Transition to turbulence in wind-drift layers

TL;DR

This study uses wave-averaged simulations to explore how wind-driven shear layers over water transition from laminar flow to turbulence, highlighting the sensitivity of results to initial wave conditions.

Contribution

It introduces a wave-averaged modeling approach to simulate the evolution of wind-drift layers and examines its predictive limitations due to wave amplitude sensitivity.

Findings

Model reproduces qualitative features of laboratory observations.

Results are highly sensitive to initial wave amplitude.

Raises questions about the predictive capability of wave-averaged models.

Abstract

A light breeze rising over calm water initiates an intricate chain of events that culminates in a centimeters-deep turbulent shear layer capped by gravity-capillary ripples. At first, viscous stress accelerates a laminar wind-drift layer until small surface ripples appear. Then a second "wave-catalyzed" instability grows in the wind-drift layer, before sharpening into along-wind jets and downwelling plumes, and finally devolving into three-dimensional turbulence. This paper elucidates the evolution of wind-drift layers after ripple inception using wave-averaged numerical simulations with a random initial condition and a constant-amplitude representation of the incipient surface ripples. Our model reproduces qualitative aspects of laboratory measurements similar those reported by Veron & Melville (2001), validating the wave-averaged approach. But we also find that our results are disturbingly sensitive to the amplitude of the prescribed surface wave field, raising the question whether wave-averaged models are truly "predictive" if they do not also describe the evolution of the coupled evolution of the surface waves together with the flow beneath.