Abstract

In a statically stratified fluid where relatively light parcels of fluid lie on top of denser ones, fluid parcels perturbed vertically feel restoring buoyancy forces that tend to bring them back to their equilibrium position. If on top of that, the fluid velocities vary in space, these fluid parcels will be deformed by velocity gradients and form elongated structures in the density field (or lamellae) on which diffusion will be enhanced. Thus, in the presence of velocity gradients, the equilibrium position of a vertically perturbed (initially at rest) fluid parcel will not be its initial position. By analysing the settling dynamic of a single lamella in a stratified Taylor-Couette flow and its equilibrium position, we present experimental evidence of this coupled effect between density stratification and stretching-enhanced diffusion (i.e. mixing) and some implications for mixing in stratified shear flows.