Supersonic Air Flow due to Solid-Liquid Impact

TL;DR

This study reveals that a solid impact on liquid can generate a supersonic air-jet from the collapsing cavity, resembling compressible flow through a liquid-shaped nozzle, even at low impact velocities.

Contribution

The paper demonstrates, through experiments and simulations, that impact-induced liquid cavities can produce supersonic air jets, a novel insight into impact dynamics and fluid flow behavior.

Findings

Supersonic air-jet speeds are achieved at impact velocities as low as 1 m/s.

The air flow structure closely resembles compressible flow through a nozzle.

A liquid cavity acts as a rapidly shrinking nozzle, producing high-speed air jets.

Abstract

A solid object impacting on liquid creates a liquid jet due to the collapse of the impact cavity. Using visualization experiments with smoke particles and multiscale simulations we show that in addition a high-speed air-jet is pushed out of the cavity. Despite an impact velocity of only 1 m/s, this air-jet attains \emph{supersonic} speeds already when the cavity is slightly larger than 1 mm in diameter. The structure of the air flow resembles closely that of compressible flow through a nozzle -- with the key difference that here the "nozzle" is a \emph{liquid} cavity shrinking rapidly in time.