Vortex imprints on a free surface as proxy for surface divergence

In turbulence near a free surface, strong vortices attach to the creating surface imprints visible as nearly circular ‘dimples’. By studying these in direct numerical simulation (DNS) data we make two observations. First, of surface-attached can be very effectively distinguished from other turbulent features using physical features: they are shape, and persist for long time compared with pertinent scales. Secondly, instantaneous number dimples an area, $N(t)$ , is intimately related its mean-square divergence, $\beta ^2(t)$ . We develop simple physically transparent computer vision procedure which, properties low eccentricity longevity, detects tracks their only, sensitivity accuracy $90\,\%$ or better. compare finding normalised cross-correlation $0.90$ changes $N$ lagging around $0.8T_\infty$ behind those ^2$ ( $T_\infty$ integral scale), confirming common observation that spawned by upwelling events where large. These findings suggest rate mass flux across being closely estimated remotely some natural flows free-surface proxy.