Explosive ripple instability due to incipient wave breaking

TL;DR

This paper investigates the explosive ripple instability on steepening nonlinear waves, revealing how small ripples can rapidly grow and contribute to wave breaking phenomena, supported by analytical and numerical methods.

Contribution

It derives an explicit asymptotic expression for ripple steepness change, linking nonlinear effects to wave dynamics and identifying conditions for super-exponential instability.

Findings

Strong ripple compression leads to explosive instability.

Analytical expressions match numerical simulations.

Instability may explain wave fragmentation and whitecapping.

Abstract

Considering two-dimensional potential ideal flow with free surface and finite depth, we study the dynamics of small-amplitude and short-wavelength wavetrains propagating on the background of a steepening nonlinear wave. This can be seen as a model for small ripples developing on slopes of breaking waves in the surf zone. Using the concept of wave action as an adiabatic invariant, we derive an explicit asymptotic expression for the change of ripple steepness. Through this expression, nonlinear effects are described using the intrinsic frequency and intrinsic gravity along Lagrangian (material) trajectories on a free surface. We show that strong compression near the tip on the wave leads to an explosive (super-exponential) ripple instability. This instability may play important role for understanding fragmentation and whitecapping at the surface of breaking waves. Analytical results are confirmed by numerical simulations using a potential theory model.