Flow visualization of dynamic stall on an oscillating airfoil

Abstract

Stroboscopic schlieren photography was used to investigate the efFects of compressibihty, reduced frequency, and amphtude on the dynamic stall of a NACA 0012 airfoil subjected to sinusoidal oscillation. The Mach number was varied from M^o = 0.25 to M^o = 0.45 (corresponding to a Reynolds number variation of Re = 450,000 to Re = 810,000); the reduced frequency was varied from k = 0.025 to k = 0.10. Oscillation amplitudes of am = 5° and am = 10 were compared. Schlieren photographs are presented, which document the dynamic stall vortex formation, convection, and shedding sequence for various experimental conditions. Additionally, a preliminary examination of the flow reattachment process was conducted. Data derived from the photographs indicates that increasing the compressibility causes dynamic stall to occur at lower angles of attack; while, increasing the reduced frequency and/or the oscillation amplitude effectively delays dynamic stall effects to a higher angle of attack. Flow reattachment is sensitive to both Mach number and reduced frequency for low values of these parameters; when either the Mach number or reduced frequency is sufficiently high, the reattachment process stabilizes.