A review on gyrotactic swimmers in turbulent flows

Many marine plankton species are motile and perform daily vertical migrations, traveling across water columns over distances of tens of meters. It is intriguing that these tiny and slow swimmers can travel in a certain direction within a turbulent environment. One way to do that is by exploiting gravitaxis, which is a form of taxis characterised by the directional movement of an organism in response to gravity. Many plankton species are able to generate a gravitational torque (e.g., due to a nonuniform mass distribution) that reorients them upwards. However, the swimming direction is disturbed by the shearing motions and the velocity fluctuations that characterise oceanic turbulence: these can generate a viscous torque that may destabilize the swimmer. The directed locomotion resulting from the combination of gravitational and viscous torques in a flow is termed gyrotaxis, which is known to lead to a non-uniform spatial accumulation of swimmers in patches or layers. These phenomena depend strongly on the non-linear dynamics that originate from the fluid motions, and the study of gyrotactic swimmers in complex flows is attracting growing attention. Numerical simulations of the Navier-Stokes equations coupled with suitable models of gyrotactic swimmers have proven their capability to provide valuable insight into the dynamical and statistical properties of self-propelled organisms. In this paper, we review recent studies and key findings on gyrotactic swimmers in turbulent flows. First, we introduce the most recent results concerning the orientation and vertical migration of gyrotactic swimmers in isotropic turbulence. Second, we discuss the findings on the accumulation of the swimmers. Last, we review recent progresses concerning the behaviour of gyrotactic swimmers in free-surface turbulence. [Figure not available: see fulltext.]