Nonlinear Sound Propagation In A Liquid Medium And Cavitation Dynamics Study

Based on the properties of acoustic propagation in bubbly liquids, the standing wave and nonlinear characteristics are researched, at the same time, their effects on the distribution and intensity of cavitation field are analyzed. In order to explore the physical essence of cavitation, the physical features in bubbly liquids are explored, such as the movements of bubbles, the motion of bubble groups and the interaction of bubbles, the action on the surface of solid boundary when bubbles or bubble clusters collapse. In this paper, the primary works are outlined as follows:(1) The properties of acoustic propagation in bubbly liquids are studied. First, the acoustic radiation of bubble and bubble clusters is analyzed in liquids. Bubbles are driven to vibrate periodically when ultrasonic wave propagates in liquid, accompanied by acoustic radiation from them. Second, the nonlinear wave equation of bubbly liquids is obtained. Based on the boundary conditions of acoustic fields in bounded space, the solutions of wave equation are deduced, and the expressions of fundamental and second harmonic wave are obtained by perturbation methods. At the same time, the characteristics of quasi-standing wave are researched. Third, the nonliear wave equation was deduced in bubbly liquids where the cross sectional area is changed, and the distribution of the fundamental and second harmonic wave is analyzed. The results show that the distribution of acoustic energy can be influenced by volume fraction of bubbles, the initial states of bubbles, the shape of containers and medium nonlinearity.(2) The movements of driven bubbles are researched in the bubbly liquids of bounded space. First, the translation and radial vibration of bubbles and their interaction are studied. The results show that the radial vibration of big bubble is weaker than cavitation bubbles. The translation displacement changes violently when cavitation bubbles collapse, while micro-bubbles vibrate in smaller amplitude, and their translation is periodic because their mass centers reciprocate round their initial position regularly. Second, vibration of bubbles with the large amplitude is explored and the size range of cavitation bubbles is estimated related to certain driven condition. The phase chart of a cavition bubble is chaotic coupled with an oddness attractor, while that of a micro-bubble approximates to triangle, and that of a big bubble is ellipse approximately. Third, the vibration of a bubble near to plane or spherical rigid walls is analyzed and the dynamic equation is obtained. The resonant frequency decreases when a bubble vibrates near a rigid wall. The simulated results of bubble vibration show that its amplitude and mode are influenced by the walls to some extent. (3) The mechanism of cavitation field is explored. First, the growth law of bubble nucleus is studied. The quasi-static equilibrium equation is obtained when bubbles exist in the acoustic field, and then the lowest cavitation threshold is gained. Second, three modes of bubble clusters are assumed and the vibration of bubble clusters is explored. Based on the theroy of energy transport, the contribution of vibrating bubbles to energy convertion is analyzed. Third, the interaction forces and the stress response of boundary surface are taken into account. The shear and normal stress is produced on the surface when bubble or bubble clusters collapse, which can result in cavitation erosion. The normal and tangent components of liquid flow field change rapidly when bubble or bubble clusters collapse, which causes the environment of solid dipped into the cavitation field to change, so the effects related to cavitaton is produced, such as shock waves, micro-jets, and the deformation of solid.(4) The ultrasonic cavitation field is researched experimentally. First, the form of bubble and bubble clusters is recorded by high speed photography. The results show that the bubble nucleus outside the acoustic radiation region are tended to move to the main region of acoustic radiation. The bubble sizes are not homogeneous, and there are many big bubbles which can be distinguished easily by the naked eyes in the cavitaton field. The big bubbles vibrate with smaller amplitude and translate with higher speed in the bubble groups. Second, the average velocities are estimated relating to the translation of big bubbles and the expansion of bubble groups. Both of two average velocities are of the order of 0.1 m/s. Third, based on the spectrum analysis of received signal from hydrophone, we explored the acoustic energy distribution in the region of bubbles. The results show that there are many frequency components in the cavitaion region. The fundamental wave has the same frequency as the driving acoustic wave. Because of the nonlinearity of medium and bubble vibration, the acoustic radiation from bubbles and the interaction of different frequency acoustic waves, the second harmonic and other frequency components such as subharmonics are produced.The main contributions of this thesis are as follows:(1) The mode of acoustic propagation is developed in the bubbly liquids holden in the containers of different shape with varying cross sectional area. Based on the mode, we analyze the nonlinear properties of standing wave. The conclusions have some applicative value for the acoustic field analysis of various reactor vessels or pipes, which are usually employed in the course of the vessels design for ultrasonic cleaning or sonochemical reaction and the ultrasonic cleaning of pipes.(2) The size range of cavitation bubbles is estimated, which can provide the theoretical basis for regulation of cavitation field. (3) Based on the physical properties of mass interaction, the mechanism of cavitation field is explored. The theroy of bubbles clusters collapse is developed.(4) The form and evolution of bubble field is recorded by high speed photography in real-time mode when the transducer works under high-power. Based on the video recordings and pictures, the movements of big bubbles and small local bubble clusters are researched. At the same time, we take into account the effects of solid surface on the distribution of bubbles.