Laboratory measurements of the generation and evolution of Langmuir circulations

TLDR

Prior to Langmuir circulation onset, the shear‑layer depth scales with (νt)^1/2, indicating molecular transport, and the observations are compared with theory, though no model directly applies to the experimental conditions. The study presents laboratory measurements of the generation and evolution of Langmuir circulations as an instability of a wind‑driven surface shear layer and discusses their implications for natural water bodies. Laboratory experiments used an accelerating wind from rest over a quiescent water surface to generate a wind‑driven surface shear layer, whose instability produces Langmuir circulations. The experiments show that the shear layer accelerates and deepens until Langmuir circulations form, which then closely follow wind‑wave growth, induce vertical mixing and surface deceleration, and for final wind speeds of 3–5 m s⁻¹ the most unstable circulation wavenumber is 0.68 ± 0.24 of the surface wavenumber at a reciprocal Langmuir number of 52 ± 21.

Abstract

We present laboratory measurements of the generation and evolution of Langmuir circulations as an instability of a wind-driven surface shear layer. The shear layer, which is generated by an accelerating wind starting from rest above a quiescent water surface, both accelerates and deepens monotonically until the inception of the Langmuir circulations. The Langmuir circulations closely follow the initial growth of the wind waves and rapidly lead to vertical mixing of the horizontal momentum and a deceleration of the surface layer. Prior to the appearance of the Langmuir circulations, the depth of the shear layer scales with ( vt ) 1/2 ( v is the kinematic viscosity and t is time), in accordance with molecular rather than turbulent transport. For final wind speeds in the range 3 to 5 m s −1 , the wavenumber of the most unstable Langmuir circulation normalized by the surface wavenumber, k * lc , is 0.68±0.24, at a reciprocal Langmuir number, La −1 , of 52±21. The observations are compared with available theoretical results, although none are directly applicable to the conditions of the experiments. The implications of this work for the generation and evolution of Langmuir circulations in the ocean and other natural water bodies are discussed.