EFFECTS OF DIFFERENTIAL DIFFUSION ON MIXING EFFICIENCY IN A DIFFUSIVELY-STABLE, TURBULENT FLOW

Abstract eng:
Laboratory experiments are used to evaluate the conditions under which differential diffusion exists and its effect on the mixing efficiency. Experiments are conducted using a linearly stratified system, which is stably stratified with both heat and salt to ensure equal Richardson numbers RiT = (NLT/u) and Reynolds numbers ReT = uLT/ν are seen for both scalars as the system is stirred using horizontally oscillating vertical rods. Both the work done on the system and the potential energy change of the system are directly measured to compute the mixing efficiency—defined as the ratio of the potential energy change to the work done. The eddy diffusivities of heat and salt are presented as a function of εa/νN , where εa is a volume and time averaged dissipation. The results of this study suggest that differential diffusion exists below εa/νN = 300 and above a turbulent Richardson number of 0.1. For relatively strong stratification and weak turbulence (RiT > 0.1, εa/νN < 300) the high density ratio experiments have a significant increase in the mixing efficiency over their low density ratio counterparts. These experiments verify that the density ratio can have as large an influence on the mixing efficiency as the stratification strength and the process generating the turbulence.

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