Diapycnal mixing efficiency in lock-exchange gravity currents

In this report, results of irreversible mixing efficiency, Ri(f)*, for a slumping gravity current is presented for a range of Reynolds numbers, Re = 485-12 270. Simultaneous velocity and density fields from particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) are used to measure the viscous dissipation (epsilon) and scalar gradients (del rho, del rho') needed to calculate Ri f *. It is observed that Ri(f)* plateaus to Ri(f)* approximate to 0.12, for the range of the buoyancy Reynolds number, Re-b, and the gradient Richardson number, Ri(g), used in our study. Owing to ambiguities in parametrizing Ri(f)* as a function of a single flow parameter, like Re-b or Ri(g), a multiparameter space is considered using a turbulent Froude number, Fr-k, and a turbulent Reynolds number, Re-L. New insights are achieved, wherein it is observed that for a given Re-b or Ri(g), Ri(f)* can assume different values owing to the turbulent regimes set up by Fr-k and Re-L and the effects of the molecular Prandtl number, Pr. It is also seen that the plateauing value of Ri(f)* can be greatly influenced by the flow parameters and fluid properties. We document that mixing efficiency can be increased in strongly stratified flow (Fr-k < 0.6), provided Re-L is sufficiently high. Also, for the parameter range in our study, 3 <= Re-b <= 48 and 0.26 <= Ri(g) <= 0.9, we did not notice a drop in the value of Ri(f)* This is attributed to the fact that our experiments are in the regime of strongly stratified turbulence with sufficiently high Re-L that could sustain turbulence and mixing. Our study suggests a need for a different parametrization of mixing efficiency in the diffusive (1 <= Re-b <= 10) and intermediate (10 <= Re-b <= 100) regimes for a gravity current.

Automated summary

Results of irreversible mixing efficiency for slumping gravity currents in a range of Reynolds numbers were presented, showing the plateauing value of the mixing efficiency is influenced by multiple parameters. New insights were achieved, indicating that different values of mixing efficiency can be observed for the same Reynolds number or gradient Richardson number due to the effects of turbulent regimes set up by different parameters. Increasing mixing efficiency in strongly stratified flow is possible by maintaining a sufficiently high turbulent Reynolds number. There was no drop in mixing efficiency observed in the parameter range studied, attributed to the presence of strongly stratified turbulence sustained by high turbulent Reynolds numbers.