Low Reynolds number effects on turbulent flows over a backward facing step (BFS) in an open channel were investigated. The Reynolds numbers based on momentum thickness (θ) and step height (h) are in the range 590 ≤ Reθ ≤ 1950 and 950 ≤ Reh ≤ 2900, respectively. The Froude number based on the approach water depth and freestream velocity varied from 0.12 to 0.37. A particle image velocimetry technique was used to measure the velocity field. The flow patterns in the reattachment and redevelopment regions are qualitatively similar for all the three Reynolds numbers studied. The mean velocity profiles in outer coordinates do not exhibit significant Reynolds number effects downstream of the BFS. On the contrary, the turbulence intensities and Reynolds shear stress do not show Reynolds number similarity. As expected, similarity with the upstream profile improves with increasing streamwise distance from the reattachment point. Data obtained in this study were also compared with previous measurements made over backward facing step in a closed channel to study free surface effects. The results showed that deviation of flow over BFS in open channel from flow over BFS in a closed channel is more significant in the immediate vicinity of the step.

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