Time-dependent nonlinear gravity-capillary surface waves with viscous dissipation and wind forcing

TL;DR

This paper introduces a time-dependent conformal method to analyze viscous effects on steep gravity-capillary waves, revealing how viscosity influences wave stability, bifurcation structures, and ripples without solving full Navier-Stokes equations.

Contribution

The authors develop a novel numerical approach to study viscous gravity-capillary waves, demonstrating viscosity's role in wave stability and bifurcation behavior, and capturing parasitic ripples efficiently.

Findings

Viscosity suppresses intricate bifurcation structures in steep waves.

Solutions are temporally stable under various perturbations.

The method simplifies analysis of viscous water waves without full Navier-Stokes computations.

Abstract

We develop a time-dependent conformal method to study the effect of viscosity on steep surface waves. When the effect of surface tension is included, numerical solutions are found that contain highly oscillatory parasitic capillary ripples. These small amplitude ripples are associated with the high curvature at the crest of the underlying viscous-gravity wave, and display asymmetry about the wave crest. Previous inviscid studies of steep surface waves have calculated intricate bifurcation structures that appear for small surface tension. We show numerically that viscosity suppresses these. While the discrete solution branches still appear, they collapse to form a single smooth branch in limit of small surface tension. These solutions are shown to be temporally stable, both to small superharmonic perturbations in a linear stability analysis, and to some larger amplitude perturbations in different initial-value problems. Our work provides a convenient method for the numerical computation and analysis of water waves with viscosity, without evaluating the free-boundary problem for the full Navier-Stokes equations which becomes increasingly challenging at larger Reynolds numbers.