An experimental study on CL2 mechanism of Langmuir circulation

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Laboratory experiments have been undertaken to understand the wave-current interaction under the wind waves. The primary purpose is to answer the following question. How do swell waves interact with the wind-driven current in a wind tank? The effects of swell (monochromatic mechanical waves) on the subsurface flow were examined to test the validity of the CL2 mechanism proposed by Craik (1977) and Leibovich (1977). In order to explore the response of the wind-driven flow to mechanical-waves, we used two sets of mechanical waves : One is referred to as the wind-following swell (case FS), and the other the wind-opposing swell (case OS). For this purpose, a vertical cross-sectional plane normal to the wind was systematically and in detail measured both vertically and laterally using a two-component LDV in each of the 2 sets of swell cases of FS and OS under the action of a constant wind speed of 7-8 m/s.A strong wave-current interaction was found in a tank flow when the propagation direct … More ion of mechanical-waves (wave frequency = 1Hz, initial wave amplitude H_0 = 2.5 cm) was reversed. First, for case OS the mechanical-waves highly speeded up the primary circulation in the tank but slightly decayed a pair of 2-dimensional (vertical) Langmuir circulation (2D-LC). Second, for case AS they dramatically slowed down the primary circulation, but enhanced the 2D-LC, leading to the generation of vigorous vertical mixing.First, it is found that the response of the vertical circulation to the monochromatic swell wave is consistent with the CL2 mechanism ; that is, the 2D-LC is enhanced by the wind-following swell but attenuated by the wind-opposing swell. In particular, it tamed out that the wind-following swell caused vigorous turbulent mixing in the whole flow, leading to the slowdown of the primary circulation, which appears to be characteristic of the CL2 mechanism.Second, the swell-induced Reynolds shear stress was negative in case FS but became positive in case OS. This fact appears to be closely related to the slowdown in case FS and the speedup in case OS of the primary circulation. That is, we may conclude that the swell-induced Reynolds shear stress plays an important role in the acceleration and deceleration of the primary circulation, when it is positive and negative, respectively. Less