Reducing water entry impact forces

TL;DR

This study demonstrates that pre-jetting water before an object impacts can significantly reduce impact forces, with a critical jet volume identified for optimal force mitigation, supported by experimental and scaling analyses.

Contribution

The paper introduces a novel method of reducing water entry impact forces by using a pre-jet of water, supported by experimental validation and scaling analysis.

Findings

Pre-jetting reduces impact forces on spheres.

A critical jet volume maximizes force reduction.

Scaling arguments explain the critical volume.

Abstract

The forces on an object impacting the water are extreme in the early moments of water entry and can cause structural damage to biological and man-made bodies alike. These early-time forces arise primarily from added mass, peaking when the submergence is much less than one body length. We experimentally investigate a means of reducing impact forces on rigid spheres by making a jet of water strike the quiescent water surface prior to the object impacting. The water jet accelerates the pool liquid and forms a cavity into which a sphere falls. Through on-board accelerometer measurements and high speed imaging, we quantify the force reduction compared to the case of a sphere entering a quiescent pool. Finally, we find the emergence of a critical jet volume required to maximize force reduction; the critical volume is rationalized using scaling arguments informed by near-surface particle image velocimetry (PIV) data.