Slamming Forces During Water Entry of a Simple Harmonic Oscillator

TL;DR

This study investigates how the elasticity of an impactor affects the hydrodynamic forces during water entry, revealing that softer impacts can sometimes increase forces due to a hydroelastic transition.

Contribution

It introduces a systematic analysis of impactor elasticity effects on water entry forces using combined experiments and theory, highlighting a critical hydroelastic factor.

Findings

Softening impactors does not always reduce impact forces.

A hydroelastic factor determines force transition from reduction to amplification.

Experimental and theoretical results agree on the critical role of impactor elasticity.

Abstract

When a blunt body impacts an air-water interface, large hydrodynamic forces often arise, a phenomenon many of us have unfortunately experienced in a failed dive or "belly flop." Beyond assessing risk to biological divers, an understanding and methods for remediation of such slamming forces are critical to the design of numerous engineered naval and aerospace structures. Herein we systematically investigate the role of impactor elasticity on the resultant structural loads in perhaps the simplest possible scenario: the water entry of a simple harmonic oscillator. Contrary to conventional intuition, we find that "softening" the impactor does not always reduce the peak impact force, but may also increase the force as compared to a fully rigid counterpart. Through our combined experimental and theoretical investigation, we demonstrate that the transition from force reduction to force amplification is delineated by a critical "hydroelastic" factor that relates the hydrodynamic and elastic timescales of the problem.