Air flow in a collapsing cavity

TL;DR

This study investigates the airflow dynamics in a collapsing cavity caused by a disk impacting water, combining experiments and simulations to understand the role of air compressibility near pinch-off.

Contribution

It introduces experimental measurements of air flow in collapsing cavities and compares them with boundary integral simulations, highlighting the importance of air compressibility.

Findings

Airflow rate depends on the Froude number.

Compressibility becomes significant near pinch-off.

Experimental results align with the theoretical model.

Abstract

We experimentally study the airflow in a collapsing cavity created by the impact of a circular disk on a water surface. We measure the air velocity in the collapsing neck in two ways: Directly, by means of employing particle image velocimetry of smoke injected into the cavity and indirectly, by determining the time rate of change of the volume of the cavity at pinch-off and deducing the air flow in the neck under the assumption that the air is incompressible. We compare our experiments to boundary integral simulations and show that close to the moment of pinch-off, compressibility of the air starts to play a crucial role in the behavior of the cavity. Finally, we measure how the air flow rate at pinch-off depends on the Froude number and explain the observed dependence using a theoretical model of the cavity collapse.