On the structure of parasitic gravity-capillary waves in the small surface tension limit

TL;DR

This paper investigates the formation and structure of parasitic gravity-capillary waves in the small surface tension limit, demonstrating conditions under which solutions can be smoothly continued to zero surface tension.

Contribution

It provides a numerical analysis of the low surface tension limit, clarifies the bifurcation structure, and introduces wave energy as a key amplitude condition for continuous deformation.

Findings

Different amplitude choices affect continuation procedures.

Wave energy condition enables smooth transition to zero surface tension.

Classifies bifurcation structures in gravity-capillary wave problem.

Abstract

In this paper, we examine the formation of small capillary waves (parasitic ripples) on the surface of steep steadily-travelling gravity waves. Previously, authors have developed ad-hoc analytical procedures for describing the formation of such parasitic ripples in potential flows; however, it has not been clear whether the small-surface tension limit is well-posed -- that is, whether it is possible for an appropriate travelling gravity-capillary wave to be continuously deformed to the classic Stokes wave in the limit of vanishing surface tension. The work of Chen & Saffman (1980) had suggested smooth continuation was not possible. In this paper, we numerically explore the low surface tension limit of the steep gravity-capillary travelling-wave problem. Our results allow for a classification of the bifurcation structure that arises, and serve to unify a number of previous numerical studies. Crucially, we demonstrate that different choices of solution amplitude can lead to subtle restrictions on the continuation procedure; the use of wave energy as an amplitude condition allows solution branches to be continuously deformed to the zero surface tension limit.