Acoustic radiation of collapsing spark-induced cavitation bubbles under various boundary conditions

Cavitation bubble collapse-induced acoustic radiation is closely associated with underwater propeller noise and structural material erosion in marine engineering. Boundary effects can significantly alter bubble collapse dynamics and acoustic radiation, yet the quantitative effects of boundaries on non-spherical bubble evolution and acoustic characteristics remain unclear. In this work, centimeter scale cavitation bubbles are generated by spark discharge, and the acoustic radiation is investigated under three conditions: in the free field, near a rigid wall, and near a free surface. The results show that the first collapse peak pressure broadly follows the energy-based reference scaling P∝Rmax3/2 over the investigated bubble size range. For bubbles adjacent to a rigid wall, the peak pressures of the first and second collapses exhibit distinct dependences on the dimensionless stand-off distance. For bubbles near a free surface, the measured acoustic directivity is quantitatively consistent with the dipole image-source model. In addition, a pressure-based acoustic inversion method is developed to estimate the early bubble radius evolution that cannot be directly resolved by high-speed imaging because of spark luminescence. These findings establish a quantitative relationship among boundary conditions, collapse dynamics, and acoustic loading, and provide a systematic experimental benchmark for cavitation noise modeling and control.