Control of laminar separation bubble over wind turbine airfoil using partial flexibility on suction surface

In order to suppress the laminar separation bubble and its negative effects on airfoil at low Reynolds numbers, a partially flexible membrane was utilized on the suction surface of a NACA4412 wind turbine airfoil. Location of the partially flexible membrane was decided between x/c = 0.2 and x/c = 0.7 according to experimental results. This partially flexible airfoil was tested using various experimental methods at Re = 2.5 × 104, Re = 5 × 104, Re = 7.5 × 104. Utilizing the partially flexible membrane over the suction surface of the airfoil enabled the bubble to be mitigated or suppressed, therefore the aerodynamic performance improved with lift enhancement and drag reduction. The values of turbulence kinetic energy and Reynolds stress decreased with the partial flexibility, as a result of reducing fluctuations in the flow. With the interaction of the partially flexible membrane and the bubble; the vortices due to the bubble caused the membrane to deform and vibrate. Vibration mode numbers of the membrane deformation were high at lower incidences due to the small vortices, whilst the mode numbers were reduced at higher incidences in the presence of growing vortices. Meanwhile, the membrane caused the vortices to damp down. However, it was observed that the effectiveness of the membrane was decreased when Reynolds number increased.