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Sedimentation of disks in a linearly stratified fluid

Abstract : We consider the unbounded settling dynamics of a circular disk of diameter d and finite thickness h evolving with a vertical speed U in a linearly stratified fluid of kinematic viscosity v and diffusivity k of the stratifying agent, at moderate Reynolds numbers (Re=Ud/v ). The influence of the disk geometry (diameter d and aspect ratio x = d/h ) and of the stratified environment (buoyancy frequency N, viscosity and diffusivity) are experimentally and numerically investigated. Three regimes for the settling dynamics have been identified for a disk reaching its gravitational equilibrium level. The disk first falls broadside-on, experiencing an enhanced drag force that can be linked to the stratification. A second regime corresponds to a change of stability for the disk orientation, from broadside-on to edgewise settling. This occurs when the non-dimensional velocity U/√vN becomes smaller than some threshold value. Uncertainties in identifying the threshold value is discussed in terms of disk quality. It differs from the same problem in a homogeneous fluid which is associated with a fixed orientation (at its initial value) in the Stokes regime and a broadside-on settling orientation at low, but finite Reynolds numbers. Finally, the third regime corresponds to the disk returning to its broadside orientation after stopping at its neutrally buoyant level.
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Submitted on : Monday, March 30, 2020 - 10:29:54 AM
Last modification on : Thursday, October 21, 2021 - 3:09:36 AM

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Matthieu Mercier, Shiyan Wang, Justin Péméja, Patricia Ern, Arezoo Ardekani. Sedimentation of disks in a linearly stratified fluid. Journal of Fluid Mechanics, Cambridge University Press (CUP), 2020, 885, pp.1-30. ⟨10.1017/jfm.2019.957⟩. ⟨hal-02524107⟩

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