Self-sustained shock wave oscillations on airfoils, commonly defined as shock buffet, can occur under certain combinations of transonic Mach number and angle of attack due to the interaction between the shock and the separated boundary layer. To help understanding buffet physics, a rigid supercritical wing model (OAT15A) was investigated in pre-buffet and buffet conditions using a combined application of BOS (Background Oriented Schlieren), deformation and force measurements. From the observation via BOS of the change of the shock location and the extent of the boundary layer separation with the AoA (angle of attack), the transition from stable shock to buffet was detected. A comparison with other research groups at supposedly similar aerodynamic conditions highlighted a great disparity among them in terms of buffet onset, amplitudes of buffet oscillations, and flow development (motion of the mean shock location with the AoA) after the onset. The average and rms (root mean square) of the surface displacement were computed together with the effective geometric AoA, taking into account the static torsional deformation of the model and its support. Moreover, the spectra of the balance and deformation data showed the same buffet peak as in the BOS spectrum, indicating a coupling between structure and flow, which increased with the AoA.

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An experimental investigation was performed in order to better understand the transonic shock buffet phenomenon and determine the dominant flow interactions at specific flow conditions. A rigid wing in the shape of an OAT15A airfoil was placed in the Trisonic Wind Tunnel Munich, where both the Mach number and the angle of attack were varied between $$0.65< M_\infty < 0.77$$ 0.65 < M ∞ < 0.77 and $$3.8< \alpha < 6.3^\circ$$ 3.8 < α < 6 . 3 ∘ respectively. With the use of high-speed imaging, high-quality optics and state-of-the-art laser equipment, highly resolved velocity field measurements were obtained via particle image velocimetry, where the streamwise and vertical velocity components were computed over the suction side of the wing center plane. It was shown that sustained buffet first occurs at $$M_\infty \ge 0.74$$ M ∞ ≥ 0.74 when maintaining the angle of attack constant at $$\alpha =5.8^\circ$$ α = 5 . 8 ∘ . Similarly, an increase in $$\alpha$$ α for a fixed $$M_\infty =0.74$$ M ∞ = 0.74 also led to the development of shock buffet. Instantaneous snapshots confirmed the presence of a recirculation region downstream of the moving shock, where an increase in the wake size was confirmed when the shock was located most upstream. Streamwise correlations were also computed near the airfoil’s upper surface in order to extract the characteristic convective velocity of flow structures. The convective velocity appeared to increase with streamwise distance, ranging on average between 50 and 150 m/s. Overall, these time-resolved velocity field measurements allow for the investigation of the flow dynamics during shock buffet and highlight the independent effect of Mach number and angle of attack on this complex phenomenon.

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