Centrifugal instability of Taylor–Couette flow in stratified and diffusive fluids

The linear and nonlinear dynamics of centrifugal instability in Taylor–Couette flow are investigated when fluids are stably stratified and highly diffusive. One-dimensional local linear stability analysis (LSA) of cylindrical Couette flow confirms that the stabilising role of stratification in centrifugal instability is suppressed by strong thermal diffusion (i.e. low Prandtl number Pr). For Pr « 1, it is verified that the instability dependence on thermal diffusion and stratification with the non-dimensional Brunt–Väisälä frequency N can be prescribed by a single rescaled parameter P _N = N ² Pr. From direct numerical simulation (DNS), various nonlinear features such as axisymmetric Taylor vortices at saturation, secondary instability leading to non-axisymmetric patterns or transition to chaotic states are investigated for various values of Pr ≤ 1 and Reynolds number Re _i. Two-dimensional bi-global LSA of axisymmetric Taylor vortices, which appear as primary centrifugal instability saturates nonlinearly, is also performed to find the secondary critical Reynolds number Re _{i, 2} at which the Taylor vortices become unstable by non-axisymmetric perturbation. The bi-global LSA reveals that Re _{i, 2} increases (i.e. the onset of secondary instability is delayed) in the range 10 ⁻³ < Pr < 1 at N = 1 or as N increases at Pr = 0.01. Secondary instability leading to highly non-axisymmetric or irregular chaotic patterns is further investigated by three-dimensional DNS. The Nusselt number Nu is also computed from the torque at the inner cylinder for various Pr and Re _i at N = 1 to describe how the angular momentum transfer increases with Re _i and how Nu varies differently for saturated and chaotic states.