Additional spanwise vortices near the free surface in open channel flows

This paper reports a new phenomenon in the near-free-surface region of open channel flows (OCFs): namely that higher relative population densities of spanwise vortices are observed than are seen at equivalent positions in turbulent boundary layers. The presence of additional retrograde spanwise vortices in OCFs throughout the region y/h > 0.2 (where y is the distance to the wall and h is the water depth) prompts us to re-examine the wall-normal extent that the free-surface effect can reach, which is classically expected to be limited within the surface and blockage layers. Finally, possible mechanisms for the phenomenon of additional spanwise vortices in OCFs are provided.

Automated summary

This paper reports a new phenomenon in open channel flows, where higher population densities of spanwise vortices are observed near the free surface compared to turbulent boundary layers. Additional retrograde spanwise vortices are present in OCFs beyond y/h > 0.2, prompting a re-examination of the wall-normal extent of the free-surface effect. Possible mechanisms for this phenomenon are also provided.