Airfoil Aerodynamic Response to Sinusoidal Streamwise Gusts: Experiments and Computations

This work combines wind tunnel experiments and numerical simulations to investigate the aerodynamic behavior of a symmetrical airfoil subjected to streamwise gusts. Specifically, this research examines the effect of sinusoidal freestream velocity oscillations on the unsteady lift of a NACA 0012 airfoil at a mean Reynolds number of 120,000. Experiments were conducted in a large closed-loop wind tunnel equipped with a louver system to generate a sinusoidal freestream velocity around the stationary wing model. Computational fluid dynamics simulations were conducted in parallel, although with the airfoil undergoing a streamwise oscillatory motion while the freestream velocity was held constant. This study investigates the impact of the surging frequency in pre-stall, post-stall, and near-stall flow conditions, considering velocity oscillations with an amplitude of 25% of the mean flow. For each scenario, quasi-steady cases were also analyzed to isolate steady effects from those due to flow acceleration. Results revealed lift enhancement during the deceleration phase under attached-flow conditions, with the converse occurring post-stall. The former was associated with cyclic shifts in the laminar separation bubble, the dynamics of which were largely driven by the time-varying pressure gradient. At the near-stall angle of attack, bubble bursting was observed during deceleration, leading to the formation and shedding of a leading-edge vortex. While Greenberg’s theoretical model was seen to be sufficient for predicting the lift coefficient magnitude and phase under attached-flow conditions, it proved ineffective when flow separation was present.