Transonic buffet is a class of shock wave–boundary layer interaction known to exhibit self-sustained two-dimensional (2D) chordwise shock wave oscillations (Strouhal number $\mathrm{St}\approx 0.05–0.1$), and three-dimensional (3D) spanwise-modulated flow separation and reattachment $(\mathrm{St}\approx 0.2–0.4)$. Due to computational cost, scale-resolving simulations of span-periodic configurations to date have been limited to narrow airfoils (aspect ratio $AR=L_z/c\approx 0.05–0.25$, for span width $L_z$ and chord $c$). These ratios are insufficient to accommodate the 3D “buffet cell” instability reported in low-fidelity simulations and experiments. In this work, implicit large-eddy simulations and modal analysis are performed on infinite wings up to $AR=3$ with sweep angles between $\lambda =[0^{\circ },{35}^{\circ }]$. The sensitivity of the 2D and 3D modes to crossflow is detailed. Two flow conditions are examined, corresponding to minimally and largely separated mean flow at the shock location. For the minimally separated case, the shock dynamics remain essentially spanwise uniform (quasi-2D), with only weak and intermittent separation cells confined to the trailing edge region and exhibiting negligible interaction with the shock. In contrast, increased mean separation leads to the emergence of pronounced 3D buffet cells with a characteristic spanwise wavelength ${\lambda }_z\approx 1–1.5c$. Spectral proper orthogonal decomposition reveals that a quasistationary low-frequency 3D separation mode previously identified on unswept wings $(\mathrm{St}\approx 0.02)$ becomes a spanwise traveling mode as sweep is imposed, shifting monotonically to intermediate frequencies $(\mathrm{St}=0.06–0.35)$. The 2D shock mode is largely insensitive to sweep, whereas the frequency and energy content of the 3D mode increase with sweep while its wavelength remains unchanged. The results demonstrate that transonic buffet on infinite wings arises from the superposition of distinct but coexisting 2D shock motion and separation-driven 3D instabilities, with mean flow separation at the shock identified as a necessary condition for dominant 3D buffet dynamics to emerge.

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• Aerodynamics
• Compressible boundary layers
• Compressible flows
• Compressible turbulence
• High-speed flow
• Shock waves
• Three-dimensional turbulence
• Turbulence
• Turbulence simulations