Abstract

Experimental evidence regarding the response of a two-density-layer fluid due to the application of a surface shear stress is presented and discussed. The experiments were conducted in a laboratory tank provided with a conveyor belt to exert the surface shear stress. An experimental technique, based on the analysis of video images, was developed in order to measure instantaneous two-dimensional velocity and density fields of the flow. Most of the response of the stratified flow is determined by the Wedderburn number, W. For values of W larger than 5 no tilting of the isopycnals is observed and mixing processes are essentially one-dimensional. For values of W lower than 5 tilting is observed and upwelling occurs for values of W less than about 1.7. In this case mixing rates are uniform along the tank and mixing processes appear also as one-dimensional except when upwelling occurs, case in which mixing is clearly two-dimensional. Despite the variety of responses associated to different values of W, the entrainment velocity can be modeled by a unique relationship, depending on W and the aspect ratio of the water basin, in the whole range of values of W studied here (0.5 < W < 20). The bulk mixing efficiency in the present flows is maximum at the beginning of the mixing process, with values always less than about 20%.