Research On The Sound Propagation And Heat Production In Cavitation Clouds

We calculate the sound field pressure and the bubble radius in cavitation cloud by solving the nonlinear sound wave equation together with the bubble dynamic equation.In this paper we study the spectral gap of the acoustic wave in cavitation clouds,the heat production characteristics of cavitation bubbles and how the coated bubbles enhance the heating process in the tissue.We studied the phenomenon of spectral gap of the acoustic wave in cavitation cloud by numerical calculation,experiment and theoretical analysis respectively.The calculation results show that when the radius and the number density of the bubbles meet some certain conditions,there will be a spectral gap in the low frequency range and the acoustic wave of these frequencies cannot penetrate the cavitation cloud.In the experiment,we used liquids of different gas content to verify the calculation results and studied how the bubble radius and number density will influence the formation of the spectral gap.Through a linear analysis of the nonlinear soundwave equation in bubbly liquids,we interpreted the spectral gap and distinguished the forbidden band and the localization phenomenon in the cavitation clouds theoretically.In the study of the heat production of cavitation clouds,we first obtained the heat generated at bubble walls caused by the liquid viscosity and the heat produced by liquid absorption through numerical calculation.We found that when the intensity of the driving ultrasound is high,the heat generation at bubble walls dominants the heating process.In addition,the heating peak of cavitation clouds will deviate from their resonance point when the acoustic localization appears.In the experiment,we used thermocouple to measure the temperature rise in the cavitation cloud,and the experiment results are in good agreement with the calculation results under the condition of weak driving ultrasound.However,when the intensity of the driving ultrasound is high,the temperature rising in the experiment is lower than the calculation results,because the liquid flow in the experiment causes some heat loss.Then we adjusted the experiment condition and studied the influence of liquid flow and the bubble number density on the temperature rising in the cavitation clouds.The thermal effect of microbubbles plays an important role in HIFU ablation.In numerical calculation,we calculated the temperature rise caused by the bubbles at the focus point of HIFU by solving the dynamic equation of coated bubbles together with the nonlinear acoustic wave equation.In the experiment,we injected the Sono Vue solution to the tissue phantom and used the thermal strain method to measure the temperature rise at the focus point of HIFU.The results showed that the bubbles can lead to enhanced heating and the temperature rising in experiments are consistent with the calculation results.In addition,we used the theoretical method proposed in this paper to calculate the variation of the bubble radius with time for three different coated-bubble dynamic models.The results showed that when the driving ultrasound is weak,the dynamic responses of a single bubble obtained by the three models are basically the same.However,under the condition of high intensity driving ultrasound and multiple bubbles,the bubble dynamic responses under the three models are different.