LABORATORY EXPERIMENTS ON THE EFFECTS OF MECHANICALLY GENERATED WAVES ON THE CORE FLOW UNDER WIND WAVES

摘要

Response of the core flow in a wind-wave tank to two sets of mechanically generated waterrnwaves is studied to extract an organized motion from the subsurface flow structure. Currentrnvelocities were measured in a vertical cross-sectional plane normal to the wind using a twocomponentrnlaser-Doppler anemometer. The two-dimensional structure of the core flow wasrndramatically modified by changing propagation direction of the mechanical-waves. For thernmechanical-waves propagating parallel to and opposite to the wind, (1) the Reynolds shearrnstress under the surface decreased and increased by about 40 % of the wind stress above thernsurface, (2) the primary circulation slowed down and speeded up, and (3) the secondary flow,rncharacterized by span-scale Langmuir circulations, slightly increased and decreased,rnrespectively. The response of the secondary flow is consistent with prediction of the CL2rnmechanism of Langmuir circulations, proposed by Craik (1977) and Leibovich (1977). For thernresponse of the primary circulation, it appears that the growth and attenuation of thernmechanical-waves due to the wind affected the Reynolds shear stress under the surface,rnresulting in its slowdown and speedup, respectively.