Transient Dynamics of the Flow Over a Pitching Wing at Low Reynolds Number*

At aerodynamically low Reynolds numbers, lifting surfaces often experience an abrupt stall due to the bursting of a laminar separation bubble. This study examines laminar separation bubble formation and bursting on a NACA 0018 semi-span finite wing model with a semi-aspect ratio of 2.5 at a Reynolds number of 1×10⁵. Bubble formation and bursting are induced by pitch-up and pitch-down motions between angles of attack of 10° and 13°. The effect of pitch rate is examined by varying the duration of the motion from 4 to 80 convective timescales. A second model approximating a two-dimensional airfoil is employed as a baseline for the evaluation of three-dimensional end effects on the transient flow over the finite wing model during the pitching motion. Simultaneous time-resolved force and two-component particle image velocimetry measurements are performed to characterise the transient aerodynamic loads resulting from the pitching motion and relate these loads to the flow-field development. For a constant pitch rate, substantial variations in the durations of the flow-field transient occur. The dynamics of separation bubble formation and bursting during the transient change of angle of attack is also compared to the dynamics observed during a change in Reynolds number investigated previously.