Fold catastrophe in breaking waves

TL;DR

This paper models wave breaking as a fold catastrophe in a dynamical system, revealing the geometric and energetic thresholds that lead to breaking in ideal free-surface flows.

Contribution

It introduces a novel dynamical-systems framework that captures the onset of wave breaking through a fold catastrophe and identifies critical geometric and energetic thresholds.

Findings

Wave breaking onset is characterized by a fold catastrophe in (m, A) space.

The critical slope angle for breaking inception is approximately 22.5 degrees.

Crest height is limited by maximum kinetic energy excess before breaking occurs.

Abstract

We present a dynamical-systems perspective on wave breaking for ideal incompressible free-surface flows. By tracking the most energetic hotspot on the wave surface, we find that near breaking the surface slope m evolves on a fast timescale governed by the small parameter epsilon = (partial_z u)^(-1), the inverse vertical velocity gradient at the hotspot, while the focusing parameter A = (U - Ce)/(U - Creq) varies slowly and adiabatically. Here U is the horizontal fluid velocity at the energetic point, Ce its propagation speed, and Creq the equivalent crest speed. This slow-fast structure reveals a fold catastrophe in the (m, A) space whose boundary forms the geometric skeleton organizing the dynamics near breaking. Finite-time blowup occurs when the trajectory crosses this boundary, marking the onset of breaking. The inception of breaking is further characterized by crossing the slope threshold theta* = arctan(sqrt(2) - 1) = 22.5 degrees. This critical angle marks the maximum anisotropy that can be sustained between the Hessians of the velocity and pressure fields, reflecting an imbalance between kinetic and potential energy fluxes. The anisotropy of the velocity Hessian also gives rise to the classical 30-degree slope observed at the inflection point of steep waves near breaking inception. The crest height is limited by the maximum excess of kinetic over potential energy that the flow can sustain, beyond which breaking becomes inevitable. Wave breaking can also be interpreted as a gravity analogue of a collapsing black hole, with apparent and event horizons representing the onset and inception of breaking.