Control of flow separation around an airfoil at low Reynolds numbers using periodic surface morphing

The paper investigates experimentally the low Reynolds number flow (Rec=50,000) around a model that approximates a NACA 4415 airfoil and the control of separation using periodic surface motion. Actuation is implemented by bonding two Macro Fiber Composite patches to the underside of the suction surface. Time-resolved measurements reveal that the peak-to-peak displacement of the surface motion is a function of both the amplitude and frequency of the input voltage signal but the addition of aerodynamic forces does not cause significant changes in the surface behavior. The vibration mode is uniform in the spanwise direction for frequencies below 80 Hz; above this frequency, a secondary vibration mode is observed. The flow around the unactuated airfoil exhibits a large recirculation region as a result of laminar separation without reattachment and consequently produces relatively high drag and low lift forces. Various actuation frequencies were examined. When actuated at Vf+=2.0, the spectra in the vicinity of the trailing edge and near-wake were found to be dominated by the actuation frequency. Sharp peaks appear in the spectra suggesting the production of Large Coherent Structures at this frequency. The increased momentum entrainment associated with these enabled a significant suppression of the separated region. The result was a simultaneous increase in CL and decrease in CD and therefore a large increase in the L∕D ratio. In addition, a delay in the onset of stall results in a significant increase in the maximum achievable lift.