Laboratory Observation of Instabilities in Stratified Taylor-Couette Flow

In 2001 Molemaker et al. (\textit{J. Fluid. Mech.} {\bf 448}, 1) predicted a new class of instabilities in a system of concentric rotating cylinders that contains a fluid with a vertically varying density. Dubrulle et al. (\textit{Astron. Astrophys}. {\bf 429}, 1, 2005) then showed that this phenomenon, which they named stratorotational instability (SRI), could be a source of instability and angular momentum transport in astrophysical accretion disks. Subsequent work by Shalybkov and R\"udiger (\textit{Astron. Astrophys.} {\bf 438}, 411, 2005) hypothesized that such stratified flow is stable when the ratio of outer and inner cylinder rotation rates $\mu$ is less than the ratio of the inner and outer cylinder radii $\eta$. Previous laboratory measurements by Le Bars and Le Gal (\textit{Phys. Rev. Lett.} \textbf{99}, 064502, 2007) confirmed this prediction for $Re<1200$ with $Re\equiv (r_o-r_i)\Omega_i r_i/\nu$. However, we find SRI exists for $\mu>\eta$ when the density gradient is large. We also find that the onset of SRI is suppressed for Reynolds numbers $Re>4000$, a region previously unexplored in experiments. For $Re>8000$, we find that the fluid does not exhibit SRI but transitions to a previously unreported chaotic state that mixes the fluid.