Acoustic radiation of tail bubble dynamics in underwater cylinder launch.

This study experimentally and numerically investigates the acoustic radiation from high-pressure tail bubbles generated during underwater cylinder launch. Synchronous high-speed imaging and hydrophone measurements in a water tank capture the transient bubble evolution and the acoustic signatures, clarifying the relationship between tail bubble dynamics and acoustic radiation. A boundary integral model reproduces the experimental observations, and a natural frequency model is developed to characterize the acoustic response of non-spherical bubbles. A systematic parametric study is then performed to evaluate the effects of ventilated pressure and free surface on tail bubble dynamics and acoustic radiation. The results reveal a stage-dependent acoustic radiation feature: acoustic radiation is negligible while the cylinder moves inside the launcher. Upon exit, significant acoustic radiation is produced by the tail bubble. After pinch-off, oscillations of two residual bubble structures dominate late-stage acoustic signals. The proposed model shows good predictive accuracy for the tail bubble natural frequency. Furthermore, the dominant acoustic frequency exhibits a non-monotonic depth dependence with a distinct minimum at intermediate depths. Beyond a critical depth, it is primarily governed by the ambient pressure. These findings constitute a valuable reference for underwater launch bubble acoustics.