The collapse behavior and acoustic characteristics of bubble clusters with varied layers and fractions under the wall's shielding effect

This paper studies the effects of the wall on the noise spectrum of bubble clusters during near-wall collapse. Applying direct numerical simulations and the Ffowcs Williams–Hawkings method to evaluate sound pressure (SP), the acoustic characteristic of layered clusters with varied layers and fractions is investigated. Preliminarily knowing about “the wall's shielding effect” on cavitation bubble collapse, we discussed how its strength and action mechanism change in detail. The results reveal that the greater the number of layers is, the more obvious “the wall's shielding effect” will exceed the interlayer shielding effect, resulting in more significant collapse asymmetry and larger disparities of sound pressure (SP) between internal bubbles. The wall imposes minor enhancement or suppression on the time-domain SP peaks and improves the overall sound pressure levels (SPLs) of the multi-layer bubble clusters. For the different-fraction clusters, the wall significantly delays the collapse of the dense cluster, as well as increasing its frequency-domain sound pressure level (SPL), but has slight effects on the SP peaks and SPL decay rates of both the dense and sparse clusters. The decay rates of α = 37.4%, 7.16%, and 0.98% bubble clusters with more than four layers roughly stabilize at 2.4, 4.0, and 5.2 dB/octave, respectively. Inside the sparser cluster, “the wall's shielding effect” is more even but predominant for individual bubbles. These conclusions provide a concrete discussion of the effect mechanism of the wall on the acoustic radiation of bubble clusters with varied layers and fractions, thereby laying a solid foundation for the practical evaluation of cavitation noise.