  Turbulent transonic buffet is an aerodynamic instability causing periodic
oscillations of lift/drag in aerospace applications. Involving complex coupling
between inviscid and viscous effects, buffet is characterised by shock-wave
oscillations and flow separation/reattachment. Previous studies have identified
both 2D chordwise shock-oscillation and 3D buffet/stall-cell modes. While the
2D instability has been studied extensively, investigations of 3D buffet have
been limited to only low-fidelity simulations or experiments. Due to
computational costs, almost all high-fidelity studies to date have been limited
to narrow span-widths around 5% of aerofoil chord length (aspect ratio, $AR =
0.05$), which is insufficiently wide to observe large-scale
three-dimensionality. In this work, high-fidelity simulations are performed up
to $AR=3$, on infinite unswept NASA-CRM wing profiles at $Re=5\times 10^{5}$.
At $AR \geq 1$, intermittent 3D separation bubbles are observed at buffet
conditions. While previous RANS/stability-based studies predict simultaneous
onset of 2D- and 3D-buffet, a case with buffet that remains essentially-2D
despite span-widths up to $AR=2$ is identified here. Strongest
three-dimensionality was observed near the low-lift phases of the buffet cycle
at maximum flow separation, reverting to essentially-2D behaviour during
high-lift phases. Buffet was found to become three-dimensional when extensive
mean flow separation was present. At $AR \geq 2$, multiple 3D separation
bubbles form, in a wavelength range of $\lambda=\left[1c-1.5c\right]$. SPOD and
cross-correlations were applied to analyse the spatio/temporal structure of 3D
buffet-cells. In addition to the 2D chordwise shock-oscillation mode (Strouhal
number $St \approx 0.07-0.1$), 3D modal structures were found in the shocked
region of the flow at $St \approx 0.002-0.004$.
