Clustering of settling microswimmers in turbulence

Abstract. Clustering of plankton plays a vital role in several biological activities including feeding, predation and mating. Gyrotaxis is one of the mechanisms that induces clustering. A recent study reported a fluid inertial torque acting on a spherical micro-swimmer, which is analogous to a gyrotactic torque. In this study, we model plankton cells as micro-swimmers that are subject to gravitational sedimentation as well as a fluid inertial torque. We use direct numerical simulations to obtain the trajectories of swimmers in homogeneous isotropic turbulence, and investigate their clustering by Voronoï analysis. Our findings indicate that fluid inertial torque leads to notable clustering, with its intensity depending on the swimming and settling speeds of swimmers. By Voronoï analysis, we demonstrate that swimmers preferentially sample downwelling regions where clustering is more prevalent.