Role of wave scattering in instability-induced Langmuir circulation

Abstract

We consider a classical problem about dynamic instability that leads to the Langmuir circulation. The problem statement assumes that there is initially a wind-driven shear flow and a plane surface wave propagating in the direction of the flow. The unstable mode is a superposition of i) shear flow and ii) surface waves both modulated in the horizontal spanwise direction and iii) circulation that is made up with vortices forming near-surface rolls whose axis are coaligned along the shear flow streamlines and whose transverse size corresponds to the modulation period. Usually, the Langmuir circulation is understood as the vortical part of the mode slowly varying in time, which is the combination of the first and the last flows. The novelty of our approach is that we, firstly, take into account the scattering of the initial surface wave on the slow current. Second, we find the interference of the scattered and the initial waves generating a Stokes drift modulated in the same direction. Third, we establish the subsequent affect of the circulation by the vortex force created by the nonlinear interaction of the initial shear flow and the modulated part of the Stokes drift. S. Leibovich & A.D.D. Craik previously showed that the third part of the mechanism could maintain the Langmuir circulation. We calculate the growth rate which is approximately twice smaller than that obtained by A.D.D. Craik. The vertical structure of the circulation in the mode consists of two vortices, that corresponds to the next mode in Craik's model. Considering the wave scattering, we describe the fast-wave motion as a potential flow with relatively weak vortical correction. Application of the technique can be expanded on other flows where fast oscillating surface waves coexist with a slow current.