The effects of the height ratio and spacing between two 2-dimensional wall-mounted roughness elements in a turbulent boundary layer were studied using 2-dimensional particle image velocimetry. The two roughness elements were arranged with spacings of l/h2 = 0.8, 1.6, 2.4, and 4.8, and height ratios, h1/h2 = 1.0, 0.75, and 0.5 where h1 is the upstream element’s height and h2 is the stationary downstream elements height. At the largest spacing, the height ratio had little to no effect on the flow as there were only small changes in the reattachment length, turbulent kinetic energy (TKE), and shape factor. The flow characteristics at the closest spacing were highly dependent on the height ratio as there were large differences in the reattachment length, TKE, and shape factor. Based on the 13 roughness element configurations tested, a height ratio of h1/h2 = 0.75 and a spacing of l/h2 = 0.8 created the smallest recirculation zone, generated the least amount of TKE, and the flow was closest to being fully recovered at x/h2 = 14. The highlighted roughness element configuration is useful in engineering applications as the roughness elements induce turbulence in the flow, therefore increasing mixing and convective heat transfer.
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