Lower drag and higher lift for turbulent airfoil flow by moving surfaces

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Large-eddy simulations of the flow over an actuated NACA4412 airfoil at a chord-based Reynolds number Rec=400,000 are conducted. These solutions extend the previous analysis of an actuated DRA2303 airfoil flow since both flow configurations possess completely different pressure distributions. The technique of spanwise traveling transversal surface waves is used to improve the aerodynamic efficiency, i.e., to decrease the drag and increase the lift. About 65% of the NACA4412 surface on the upper and lower side of the wing section are fully actuated. Two parameter combinations are tested, the first setup leads to an overall drag reduction of Δcd=-8.3% and an increase of the lift by Δcl=2.4% and the second combination yields a mild net power saving of ΔPnet=-1.4%. Strong reductions of the wall-shear stress up to Δcf=-31% are achieved in the regions where the actuation parameters are optimum and the boundary layer growth is damped such that a reduced boundary layer thickness is observed at the trailing edge. Detailed boundary layer statistics are discussed for two positions on the suction and pressure side for both cases. The velocity fluctuations are strongly reduced across the boundary layer and a reduction of the streamwise fluctuations in the near-wall region is also observed in the problem specific inner scaling. Especially for the streamwise velocity a shift of energy away from the wall and from the smaller scales is obtained. The changes of the boundary layers persist beyond the actuated region, i.e., reduced turbulent kinetic energy is determined in the wake downstream of the trailing edge. The comparison of the actuated NACA4412 flow with data from an actuated DRA2303 flow shows similar modifications of the flow field, i.e., positive drag reduction is achieved for both airfoils. This clearly indicates the robustness of the transversal-traveling-wave drag reduction technique.