm,and the evolution curve of bubble radius in single bubble and bubble group under the action of sound field is studied systematically.The results show that the coupling effect between bubbles in a bubble group can inhibit the growth of bubbles,which reduces the expansion ratio of bubbles and changes the growth and collapse time of bubbles.Considering the coupling effect between bubbles,the radius evolution relationship of bubbles in a bubble group is different from that of a single bubble,which is closely related to the number of bubbles in a bubble group and the distance between bubbles in addition to the influence of initial radius of bubbles and driving sound parameters.The larger the number of bubbles in the bubble group and the closer the distance between bubbles,the greater the interaction force between bubbles,and the smaller the maximum radius that bubbles can reach in the expansion process,which shows that bubbles are more difficult to grow.The dynamic equation of bubbles in a bubble group is a nonlinear differential equation,and it is difficult to get its analytical solution.Through linear reduction of the equation,the forced vibration equation of cavitation bubbles in different shapes of bubble groups after reduction is derived,and the resonance frequency of bubbles in the bubble group is given,and the analytical solution of bubbles in small amplitude vibration is obtained.By comparing with the resonant frequency of a single bubble,the correction coefficients of the resonant frequency of a bubble in a bubble group and that of a single bubble are given.The results show that there is weak coupling between bubbles in the bubble group,and the coupling vibration changes the resonant frequency of bubbles.The resonance frequency of bubbles in a bubble group is not only related to the initial radius,number and spacing of bubbles in the bubble group,but also to the shape distribution of bubbles in the bubble group.Among these parameters,the initial radius of bubbles and the distance between bubbles have the most significant effect on the resonant frequency of bubbles.When the number of bubbles is the same,the larger the distance between bubbles,the weaker the interaction between bubbles.At the same time,it is found that the coupling force between bubbles in different shapes is different,which results in different resonance frequency and frequency correction coefficient.Under the same initial conditions,the interaction of bubbles in the spherical bubble group is obvious,and the resonance frequency of bubbles varies widely.Through the fluid dynamics software to simulate the cavitation process,the micro bubble growth process and the rapid collapse process can be well demonstrated,at the same time,the coupling process of bubbles in the bubble group and the non spherical change of bubbles can be well reflected to achieve better visualization research.The growth and collapse of five or seven equal bubble groups with different shapes under the action of sound field are simulated.The simulation results show that the bubbles in different shapes can be under the action of sound field.Because of the uniform pressure distribution inside and outside the bubbles,the bubbles can keep the spherical growth well until they reach the maximum bubble volume.The simulation results are in good agreement with the theoretical results.With the passage of time,all bubbles begin to contract almost at the same time.In the process of contraction,in the spherical bubble and the cylindrical bubble group,the contraction speed of the outer bubble is slightly faster,which changes from the spherical to the ellipsoid slowly,and then keeps the ellipsoid shrinking all the time.When the bubble shrinks to a certain extent,the peripheral bubble begins to collapse first,and in the process of collapse,a micro jet is formed which points to the interior of the bubble,and breaks the bubble down from the middle along the axial direction of the center of the linear bubble group.The simulation results show that the bubble can only be spherical in the period of expansion,and its non spherical effect will become significant in the period of contraction and collapse.Based on the perturbation theory,the dynamic equation and the form instability equation of two non spherical bubbles in the sound field are derived,and the influence of sound parameters on the bubble shape instability is analyzed.Finally,the growth and collapse process of two interacting spherical and non spherical bubbles in the sound field is simulated by the hydrodynamics software.The shape instability of bubble is mainly affected by the velocity and acceleration of bubble wall.If the acceleration of one side of bubble is larger than that of the other side in the process of expansion or contraction,then the bubble will deviate from the sphere.For the spherical and non spherical bubbles,the unstable time is mainly concentrated in the bubble contraction and collapse time.At this time,due to the sharp change of the internal and external pressure of the bubble,the acceleration on both sides of the bubble wall will be greatly different,and the horizontal and longitudinal contraction speed will be inconsistent,and the bubble will seriously deviate from the sphere.The growth and collapse process of two spherical and ellipsoidal bubbles are calculated by numerical simulation and VOF comparison.It is found that the bubble of spheroid bubble and ellipsoid bubble is less affected by the shape instability in the bubble expansion stage,and can basically maintain its original shape expansion.