Sedimentation of disks in a linearly stratified fluid

Matthieu Mercier (1) , Shiyan Wang (2) , Justin Péméja (1) , Patricia Ern (1) , Arezoo Ardekani (2)

1 IMFT - Institut de mécanique des fluides de Toulouse
2 Purdue University [West Lafayette]

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.

Mots clés: Stratified flows; Prticle/fluid flows; Instability

Domaines: Mécanique des fluides [physics.class-ph]

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