A pair of Langmuir cells in two laboratory tanks (II) on generation mechanism |
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Authors: | Shinjiro Mizuno Hideaki Noguchi Yasutaka Kimura |
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Institution: | (1) Department of Civil Engineering, Hiroshima Institute of Technology, Miyake 2-1-1, Saeki-ku, 731-51 Hiroshima, Japan;(2) Chugoku National Industrial Research Institute, Hiro-Suehiro 2-2-2, 737-01 Kure, Hiroshima, Japan;(3) FUKKEN Co., Ltd., Hikarimachi 2-20-11, Higashi-ku, 732 Hiroshima, Japan |
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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 sidewalls. 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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Keywords: | Side-wall effects secondary circulation parabolic cross-wind current profile advective Reynolds stresses |
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