(470e) Vertical Migration of Small Organisms in a Stratified Fluid

Significant progress has been made in analyzing low-Reynolds number locomotion in homogeneous fluids. Even though density interfaces due to temperature or salinity gradients (pycnoclines), ubiquitously occur in oceans and lakes, the effects of stratification on the hydrodynamics of swimming of small organisms, their interaction with each other and their migration are poorly understood. Our recent computational results reveal the full nonlinear effects of stratification on the locomotion of small organisms. We implement a direct numerical simulation of the migration of small organisms at pycnoclines and illustrate the role of the diffusivity of the stratified agent on their motility, nutrient uptake, and energy expenditure. Understanding the effects of density stratification on the propulsion of small organisms is a pertinent step in explaining accumulation of particles and organisms and intense biological activity associated with pycnoclines and the occurrence of important environmental processes that are correlated with stratification. We demonstrate that for an archetypal swimmer model, squirmer, the migration velocity in a density stratified fluid can be largely influenced by buoyancy effects. The effects of density stratification are increased as the diffusivity of the stratified agent is reduced. The results demonstrate that the stratification suppresses the vertical migration and consequently affects the life of low Reynolds number swimmers at pycnoclines.