Direct numerical simulations of transonic turbulent buffet over a supercritical airfoil

Transonic buffet is an aerodynamic instability that manifests on supercritical airfoils as low-frequency shock oscillations coupled with flow unsteadiness, producing large lift fluctuations and limiting the flight envelope. The mechanisms sustaining the buffet cycle remain unclear, hindering accurate onset prediction and control development. In this work, we perform direct numerical simulations (DNS) of transonic flow over a supercritical airfoil with the boundary layer tripped at 10% of the chord. Eight cases are investigated at Mach number 0.7 and Reynolds numbers (Re) of 300,000 and 600,000, with the angle of attack (α) ranging from stable (4°, 5°) to buffet conditions (6°, 7°). As α increases, the unsteadiness intensifies and transitions to the buffet state, with corresponding formation of a separation bubble. Further increase in α causes the bubble to grow, periodically resulting in a fully separated boundary layer. Spectral proper orthogonal decomposition (SPOD) shows that the spatio-temporal coherence at the shock buffet frequency exhibits low-rank behavior projected to the first SPOD mode. Finally, we leverage the DNS dataset to benchmark Reynolds-averaged Navier-Stokes (RANS) predictions with the Spalart–Allmaras model. While RANS slightly overpredicts the buffet onset angle, it accurately reproduces the modal structure of the self‑sustained shock oscillations.