Surfacing of gyrotactic micro swimmers in free surface turbulence.

We investigate the surfacing of gyrotactic swimmers in free-surface turbulence. This setup mimics an environmentally-plausible situation (dynamic behavior involving different types of phytoplankton) in water bodies when surface waves and ripples are smooth or absent. We perform direct numerical simulation of open-channel turbulence at varying shear Reynolds number (from 170 to 1018 based on the channel height) and we track inertia less 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 vertical migration of the swimmers, which are initially distributed randomly throughout the flow, depends on both their re-orientation ability and shape. This effect reflects in the velocity, orientation and concentration statistics. 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. This indicates that free-surface turbulence has a strong influence on the swimmers’ spatial distribution and, in turn, on the carbon-dioxide exchange cycle across the water/atmosphere interface.