Non-invasive turbulent mixing across a density interface in a turbulent Taylor-Couette flow

We present experimental measurements of the turbulent transport of salt across an interface between two layers of fluid of different salinities, confined to a cylindrical annulus with gap $L$ where the inner cylinder rotates to produce an approximately irrotational mean azimuthal flow, with narrow boundary layers. We focus on the limit of high Richardson number flow, defined as $Ri=g \Delta \rho H /(\rho_0 u_{rms}^2)$ where $\rho_0$ is a reference density, $\Delta \rho$ is the time-dependent difference of the layers' mean densities, $u_{rms}$ is the rms of the turbulent velocity fluctuations and $H$ is the layer depth. The mean flow has $Re \sim 10^4-10^5$, and the turbulent fluctuations in the azimuthal and radial directions have rms speed of order $10\%$ of the mean azimuthal flow. The interface between the two layers remains sharp, each layer remains well-mixed, and the vertical flux of salt between the layers, ${F}_s \sim (1.15 \pm 0.15) Ri^{-1}{\cal A}(H/L) u_{rms} \Delta S$, where $\Delta S$ is the spatially-averaged time-dependent salinity difference between the layers and ${\cal A}(H/L)$ is a function of the aspect ratio. The salt transport appears to be caused by turbulent eddies scouring and sharpening the interface and implies a constant rate of conversion of the turbulent KE to PE, independent of the density contrast between the layers.