Wave breaking and velocity fields induced by large amplitude harmonic forcing

ptv_vectorsOn Tuesday afternoon, 29 July, John Grue talked about recent experiments at Oslo on particle paths, velocity fields, and geometry of breaking waves induced in a wave tank generated by harmonically forcing a paddle at one end.  The motivation was an interest on the part of industry and research councils on wave dynamics near proposed offshore wind turbine fields.  In the wave tank, the water depth was 0.2m, the end paddle was oscillated at 0.8hz, and the wave characteristics were measured at a distance of 2.8m and 4.43m downstream.  The waves were typically around kh ~ 0.78. The main question was to determine the maximum velocity amplitude induced by wave breaking.  In almost all cases, the maximum height before breaking was H=0.5h where H is the wave height and h is the depth.  The waves were predominantly irrotational, with a small layer of vorticity near the bottom boundary. Even very large waves compared well with steady waves computed by Fenton (1990).  The second part of the talk concentrated on particle paths.  He started by recalling the observations of Bagnold (1947) on drift in laboratory experiments.  Then results were reported on particle drift measurements.  They found forward drift along the bottom, backward drift in the middle, and forward drift near the surface. The drift results compared favorably with the second order theory of Longuet-Higgins (1953).

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