Abnormal Attenuation Of Powerful Ultrasound Propagating In Cavitating Liquids

Compared with the propagation of sound in solids or gases, the propagation of sound waves in liquids has its unique mechanism, which is the acousitc cavitation. When the intensity of ultrasound waves in liquids is large enough, large amounts of micron-size cavitation bubbles appear and affect the propagation of ultrasound in return.Owing to bubbles’ scattering and dissipating effects, propagation of ultrasound waves will weaken,which is called cavitation shielding.In this paper, we mainly study the acoustic attenuation of high-intensity ultrasonic in cavitating liquid. We set up an experimental system composed of three parts:ultrasound irradiation,sound signal acquistion and spatial positioning. The overall system connected with GPIB control and RS-232 serial interface is monitoed by a computer and can realize digital program-control.With the experimental system,We measure the distribution of the sound pressure in the tank under different driving pressure and compare the relationship between the sound pressure and the driving pressure in the near-field and far-field. We find that,in the near-field,the acoustic attenuation is is stronger under high driving sound pressure and weaker under low driving sound pressure.Nonetheless, the sound pressure corrsponding to high driving sound pressure is still higher than that of low driving sound pressure.However,in the far-field,we obtain the opposite result that the sound pressure corrsponding to high driving sound pressure is lower than that of low driving sound pressure.We call it the abnormal attenuation.We divide the cavitation field into two parts, attenuation region and abnormal attenuation region based on the relative value between the sound pressure corrsponding to high driving sound pressure and the sound pressure corrsponding to low driving sound pressure.In the attenuation region,the cavitation is more violent under high driving sound, the sound pressure drived by high driving sound pressure is still higher than that of low driving sound pressure.However, in the abnormal attenuation region, high driving sound pressure leads to low sound pressure.Owing to relative far distance away from the ultrasound irradiation source and the cavitation shielding caused by cavitation bubbles,in the far-field,the sound pressure under different driving sound pressure and the extent of cavitation are almost same.Thus,the liquids in the far fields under dfferent driving sound pressure should have the same physical characteristics.But the experiment results show that the sound pressure corrsponding to high driving sound pressure attenuates faster so that it is lower than that of low driving sound pressure abnormally.In order to understand the abnormal attenuation, we acquire the sound pressure’s time domain signal at several points in a one-dimension sound field.Then,we perform the FFT method to obtain the spectrum along the one-dimension field.The result shows that, in the near-field,stronger driving sound pressure leads to more violent cavitation effect and much more sound energy transfers to high harmonics.The high harmonics energy decays more rapidly in the liquids than the fundamental,which leads to the abnormal attenuation in the far-field-the sound pressure corrsponding to high driving sound pressure is lower than that of low driving sound pressure.On the basis of experimental results that attenuation factor varies with different driving sound pressures,we obtain numerical simulation of sound pressure distribution under different driving sound pressures.