Some aspects of mixing in a stratified turbulent patch

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Abstract
Mixing in a turbulent patch, that was generated within a linearly stratified fluid of buoyancy frequency No, was studied experimentally. The turbulence within the patch was induced by sustained oscillations of a mono-planar grid. Grid oscillations produced a local turbulent region, which initially grows rapidly as in a non-stratified fluid and then assumes a quasi-stationary thickness (Lp)c when the stratification inhibits its vertical growth at a time Not ≈ 4. The growth beyond (Lp)c occurs slowly via breaking of interfacial waves at the entrainment interface. As mixing proceeds, the buoyancy frequency within the patch N decreases. The time evolution of N, the buoyancy lengthscale, the Thorpe lengthscale, the maximum Thorpe displacement, the overturning lengthscale and the available potential energy fluctuations were measured and their relationships were investigated. Various aspects of the wave radiation from the patch to the outer stratified layer, the trapping of interfacial waves at the entrainment interface, and the effects of grid parameters on the evolution of the patch were also studied.