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Laboratory experiments with tilted convective plumes on a centrifuge: a finite angle between the buoyancy force and the axis of rotation

  • VITALII A. SHEREMET (a1) (a2)

The effect of both vertical and horizontal components of the Earth's rotation on plumes during deep convection in the ocean is studied. In the laboratory, the misalignment, characterized by the angle $\alpha$, between the buoyancy force (‘effective’ free-fall acceleration ${\bm g}_e$) and the rotation axis ${\bm \Omega}$ is produced by using the centrifugal force: an experimental tank was placed at a large distance from the centre of the turntable. The mathematical analogy between the laboratory model and the oceanic environment is presented. For $\alpha\,{=}\,30^\circ$, a number of laboratory experiments spanning a wide range of the buoyancy flux parameter, and correspondingly Reynolds number, is used to illustrate the development of the convective plume from a point source in regimes ranging from weakly to highly turbulent. New features of the flow, as compared to $\alpha\,{=}\,0$, are documented and explained.

The incoming heavier dyed fluid jet disintegrates into fast-sinking coherent blobs (in a low-Reynolds-number regime) or turbulent billows (in a high-Reynolds-number regime) and a more diffuse cloud of highly diluted dyed water. An analysis of the forces acting on an ellipsoid moving in a rotating fluid with the main balance including the buoyancy, Coriolis forces, and the hydrodynamic reaction due to generation of inertial waves correctly predicts the trajectory of a descending blob. It also explains the tendency of the plume to develop in the direction intermediate between ${\bm g}_e$ and ${\bm \Omega}$ and to shift ‘eastward’ (lagging the rotation of the centrifuge) if the plume is envisaged as an ensemble of blobs.

The stretching of the highly diluted dyed water along the absolute vorticity tubes with simultaneous shearing by horizontal quasi-two-dimensional flow produces conspicuous tilted structures or tilted Taylor ‘ink walls’. The misalignment between ${\bm g}_e$ and ${\bm \Omega}$ enhances the turbulent mixing and development of tilted structures by breaking the symmetry and producing motions directed away from the rotation axis.

We argue that the conditions at the sites of ocean deep convection are favourable for the development of tilted structures because of the smallness of the Rossby number and an extreme homogenization of the mixed layer. We hypothesize that the homogenized sublayers observed within actively convecting regions in the ocean may not be horizontal, but in fact analogous to the tilted ‘ink walls’ observed in the laboratory experiments and that they represent the internal structure of a plume on horizontal scales smaller than its depth.

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Journal of Fluid Mechanics
  • ISSN: 0022-1120
  • EISSN: 1469-7645
  • URL: /core/journals/journal-of-fluid-mechanics
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