TERRAPUB Journal of Oceanography

Journal of Oceanography, Vol. 54 (No. 1), pp. 77-100, 1998

A Pair of Langmuir Cells in Two Laboratory Tanks (II) On Generation Mechanism

Shinjiro Mizuno1, Hideaki Noguchi2 and Yasutaka Kimura3

1Department of Civil Engineering, Hiroshima Institute of Technology, Miyake 2-1-1, Saeki-ku, Hiroshima 731-51, Japan
2Chugoku National Industrial Research Institute, Hiro-Suehiro 2-2-2, Kure, Hiroshima 737-01, Japan
3FUKKEN Co., Ltd., Hikarimachi 2-20-11, Higashi-ku, Hiroshima 732, Japan

(Received 5 August 1996; in revised form 17 June 1997; accepted 29 September 1997)

Abstract: Vertical and cross-wind profiles of mean currents were measured systematically in vertical cross-sections of two wind-wave tanks with aspect ratios of order one to study the secondary flow in the tanks. A pair of Langmuir cells turned out to be driven by a close combination of the pressure gradient along the tank and the side-wall effects. That is, part of the adverse pressure gradient produced a parabolic cross-wind profile with the smallest downwind current at the centerline and the largest current along the two side-walls. As a result, upwelling occurred in the center zone where the return flow was strongest, probably because of the entrainment action of the wind-driven current. In order to compensate for this upwelling, downwelling occurred along the two side-walls from the flow continuity. The resulting vertical circulation formed a pair of Langmuir circulations across the span and served to maintain the parabolic profile formed by the pressure gradient. A positive feedback mechanism is thus found between the primary and secondary circulations through upwelling of the return flow in the center zone. Vertical shears of the span-averaged downwind current measured in two tanks were found to be systematically different from each other. This difference seems to depend on the magnitude of the advective Reynolds stresses in the two tanks.

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