Benjamin-Feir instability of Stokes waves in finite depth

TL;DR

This paper rigorously analyzes the Benjamin-Feir instability of Stokes waves in finite depth, identifying the critical depth where the transition from stability to instability occurs and describing the eigenvalue behavior near this threshold.

Contribution

It provides a complete characterization of the eigenvalues near zero for Stokes waves in finite depth, confirming the Benjamin-Feir instability threshold and detailing the eigenvalue dynamics across it.

Findings

Existence of a critical depth $ exttt h_{WB} \

,

the eigenvalues remain purely imaginary below $ exttt h_{WB}$ and form a figure "8" above it,

Abstract

Whitham and Benjamin predicted in 1967 that small-amplitude periodic traveling Stokes waves of the 2d-gravity water waves equations are linearly unstable with respect to long-wave perturbations, if the depth $\mathtt h$ is larger than a critical threshold $\mathtt h_{WB} \approx 1.363$. In this paper we completely describe, for any value of $\mathtt h > 0$, the four eigenvalues close to zero of the linearized equations at the Stokes wave, as the Floquet exponent $\mu$ is turned on. We prove in particular the existence of a unique depth $\mathtt h_{WB}$, which coincides with the one predicted by Whitham and Benjamin, such that, for any $0 < \mathtt h < \mathtt h_{WB}$, the eigenvalues close to zero remain purely imaginary and, for any $\mathtt h > \mathtt h_{WB}$, a pair of non-purely imaginary eigenvalues depicts a closed figure "8", parameterized by the Floquet exponent. As $\mathtt h \to \mathtt h_{WB}^+$ this figure "8" collapses to the origin of the complex plane. The proof combines a symplectic version of Kato's perturbative theory to compute the eigenvalues of a $4 \times 4$ Hamiltonian and reversible matrix, and KAM inspired transformations to block-diagonalize it. The four eigenvalues have all the same size $O(\mu)$ - unlike the infinitely deep water case in [6]- and the correct Benjamin-Feir phenomenon appears only after one non-perturbative block-diagonalization step. In addition one has to track, along the whole proof, the explicit dependence of the entries of the $4 \times 4$ reduced matrix with respect to the depth $\mathtt h$.