The Formation Process of Vortical Structure Beneath a Free Surface by Wind Waves

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  • 風波による自由表面下の渦構造の生成過程
  • フウハ ニ ヨル ジユウ ヒョウメンカ ノ ウズ コウゾウ ノ セイセイ カテ

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Abstract

The formation process of three-dimensional vortical structure beneath a free surface by wind waves is investigated by numerical experiments. The full Navier-Stokes equation with the nonlinear free-surface condition is solved by the finite volume method. The flow field is assumed to be periodic in both wind and lateral directions. The surface shear layer with the velocity profile of an exponential function of depth and the free surface waves of small amplitude propagating in the same direction induced by wind are incorporated in the numerical model as a initial state. The model shows that the transition to three-dimensionality of flow field occurs when laterally periodic disturbance is imposed on the shear layer. Then the waves are deformed by the shear layer with the wave amplitude increasing in low speed region while the vorticity component in the wind direction is amplified by the waves propagating over it. It is shown that this interaction of the shear layer and the waves causes the formation of three-dimensional vortical structure which consists of a parallel system of counter-rotating vortices aligned with the wind. It is also confirmed that the structure has the same qualitative features as the observation of Langmuir Circulation. These results indicate that the presence of the surface shear layer and the free surface waves with small lateral anomaly is sufficient for the formation of Langmuir Circulation.

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