The mean velocity, Reynolds stress, and mean vorticity regions of a separated shear layer over a surface mounted block are investigated by 2D Digital Particle Image Velocimetry (DPIV) for three Reynolds numbers ($Rea=500$, $1000$, and $2500$) and two channel-to-block height ratios ($H∕a=1.825$ and $4.6$). The recirculation region’s height and length are determined for the separated shear layer by means of $U¯=0$ contours. It is observed that the high Reynolds stress regions lay just outside of the $U¯=0$ contours. The flow visualization and DPIV measurement of vorticity indicate that the differing normalized Reynolds stresses between $Rea=500$ and 1000 are most probably due to the initiation of the vortex shedding between these two Reynolds numbers while, differences are minimal between $Rea=1000$ and 2500. A sign change in the Reynolds shear stress distribution of the separated shear layer near the leading edge of the block was recognized for every Reynolds number and channel width.

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