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

Results from a joint experimental and direct numerical simulation (DNS) investigation are presented for the flow over a backward-facing step manipulated by low-amplitude time-periodic (harmonic) blowing/suction excitation through 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 inflow, a 33% reduction of the mean recirculation length (in comparison to the non-manipulated reference case) could be obtained with a forcing amplitude of the order of one per cent of U-o. Based on the momentum thickness, theta, 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 = 0.012. From the experimental data it can be concluded that, in our how case, the optimum frequency, f(opt) = 50 Hz, was the most amplified frequency in the transition-to-turbulence process of the separated laminar shear layer. Detailed comparison of the experimental data with data from the numerical simulation shows that DNS and experimental data agree up to second-order statistics. The joint experimental and numerical investigations exhibit a complementary nature in the sense that, on the one hand, the main advantage of the experiment was the relative ease with which a wide range of forcing parameters could be tested and, on the other hand, DNS could provide spatio-temporal details of the how which could not be so easily obtained in the experiment. (C) 2001 Editions scientifiques et medicales Elsevier SAS.