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Axisymmetric selective withdrawal in a rotating stratified fluid

  • Stephen G. Monismith (a1), N. Robb Mcdonald (a2) (a3) and Jörg Imberger (a2)


In this paper we consider the axisymmetric flow of a rotating stratified fluid into a point sink. Linear analysis of the initial value problem of flow of a linearly stratified fluid into a point sink that is suddenly switched on shows that a spatially variable selective withdrawal layer is established through the outward propagation of inertial shear waves. The amplitude of these waves decays with distance from the sink; the e-folding scale of a given mode is equal to the Rossby radius of that mode. As a consequence, the flow reaches an asymptotic state, dependent on viscosity and species diffusion, in which the withdrawal-layer structure only exists for distances less than the Rossby radius based on the wave speed of the lowest mode, R1. If the Prandtl number, Pr, is large, then the withdrawal layer slowly re-forms in a time that is O2iκ-1), such that it extends out much farther to a distance that is O(R1Prδ2iδ-2e) rather than O(R1).

Because there is no azimuthal pressure gradient to balance the Coriolis force associated with the radial, sinkward flow, a strong swirling flow develops. Using scaling arguments, we conclude that this swirl causes the withdrawal-layer thickness to grow like $(ft)^{\frac{1}{3}}$, such that eventually there is no withdrawal layer anywhere in the flow domain. Scaling arguments also suggest that this thickening takes place in finite-size basins.

These analyses of swirl-induced thickening and diffusive thinning can be combined to yield a classification scheme that shows how different types of flows are possible depending on the relative sizes of a parameter J, which we define as fQ(Nhv)-1, E (the Ekman number fh2v-1), and Pr.



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