Dynamic stall on airfoils exposed to constant pitch-rate motion

Gust-like flow behavior is simulated using pitching airfoils to represent the unpredictable nature of atmospheric turbulence on the blades of wind turbines. Angle of attack, reduced frequency and both NACA 0021 and NACA 0012 airfoils are investigated using particle image velocimetry to assess their influence on the developed flow structure resulting from the ramp-up constant angular velocity motion. The pitch motion was shown to delay the onset of the stall vortex to high angles of attack, which is linked to increased lift. Moreover, increasing the reduced frequency reduced the rate of vortex growth as the angle of attack was increased. Development of a rear separation bubble with low velocity is noted during initial development of the dynamic stall process. Once the critical angle of attack is reached, initiation of the formation of the dynamic stall vortex is observed. Increased angular displacement resulted in the annihilation of the trailing edge vortices, by the initial stall vortex, which also coincided with vortex-induced separation leading to bluff body separation. Results from the current work show the presence of delayed separation and vortex formation on the upper surface of the airfoil characteristic to the dynamic stall process. The current work highlights the flow features responsible for enhanced lift, whilst shedding light on the development process for constant-pitch-rate motion about thick and thin airfoil sections.R.R. Leknys, M. Arjomandi, R.M. Kelso and C. Birze