Experimental investigation of the transonic buffet cycle on a supercritical airfoil

Abstract Transonic buffet behaviour of the supercritical airfoil OAT15A was investigated experimentally at flow conditions $$Ma=0.7$$ M a = 0.7 and $$\alpha =3.5^\circ $$ ? 3 . 5 ? , using schlieren particle image velocimetry (PIV). The general cycle characterised with short-exposure visualisation phase-averaged PIV measurements. A spectral analysis showed that shock oscillation occurs a dominant contribution 160 Hz (St = 0.07, in good agreement literature) between 25 55 % chord airfoil. Proper Orthogonal Decomposition (POD) applied to data extract main modes connected buffet. It is found first three most energetic capture around 65 total fluctuating kinetic energy. third have frequency peak are well representing separated area oscillation. second mode was, instead, associated an asymmetrical shear layer displays 320 Hz, being double frequency. Finally, it shown by 11 POD modes, accurate reduced-order model (ROM) obtained, which when subtracted from instantaneous velocity fields allows small-scale structures present flow, such as upstream travelling waves (UTWs) vortex shedding near trailing edge. allowed estimate UTWs, obtaining values literature. In contrast, dynamics edge revealed vortices shed foot, convect downstream detached surface, cannot be considered responsible for creation UTWs view mismatch two phenomena. Graphic abstract