Near-wall investigation of backward-facing step flows

The electrodiffusion technique has been used to investigate reattaching and recirculating flows behind a backward-facing step. The instantaneous wall shear rate vectors were determined using the current signals provided by a three-segment electrodiffusion probe. The near-wall extents of two counter- rotating recirculation zones located behind the step were determined under turbulent flow conditions in a water channel. The near-wall flow inside the se recirculation zones was found to be very unsteady, with strong low-frequ ency fluctuations. T e streamwise profiles of the wall shear stress were me asured at several values of the Reynolds number and a high level of skin fr iction was obtained in the reverse-flow region. The strong dependence of th e peak value of skin friction on the Reynolds number confirms the viscous-d ominated character of the reverse flow appearing inside the recirculation z one.