Stokes drift and its discontents

TL;DR

This paper clarifies the properties of Stokes drift in incompressible fluids, decomposing it into a solenoidal part and a small remainder, and connects it to the Craik-Leibovich equation using glm theory.

Contribution

It introduces a natural decomposition of Stokes velocity into a divergence-free component and a small remainder, and relates this to the glm theory and the Craik-Leibovich equation.

Findings

Decomposition of Stokes velocity into solenoidal and remainder parts.

Reformulation of Lagrangian-mean momentum equation matching Craik-Leibovich.

Identification of the solenoidal component as the primary Stokes velocity in incompressible fluids.

Abstract

The Stokes velocity $\mathbf{u}^\mathrm{S}$, defined approximately by Stokes (1847, Trans. Camb. Philos. Soc., 8, 441-455), and exactly via the Generalized Lagrangian Mean, is divergent even in an incompressible fluid. We show that the Stokes velocity can be naturally decomposed into a solenoidal component, $\mathbf{u}^\mathrm{S}_\mathrm{sol}$, and a remainder that is small for waves with slowly varying amplitudes. We further show that $\mathbf{u}^\mathrm{S}_\mathrm{sol}$ arises as the sole Stokes velocity when the Lagrangian mean flow is suitably redefined to ensure its exact incompressibility. The construction is an application of Soward & Roberts's glm theory (2010, J. Fluid Mech., 661, 45-72) which we specialise to surface gravity waves and implement effectively using a Lie series expansion. We further show that the corresponding Lagrangian-mean momentum equation is formally identical to the Craik-Leibovich equation with $\mathbf{u}^\mathrm{S}_\mathrm{sol}$ replacing $\mathbf{u}^\mathrm{S}$, and we discuss the form of the Stokes pumping associated with both $\mathbf{u}^\mathrm{S}$ and $\mathbf{u}^\mathrm{S}_\mathrm{sol}$.