Cavitation clouds in gas-containing liquids block low-frequency components of ultrasonic waves

Abstract

Intense single-frequency ultrasonic waves generate cavitation clouds in liquids, while at the same time, the pulsating cavitation bubbles emit broadband noise. In gas-containing liquids such as soda and beer, an ultrasonic source with a driving frequency of 20 kHz is shown to easily stimulate the formation of dense cavitation clouds and large cavitation bubbles in experiments. The acoustic signal is acquired at the boundary (opposite the ultrasonic source) just outside the cavitation cloud. Analysis of this signal using the fast Fourier transform reveals a gap in the acoustic spectrum at low frequencies, which means that the low-frequency acoustic waves cannot penetrate the cavitation area. However, these low-frequency waves can be detected in the cavitation cloud, which means that they are localized there. Through a linear analysis of a nonlinear equation for soundwave propagation in bubbly liquids, we can interpret the localization observed in the experiments. When soundwave frequencies are close to the resonance frequency, the absorption is strong and the corresponding frequencies form a forbidden band in the frequency spectrum unless the void fraction of the bubbles is too small. The intense ultrasonic wave that we applied in the experiment is of a frequency that is slightly smaller than the resonance frequency and can penetrate the cavitation cloud with some attenuation, driving the bubbles to pulsate violently. In this case, noise with frequencies in the forbidden band radiated by nonlinearly oscillating bubbles cannot penetrate the cavitation cloud and can only exist around local bubbles.