Many aquatic environments are stratified, characterized by regions of vertical variation in fluid density (pycnoclines) caused by gradients in temperature (thermoclines) or salinity (haloclines). The settling and dispersion of particles, swimming motion of organisms in oceans and lakes and motion of drops during oil spills are few examples of important processes that are affected by density stratification. We use combination of perturbation  method and direct numerical simulations of motion of particles  and organisms to quantify the effect of density stratification on the settling/rising velocity, the microstructure of a suspension, and the induced mixing. The results of our direct numerical simulations of the sedimentation of particles show that the presence of vertical density gradients in the water column can substantially affect the settling dynamics of a particle/drop, interaction between a pair of particles/drops, and settling rates and microstructure of suspension of particles/drops. We show that the shape of non-spherical particles affects both the settling orientation and the settling rate of particles in stratified fluids, which will have direct consequences on the vertical flux of particulate matter and carbon flux in the ocean. We further demonstrate an unexpected effect of buoyancy, potentially affecting a broad range of biological processes at pycnoclines in oceans and lakes. In particular, stratification has a major effect on the flow field, energy expenditure, and nutrient uptake of small organisms. In addition, the role of stratification in pattern formation of bioconvection plumes of algal cells and in biogenic mixing will be discussed.

Suspension of particles, drops and organisms in density stratified fluids

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