Direct numerical simulation of airfoil separation control using a synthetic jet

Direct numerical simulations (DNS) are performed on a synthetic jet based separation control of flow over a NACA-0018 airfoil, at 10 degrees angle of attack and Reynolds number of 104 based on the airfoil chord length C and uniform inflow velocity U0. The actuator of the synthetic jet is simplified as a spanwise slot on the airfoil leeward surface with a wall-normal Poiseuille-type velocity profile and is positioned just upstream of the leading edge separation point. The momentum coefficient of the jet is chosen at a small value 2.13 × 10-4 normalized by that of the inflow, and two values of the reduced frequency F+ = fC/U0 = 1.0 and 4.0 are investigated. The DNS are conducted with an energy conservative spatially fourth-order parallel code solving the incompressible Navier-Stokes equations on a generalized curvilinear grid. We report results from both two- and three-dimensional simulations. The objectives of the present study are twofold: the first is to identify the effects of geometric variation introduced by the jet on the flow separation patterns; the second is to investigate the effects of synthetic jet on the separation control of flow over an airfoil, with the emphasis on the improvement of aerodynamic performance and related flow separation patterns. Numerical results reveal that the two-dimensional simulations significantly overpredict the lift and drag coefficients compared with the three-dimensional DNS due to differences in the prediction of separation. The geometric variation introduced by the actuator is confirmed to have statistically negligible effects on the flow separation. At proper pulsating frequency, the introduction of the synthetic jet greatly reduces the scope and duration of the separation bubble, increases the lift and may decrease the drag, thus improving the aerodynamic performance.