Numerical study of the kinematic and acoustic characteristics of bubble clusters

Direct numerical simulations are performed to study single gas/vapor bubble and spherical bubble clusters containing 13-352 vapor bubbles in compressible flow fields. The numerical results show that the single cavitation bubble keeps spherical during the collapse process, and the far-field acoustic pressure calculated by the Ffowcs William-Hawkings (FW-H) formulation is basically consistent with the analytical solution obtained based on the volume acceleration calculation. However, the spherical bubble cluster collapses layer by layer due to the strong coupling between bubbles. The closer to the center of the bubble cluster, the shorter the collapse time and the stronger the non-spherical deformation. The collapse of a bubble cluster would generate multiple acoustic pressure peaks, which cannot be accurately predicted by the volume fluctuation sound source theory. The size and volume fraction of the bubble cluster have a significant influence on the collapse time and the distribution of sound pressure. We found that when the volume fraction of a bubble cluster is large, the total collapse time is basically the same as that of its corresponding single bubble with the equal volume. The frequency distribution of sound pressure of a dense bubble cluster is also close to that of its corresponding single bubble. In addition, we found that a bubble cluster with randomly distributed bubble diameters collapses asymmetrically and rebounds in the late stage of the collapse process. The above study reveals part of the mechanism of bubble cluster collapse and sound generation, and provides a theoretical basis for the establishment of cavitation noise model.