Titre/Title : Double-diffusive sedimentation
Contact : Julien Chauchat (équipe MEIGE)
Résumé/Abstract : When a layer of particle-laden fresh water is placed above clear, saline water, both double-diffusive and Rayleigh-Taylor instabilities may arise. We investigate this configuration by means of linear stability analysis and three-dimensional direct numerical simulations. The simulations show that the presence of particles with a Stokes settling velocity modifies the traditional double-diffusive fingering by creating an unstable `nose region’ in the horizontally averaged profiles, located between the upward moving salinity and the downward moving sediment interface. The effective thickness of the salinity (sediment) interface grows diffusively, as does the height of the nose region. The ratio of nose thickness to salinity interface thickness initially grows and then plateaus, at a value that is determined by the balance between the flux of sediment into the rose region from above, the double-diffusive/Rayleigh-Taylor flux out of the nose region below, and the rate of sediment accumulation within the nose region. For small ratios, double-diffusive fingering dominates, while for larger values the sediment and salinity interfaces become increasingly separated in space and the dominant instability mode becomes Rayleigh-Taylor-like. A scaling analysis based on the results of a parametric study indicates that the thickness ratio is a linear function of a single dimensionless grouping that can be interpreted as the ratio of in- and outflow of sediment into the nose region.