Control of the flow downstream of a backward-facing step

A numerical study was performed to control the characteristics of the flow through the backward-facing step with the aid of active and passive control methods. The Coanda actuator was used as the active flow control method while the inclined double backward-facing step surfaces were used as the passive flow control method. The investigations were performed for the Reynolds Numbers of Reh=6000, 9600, and 20000 based on step height, h, and the free stream velocity. Three different turbulence models, i.e., Spalart-Almaras, realizable k-epsilon and SST k-omega turbulence models, were tested for the solution of the flow over the backward-facing step. The results, which were validated using the experimental data, shown that the Spalart-Allmaras model estimated the reattachment length, with an error of 0.95%. However, in the recirculation region, the SST k-omega estimated the axial velocity with a maximum error of 5%. In the case of an unmodified backward-facing step with no flow control, the reattachment length was found as Xr/h = 4.40, 4.16, 4.20 for , and. As the results of the active flow control via the Coanda actuator, at the momentum coefficient of CM = 0.02, the reattachment length was reduced to Xr/h = 1.0, 0.58, and 0.56 for Reh = 6000, 9600, 20000, respectively. Using a larger momentum coefficient, i.e., CM ≥ 0.06, completely vanished the recirculation region. In the case of passive flow control via inclined double backward-facing step, the maximum relative percent reduction of the reattachment length was 20.2% using the combinations of the inclination angle of upper step surface, α = 15 degree, and lower step surface, β = 70 degree. On the other hand, the other inclined surface combinations, including α = 75 degree and 90 degree, resulted longer reattachment length than the one obtained with unmodified BFS geometry