Gust–Airfoil Coupling with a Loaded Airfoil

The unsteady load response of an airfoil encountering a gust is often modeled using analytical transfer functions, which assume idealized behavior of both the flow and the airfoil. One such transfer function is the Sears function, which models a pure transverse gust interacting with a flat-plate airfoil at zero mean incidence. The function was extended by Goldstein and Atassi (“A Complete Second-Order Theory for the Unsteady Flow About an Airfoil due to a Periodic Gust,” Journal of Fluid Mechanics, Vol. 74, No. 4, 1976, pp. 741–765) to account for camber and incidence, as well as the presence of a streamwise gust component. Atassi (“The Sears Problem for a Lifting Airfoil Revisited-New Results,” Journal of Fluid Mechanics, Vol. 141, April 1984, pp. 109–122) showed that the effects of camber and incidence (that is, nonzero mean airfoil loading) are not negligible when there is a streamwise gust component. In this work, new experimental data are shown for an airfoil with nonzero loading encountering a gust with both streamwise and transverse components. As well as giving validation to the Atassi model, the flow physics behind the model is shown, including the superposition of the gust onto the airfoil potential field and the propagation of the gust along the airfoil surface.