Characteristics of Shock Unsteadiness Towards Onset and Offset of Transonic Buffet

Transonic flows over airfoils can exhibit self-sustained, periodic shock-wave motion known as transonic buffet, which can adversely impact aerodynamic performance and impose significant structural loads on aircraft. This study experimentally investigates the evolution of shock unsteadiness on a supercritical OAT15A airfoil, covering the range from incipient buffet (onset) to diminishing buffet (offset) conditions for a chord-based Reynolds number of $Re_c = 2 \times 10^6$. Measurements are performed in a transonic wind tunnel using high-speed focusing schlieren visualization, enabling detailed, time-resolved tracking of shock excursions. Parametric sweeps in Mach number ($0.68 \leq M_{\infty} \leq 0.80$) and angle of attack ($3.5 \, \textrm{deg} \leq \textrm{AoA} \leq 6.5 \, \textrm{deg}$) reveal a confined pocket of established buffet, surrounded by onset and offset regimes characterized by lower albeit still significant shock displacement. Towards onset conditions, the shock performs small-amplitude oscillations accompanied by reversed shock displacement, indicating a precursor to sustained buffet. Conversely, towards offset, the shock motion becomes more chaotic, exhibiting a broadband frequency distribution but retaining moderate oscillation amplitudes that do not revert to the low levels of the pre-buffet state. Dynamic mode decomposition illustrates the dominant buffet modes and frequencies, highlighting the interplay between shock oscillations, shear-layer flapping, and boundary-layer separation. The analysis shows a marked transition from a quasi-sinusoidal single-mode response at peak buffet to increasingly multimodal, chaotic shock motions approaching offset conditions. These findings reveal significant features of buffet are preserved well beyond peak conditions, thus underlining the need to capture both onset and offset states to promote an improved understanding of the buffet mode, and to characterize the unsteady aerodynamic loads imposed in these regimes.