Turbulence below and along a free surface

We investigate the flow below and along the non-wavy free surface of a volume of homogeneous turbulence. We leverage a two cubic meter water tank stirred by randomly actuated jets, where particle image velocimetry resolves from the dissipative to the integral scales below the surface. As the Reynolds number is increased, both the vertical and horizontal contributions to the turbulent kinetic energy approach the prediction of rapid distortion theory. At odds with the theory, however, the integral scale of the horizontal fluctuations grows as the surface is approached. Indeed, along horizontal separations, the direct cascade of horizontal energy is hindered, and an inverse cascade of vertical energy is established. This is connected to the balance of upwelling events (associated with extensional surface-parallel motions and vertical compression) and downwelling events (favoring horizontal compression and vertical stretching). This picture is reflected in the flow along the surface, where we resolve the dynamics of vorticity, strain-rate and divergence by tracking highly concentrated microscopic floating particles. We find that, in contrast with the three-dimensional flow, the compression of surface-attached vortices is as common and intense as their stretching, resulting in long-lived structures and a greatly reduced dissipation rate.