Nonlinear waves in stratified Taylor--Couette flow. Part 2. Buoyancy flux

This paper is the second part of a two-fold study of mixing, i.e. the formation of layers and upwelling of buoyancy, in axially stratified Taylor--Couette flow, with fixed outer cylinder. In a first paper, we showed that the dynamics of the flow was dominated by coherent structures made of a superposition of nonlinear waves. (Mixed)-ribbons and (mixed)-cross-spirals are generated by interactions between a pair of linearly unstable helical modes of opposite `handedness', and appear to be responsible for the formation of well-mixed layers and sharp density interfaces. In this paper, we show that these structures are also fully accountable for the upwards buoyancy flux in the simulations. The mechanism by which this occurs is a positive coupling between the density and vertical velocity components of the most energetic waves. This coupling is primarily caused by diffusion of density at low Schmidt number Sc, but can also be a nonlinear effect at larger Sc. Turbulence was found to contribute negatively to the buoyancy flux at Sc=1,10,16, which lead to the conclusion that mass upwelling is a consequence of chaotic advection, even at large Reynolds number. Artificially isolating the coherent structure therefore leads to excellent estimates of the flux Richardson numbers Ri_f from the DNS. We also used the theoretical framework of Winters et al. (1995) to analyse the energetics of mixing in an open control volume, shedding light on the influence of end effects in the potential energy budget. The potential connection with the buoyancy flux measurements made in the recent experiment of Oglethorpe et al. (2013) is also discussed.