Vertical mixing induced by wind and a rotating screen in a stratified fluidin a channel

An experimental investigation of vertical mixing induced by wind, or by a screen which moved horizontallyover the water surface, was carried out in a wind-wave channel similar to that of Wu [27]. It was found that, in the wind-wave channel, the entrainment rate due to the shear stress, which was exerted on the water surface by the moving screen, was two orders of magnitude smaller than the one obtained in similar experiments, which were carried out in an annular tank by Kato & Phillips [11], Kantha [9], and Kantha et al [10]. The entrainment rate in the wind induced mixing experiments, was one order of magnitude larger than that found in the experiments with the horizontally moving screen, both carried out in the same channel. The explanation of these differences, which is offered in this work, is based on the analysis of the flow patterns in the mixed layer in both systems. An additional important result, is the fact that, the relation between the entrainment rate, E, and the Richardson number, Ri, agrees with the one obtained by Turner[21], and other authors, i.e. E - Ri^−3/2, particularly in the experiments of wind induced mixing.