Acoustic Signatures of Pinch-Off Cavities During Water-Entry

TL;DR

This paper combines experimental, numerical, and theoretical methods to analyze the cavity and acoustic dynamics during water entry of a cylindrical projectile, revealing how cavity oscillations influence underwater sound signatures.

Contribution

It introduces a semi-theoretical model incorporating boundary effects that accurately predicts cavity oscillation frequencies during water entry, advancing understanding of underwater acoustics.

Findings

Cavity oscillation frequency increases with Froude number.

The boundary effect raises the cavity frequency above the Minnaert frequency.

Experimental and numerical results show good agreement with the theoretical model.

Abstract

This study experimentally, numerically, and theoretically investigates the cavity/bubble dynamics and radiated acoustics during the water entry of a centimeter-scale cylindrical projectile with a conical nose. Experiments were conducted in a laboratory tank, employing synchronized high-speed imaging and hydrophone measurements to characterize the cavity closure modes and their resultant acoustic signatures across a range of Froude numbers. The acoustic signal features a weak radiated signal upon impact, followed by significant pressure oscillations spanning more than 20 cycles in the flow field after cavity elongation and pinch-off. A numerical model based on the Finite Volume Method (FVM) successfully captures these physical processes. Subsequently, a semi-theoretical model that incorporates the projectile's boundary effect is developed from potential flow theory. The model not only yields a dominant cavity oscillation frequency that agrees well with experimental data, but also reveals that the boundary effect leads to a cavity oscillation frequency markedly higher than the Minnaert frequency of an equivalent-volume ellipsoidal bubble containing an internal rigid core. The dominant cavity frequency falls nearly linearly with Fr, governed by nose geometry and projectile inertia. This study clarifies the underlying physics connecting cavity dynamics during water entry to underwater acoustic radiation.