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have demonstrated that active flow control (AFC) has a potential to enable significant advances in many engineering applications. Though demonstrated experimentally, unsteady separation flow control remains a challenge for Computational Fluid Dynamics (CFD). The main goal of this work was a computational study of the effects of boundary-layer forcing on the mean flow and turbulence using various methods for turbulent flow computations: Large-eddy simulation (LES), Reynolds-averaged Navier-Stokes (RANS) and Detached-eddy Simulation (DES), aiming also at mutual comparison of their features and performance in complex flow situations. Predictive capability of various CFD methods were evaluated for the three representative complex separated flow configurations without flow control. A potential of the methods for unsteady flow computations: LES, DES and URANS was investigated by predicting the flow and turbulence field for the two experimentally investigated AFC configurations. They involve the two recent experimental works pertinent to AFC: periodically perturbed backward-facing step (BFS) flow at a low Reynolds number (Yoshioka et al., 2001) and high Reynolds number flow over a wall-mounted hump (Greenblatt et al., 2004). In general, both the LES and DES computations have reproduced all important effects observed in the BFS experiments. The imposed perturbation frequency corresponding to St=0.19 was found to be the optimum one, leading to the maximum reduction of the reattachment length. URANS underpredicts substantially the intensity of the reduction, exhibiting a very weak sensitivity to the perturbations. Beside a close agreement with the experiment concerning time-mean behaviour of the flow for all perturbation frequencies, the extracted phase-averaged LES results for the case with the optimum frequency (St=0.19) compare well with the reference experimental data. The LES and DES predictions of the main characteristics of separated flow over a wall-mounted hump, obtained on relatively coarse grids with respect to the flow Reynolds number considered (Re_c=9.36x10^5), are encouraging, outperforming significantly the examined RANS models. The numerous simulations of the flow configurations pertinent to active flow control (AFC) have been carried out providing a picture of the current status of CFD in AFC applications. |
