Experimental study of the effect of wing sweep on transonic buffet

In this study the effect of wing sweep on transonic buffet is studied experimentally to reveal the differences between two-dimensional (2D) and three-dimensional (3D) wing configurations. Background oriented schlieren (BOS) and stereographic particle image velocimetry (PIV) have been used as measurement techniques, performing experiments on: an airfoil, an unswept wing and two swept wings with a sweep angle of 15° and 30° respectively (all wings are based on the OAT15A airfoil). All wings have been tested at a constant normal Mach number (Ma∞n=0.7) with respect to the leading edge. The results show that the buffet oscillations are much stronger for the airfoil than for the three finite-span wings. A large difference in the buffet behavior can be noticed between the airfoil and the unswept wing, particularly in correspondence of the more outboard spanwise locations, suggesting that in the latter an important role could be played by finite-wing effects, notably the tip vortex. A spectral analysis has shown that for the swept wings the classical 2D buffet peak (occurring at f=160 Hz for the present conditions) is substantially attenuated, while additional contributions in the range of 450-850 Hz appear. The PIV results showed, for the 30° sweep angle wing, a periodical occurrence of a secondary supersonic area downstream of the main shockwave structure, which is absent for the other wing models. The stereographic PIV configuration allowed the reconstruction of the spanwise oriented velocity component, obtaining in the trailing edge area, spanwise outboard velocities (80-100 m/s) which are in agreement with the spanwise convection of buffet cells observed in literature in this region.