Modeling Transverse Gusts Using Pitching, Plunging, and Surging Airfoil Motions

Three model motions were developed to replicate the aerodynamic response of a transverse gust. These motions included a pure plunging and two three-degree-of-freedom motions that approximated the angle-of-attack distribution produced by the gust. Using inviscid models and viscous flow simulations, the responses of the gust and model motions were compared as a function of the nondimensional reduced frequency. The inviscid model was found to overestimate the influence of the rotational added mass in the three-degree-of-freedom motions. In contrast, the viscous flow simulations showed that the two primary sources of discrepancy between the gust and model motions lie in the nonlinear angle-of-attack distribution caused by the gust and the wake development during the model motions. Flow simulations showed that all three motions experienced greater than 90% agreement in lift for gusts with reduced frequencies less than 0.5, indicating that, under this reduced frequency, 1) the effect of the gust convection is minimal, and 2) a pure-plunging motion may suffice for modeling gusts. However, at higher reduced frequencies, the pure-plunging motion experiences greater than 10% worse agreement than the three-degree-of-freedom motions. Overall, the motions provide a good approximation with greater than 90% accuracy in lift for gusts of reduced frequencies less than k=0.75.