VERTICAL MIXING IN THERMALLY STRATIFIED FIELDS INDUCED BY COMBINATION OF MECHANICAL AND THERMAL STIRRING

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Model experiments are performed to examine the vertical mixing process in the case when the wind stress and thermal convection simultaneously act on the surface of thermally stratified water body. The stress-driven mechanical turbulence and thermally induced convective motions are simulated by means of the oscillating grid and the bottom plate heater, respectively. Introducing the characteristic velocity scale defined by linearly superimposing the energy flux of mechanical turbulence and thermal convection, the mixing rate can be expressed as a function of an overall Richardson number. Analytical solutions for the mixing layer depth, specific density anomaly, etc., are derived based on the experimental law of mixing rate. Satisfactory agreement between the theory and experiments is obtained. Additionally, the regimes where the mechanical or thermal stirring effects dominate the mixing process, respectively, are systematically classified.