Experimental study on the thickness dependence of bionics coverts for the wing stall control

Bio-inspired by the covert feathers on bird wings, the artificial coverts of different thicknesses were designed by using flexible materials, and they were installed at different locations on the upper side of a NACA0018 wing model at a high angle of attack. In the wind tunnel experiments, the hot-wire anemometer was used to measure the velocity distributions in the wake flow, and therefore the time-averaged and turbulent fluctuation velocities were obtained. The flow separation control effectiveness of the different thicknesses were analyzed by the time-averaged velocity profiles, the root-mean-square velocity

distributions of the turbulent fluctuations as well as their Power Spectral Density (PSD). The results show that, the thin coverts near the leading-edge effectively reduce the distance between the leading-edge shear layer and the upper surface, whereas installed near the trailing-edge, the flow field around the airfoil has mere change. On the other hand, for the thick coverts, the flow separation control effectiveness is better than those near the leading-edge. Based on the multi-scale wavelet analysis, the artificial coverts improve the transformation of low-frequency large-scale coherent structures to high-frequency small-scale ones by adaptively fluttering and flapping motions, which is highly effective for flow separation control.