Reverse flow phenomenon in duct corners at a low Reynolds number

In recent years, the near-wall reverse flow (NWRF) phenomenon taking place in wall-bounded turbulent flows has become the subject of comprehensive theoretical and experimental study. Currently, it is generally accepted that the NWRF events are caused by strong near-wall vortical structures located in the buffer region of the boundary layer, which are either quasi-streamwise vortices tilted with respect to a mean flow direction or transversely oriented hairpin-like vortices. In the present investigations, we demonstrate that there is at least one more mechanism that differs from the existing ones. Considering a fully developed turbulent duct flow studied by direct numerical simulations at a relatively low Reynolds number Re_τ = 204, we found the presence of the NWRF events in the corner regions. The frequency of their appearance is three orders of magnitude higher than those appearing in the central area of the wall, and their lifetime is about three times longer. The mechanism of their formation is found to be associated with streamwisely oriented vortical structures located near the corner.