Vertical Migration of the 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 very poorly understood. In this article, we implement a direct numerical simulation of the migration of swimmers at pycnoclines and illustrate the role of the diffusivity of the stratified agent on the swimming of small organisms. We demonstrate that for an archetypal swimmer model, squirmer, the migration at density stratified fluid can be largely influenced by buoyancy effects. We also show that the effects of density stratification are increased as the diffusivitty 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 across pycnoclines. Our recent computational results reveal the full nonlinear effects of stratification on the locomotion of small organisms.