Taylor-Couette flow in a two-layer stratified fluid: instabilities and mixing
Résumé
In this preliminary experimental study we investigate the mixing of a salt-stratified two-layer fluid in a Taylor–Couette flow. We focus on two flow regimes, one in which waves are present in the pycnocline and vortices in the homogeneous layers, and one with vortices also in the pycnocline. The transition between these two regimes differs significantly compared to the wave-vortex transition found in a uniform stratification, investigated by Caton et al., and is accompanied by a larger jump in fundamental frequency of the density fluctuations. Most density gradients show a strong tendency to sharpen with time, with layer formation in the case of an initially thick interface. On a long time-scale, the entrainment of a smooth pycnocline shows cascade-like bifurcations before its transition to chaos. In the latter case, a decrease of the initial density gradient is observed. We discuss the mixing efficiency, measured by the Richardson flux number Rif. Its value is a function of the Reynolds number Re$^k$, with $k\approx 3/2$, and for Reynolds numbers $O(10^3)$, the mixing efficiency is of order $O(10^{-2})$.
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