Dynamic- and post-stall characteristics of pitching airfoils at extreme conditions

Abstract

Post-stall flow structure and surface pressures are evaluated to determine the effects of large angles of attack, perching like manoeuvres on the flow about a NACA 0021 airfoil exposed to dynamic stall. Phase-averaged particle image velocimetry was performed to assess the load development during the constant angular velocity pitch-up motion and in post-stall conditions. Evaluation of the resultant aerodynamic loads indicates that initial airfoil rotation generates significant delays in force response. Furthermore, the reduced frequency is shown to influence the angle of attack at which deep stall is initiated, to the extent that fully separated flows are delayed to an angle of attack of 60°. Vortex structures are linked to lower surface pressures with increased angle of attack and also for post-stall flow conditions. Likewise, the presence of the vortex structures shifts the centre of pressure significantly along the airfoil chordline immediately after cessation of the airfoil rotation. At the maximum angle of attack, the centre of pressure is shown to move aft for fully separated flow conditions. The variation in location of the centre of pressure, not only changes the moment generation and aero-elastic characteristics of the airfoil, but also increases structural torsional loading and fluctuations that result in increased fatigue of helicopter rotor shafts and horizontal-axis wind turbines.