Effects of the separating shear layer on the reattachment flow structure part 2: Reattachment length and wall shear stress

Measurements of reattachment length of a separated flow behind a backward-facing step for a range of Reynolds numbers (8000 < Re _H < 40,000) and initial boundary-layer thickness (0 < /H < 2) were performed with the purpose of explaining the scatter in existing (high quality) data sets and to understand the effect of the initial shear-layer structure on the reattachment zone. The reattachment length for the case of laminar boundary layers upstream of the step were 30% smaller than when the boundary layer upstream of the step was turbulent. Measured values of the mean wall shear stress in the reattachment zone were also measurably affected by the upstream boundary-layer state. The (rms) levels of fluctuating wall stress were not sensitive to boundary-layer state, but rather to /H, as was the case for the pressure profiles in part 1 (Adams and Johnston 1988).List of symbols C _* ^p normalized pressure, (C _p – C _{p, min})/(1 – C _{p, min}) - C _f skin friction coefficient, /0.5 U _ref ² - C _f level (rms) of fluctuation part of skin-friction coefficient - ER duct expansion ratio; outlet to inlet width - H step height - Re _d Reynolds number based on diameter - Re Reynolds number based on inlet boundary-layer momentum thickness, and U _ref - Re _H Reynolds number based on H and U _ref - x _r distance to reattachment - X ^* normalized distance, (x – x _r)/x _r (note: different from x/x _r in part 1) - /H ratio of inlet boundary-layer thickness to step height - gq ₀ momentum thickness upstream of step