EFFECTS OF TIME SCALES ON LIFT OF AIRFOILS IN AN UNSTEADY STREAM

In unsteady aerodynamics, the reduced frequency has always been consid ered as the predominant controlling parameter because it relates to th e unsteadiness. In this study, we examined the responses of several de lta wings of varying aspect ratio and a two-dimensional wing in an uns teady periodic freestream. We found that in certain operating conditio ns, the lift of the airfoils may or may not be a function of the reduc ed frequency, depending on whether the leading-edge separation vortex can remain stationary on the wing or not. For delta wings with attache d, stationary leading-edge vortices, the lift forces are not a functio n of the reduced frequency. However, if the leading-edge vortices shed and convect downstream, then the lift force depends on the reduced fr equency.