Three-dimensional internal gravity-capillary waves in finite depth

TL;DR

This paper analyzes three-dimensional gravity-capillary waves in a two-layer fluid with finite depth, using Hamiltonian systems and bifurcation theory to identify various wave solutions including periodic and solitary waves.

Contribution

It introduces a Hamiltonian framework for 3D internal gravity-capillary waves and studies bifurcations leading to new wave solutions using center-manifold and variational methods.

Findings

Detection of bifurcation scenarios including resonance and Hamiltonian-Hopf bifurcation.

Existence of doubly periodic traveling waves.

Existence of oblique solitary waves with dark or bright profiles.

Abstract

We consider three-dimensional inviscid irrotational flow in a two layer fluid under the effects of gravity and surface tension, where the upper fluid is bounded above by a rigid lid and the lower fluid is bounded below by a flat bottom. We use a spatial dynamics approach and formulate the steady Euler equations as an infinite-dimensional Hamiltonian system, where an unbounded spatial direction $x$ is considered as a time-like coordinate. In addition we consider wave motions that are periodic in another direction $z$. By analyzing the dispersion relation we detect several bifurcation scenarios, two of which we study further: a type of $00(\mathrm{i}s)(\mathrm{i}\kappa_0)$ resonance and a Hamiltonian-Hopf bifurcation. The bifurcations are investigated by performing a center-manifold reduction, which yields a finite-dimensional Hamiltonian system. For this finite-dimensional system we establish the existence of periodic and homoclinic orbits, which correspond to, respectively, doubly periodic travelling waves and oblique travelling waves with a dark or bright solitary wave profile in the $x$-direction. The former are obtained using a variational Lyapunov-Schmidt reduction and the latter by first applying a normal form transformation and then studying the resulting canonical system of equations.