Micro-swimmers in vertical turbulent channel flows

Abstract

The behavior of micro-organisms in fluid flows, with a rich spectrum of dynamics, has been long an intriguing problem. Despite the wide presence of micro-swimmers in nature, only recently has their motility in turbulence been explored by simulation and experimental approaches. In this work, we study the effect of active swimming on motile micro-organisms in vertical turbulent channel flows by direct numerical simulations. The micro-swimmers are elongated and modeled as inertia-less prolate spheroids which are also subjected to gravity induced settling. The swimmers show a non-uniform distribution in the wall-normal direction and preferential orientation in mean flow direction. Particles with a predominant swimming velocity accumulate near the solid walls, regardless of the flow direction. This differs from the case of settling non-motile particles where flow direction changes the location of particle accumulation. The underlying mechanisms are explained by the wall-normal transport process including particle–fluid slip velocity and local clustering. The mean slip velocity makes the swimmer drift towards the wall relative to the local fluid, while the local fluid carries the swimmers away from the wall. Particle distribution reaches a stable state as the two mechanisms balance each other. In terms of orientation for prolate spheroids, swimming has been shown to increase a tendency that particles align with the streamwise direction. This suggests that swimming plays an important role in orientation, motion and accumulation of micro-organisms in wall turbulence. Our discovery may contribute to further understanding of the behavior of many aquatic creatures including algae and oyster larvae in natural environment and industrial bioreactors.