Vortex generator design for unsteady flow separation control and dynamic stall suppression on pitching thick airfoils

Abstract. This study experimentally investigates the performance of vortex generators (VGs) designed for maximising lift-to-drag ratio in steady conditions to prevent unsteady flow separation. Surface pressure measurements are conducted in the TU Delft low-speed wind tunnel on a DU-97-W-300 airfoil undergoing pitch oscillations while equipped with VGs of various vane sizes and shapes. In steady conditions, vanes with heights smaller than the local boundary layer thickness optimally balance the stall delay with maximum lift-to-drag ratio among the tested triangular vane VGs. However, these same VGs with vane heights smaller than or equal to the steady local boundary layer thickness are insufficient to suppress unsteady flow separation in all pitching cycles. VGs whose vane height exceeds the local boundary layer thickness for a larger part of the pitch cycle prevent unsteady flow separation and restrict the upstream movement of the stall vortex for a larger percentage of cycles. Flow separation is less likely at higher reduced frequencies, making the number of separated flow pitching cycles less sensitive to the VG vane size. Contrary to past literature, rectangular vanes yield a higher steady aerodynamic efficiency than triangular vanes. Rectangular vanes also suppress unsteady flow separation in all pitching cycles at all tested reduced frequencies, indicating overall stronger streamwise vortices than triangular vanes and proving to be a better VG shape for steady and unsteady stall suppression on thick airfoils.