The manipulated transitional backward-facing step flow: an experimental and direct numerical simulation investigation

Results from a joint experimental and direct numerical simulation (DNS) inv estigation are presented for the flow over a backward-facing step manipulat ed by low-amplitude time-periodic (harmonic) blowing/suction excitation thr ough a narrow slot at the edge of the step. For a Reynolds number of Re-h = 3000 (based on step height, h, and inflow velocity, U-o) and for laminar i nflow, a 33% reduction of the mean recirculation length (in comparison to t he non-manipulated reference case) could be obtained with a forcing amplitu de of the order of one per cent of U-o. Based on the momentum thickness, th eta, of the incoming laminar boundary layer (at the edge of the step), the corresponding optimum Strouhal number is St(theta) = f(opt)theta /U-o = 0.0 12. From the experimental data it can be concluded that, in our how case, t he optimum frequency, f(opt) = 50 Hz, was the most amplified frequency in t he transition-to-turbulence process of the separated laminar shear layer. D etailed comparison of the experimental data with data from the numerical si mulation shows that DNS and experimental data agree up to second-order stat istics. The joint experimental and numerical investigations exhibit a compl ementary nature in the sense that, on the one hand, the main advantage of t he experiment was the relative ease with which a wide range of forcing para meters could be tested and, on the other hand, DNS could provide spatio-tem poral details of the how which could not be so easily obtained in the exper iment. (C) 2001 Editions scientifiques et medicales Elsevier SAS.