Characterization of ice-induced low-frequency flowfield oscillations and their effect on airfoil performance

Wind tunnel measurements were used to determine the unsteady location of shear-layer reattachment on an NACA 0012 airfoil with a leading-edge horn-ice shape, along with the corresponding time-resolved unsteady iced-airfoil performance. Similar trends in lowfrequency content were identified between the unsteady shear-layer reattachment location and the iced-airfoil performance. Conditional averaging was used to determine the average relationship between the unsteady reattachment location and the iced-airfoil performance corresponding to this common low-frequency mode. A phase relationship was also identified between the shear-layer reattachment location and the iced-airfoil performance. Changes in spectral content over time in the airfoil performance and reattachment location were analyzed using wavelet transforms. The low-frequency oscillations were observed to occur at similar time instances between these signals, and appeared to originate in the separation bubble at a location corresponding to nearly half of the separation bubble length. This lowfrequency mode was then propagated throughout the surface pressure flowfield, affecting both the airfoil performance and the separation bubble length. A summary of the lowfrequency oscillation in the airfoil flowfield is presented.