Prandtl number effects on extreme mixing events in forced stratified turbulence

`Strongly' stratified turbulent flows can self-organise into a `layered anisotropic stratified turbulence' (LAST) regime, characterised by staircases of relatively deep and well-mixed density `layers' separated by relatively thin `interfaces' of enhanced density gradient. Understanding their mixing dynamics is important for parameterizing heat transport in the world's oceans. It is challenging to simulate such `LAST' mixing, which is associated with Reynolds numbers Re=U L/ν >> 1 and Froude numbers Fr=(2 π U)/(L N) << 1, (U and L being characteristic velocity and length scales, ν being the kinematic viscosity and N the buoyancy frequency). As a sufficiently large dynamic range (largely) unaffected by stratification and viscosity is still required, the buoyancy Reynolds number Re<sub>b</sub>=ε/(ν N<sup>2</sup>) >> 1, where ε is the TKE dissipation rate. This requirement is exacerbated for oceanic flows, as the Prandtl number Pr =ν/κ=O(10) in thermally-stratified water (κ is the thermal diffusivity), thus leading (potentially) to even finer denisty field structure. We report on four fully-resolved direct numerical simulations of stratified turbulence with different Fr=2,0.5 and Pr=1,7, forced so that Re=9216 and Re<sub>b</sub> =50, with resolutions up to 30240 x 30240 x 3780, investigating how variation of bulk parameters affects mixing properties. We find that as Pr increases, stably stratified interfaces are finer and their contribution to bulk mixing characteristics decreases. Neverthelesss, `extreme' mixing events (with highly elevated and exceptionally `efficient' buoyancy variance destruction rates χ, dominating the total mixing budget) are still preferentially found in strongly stratified interfaces.