Séminaire régulier IUSTI – 15 juin 2018 – 11h salle 250

Settling particles in a stratified fluid

Mathieu Mercier – IMFT, Toulouse

The settling dynamics of small objects in stratified fluids is important to understand the fate of the biomass in lakes or oceanic environments, or for industrial applications such as waste-water disposal. Research studies dedicated to vertically moving objects in a stratified fluid have mainly considered the influence of some specific background stratifications to estimate the changes in the drag experienced by objects. In linearly stratified fluids, a sedimenting sphere of diameter d feels a constant effect of the stratification, that can be expressed as a modified drag coefficient, function of the buoyancy gradient and Froude number (Fr=U/Nd) at moderate values of the Reynolds number (Re=Ud/nu), with N the Brünt-Väisälä frequency, U the settling speed of the sphere and nu the kinematic viscosity of the fluid. The specific issue of more complex objects (spheroids, disks), or of several (two or more) settling bodies is a fundamental problem, that has received much less attention. I will present experimental and numerical results on the settling dynamics of isolated spheres (cf. figure) and disks, and on the interaction of two spheres settling in a linearly stratified fluid.