Surfacing of gyrotactic swimmers in stratified/unstratified free-surface turbulence

The surfacing of gyrotactic swimmers in free-surface turbulence mimics the dynamic behavior of several phytoplankton species in water bodies in the absence of large surface waves and ripples. We examine the surfacing dynamics via direct numerical simulation of open-channel turbulence at varying shear Reynolds number (from 170 to 1018 based on the channel height) coupled with Lagrangian tracking of inertialess swimmers with stability number covering 2 orders of magnitude, which orient themselves according to Jeffery equation. We consider both spherical and elongated swimmers, the latter being modeled as prolate ellipsoids with aspect ratio up to 10. We show that the vertical migration of the swimmers, which are initially distributed randomly throughout the flow, depends on both their re-orientation ability and shape. Once at the surface, swimmers tend to stay there and organize themselves into filamentary clusters, which are in direct correlation with the topology of surface turbulence. The surfacing process is found to depend strongly on the level of thermal stratification within the water body, which we examine at varying Richardson number (from 0, corresponding to the unstartified case, to 510, corresponding to a stratfied flow characterized by the occurrence of a thermocline below the free-surface). Our results indicate that free-surface turbulence and thermal stratification have a strong influence on the swimmers' spatial distribution and, in turn, on the carbon-dioxide exchange cycle across the water/atmosphere interface.