  Transonic buffet is an unsteady flow phenomenon that limits the safe flight
envelope of modern aircraft. Scale-resolving simulations with span-periodic
boundary conditions are capable of providing new insights into its flow
physics. The present contribution shows the co-existence of multiple modes of
flow unsteadiness over an unswept laminar-flow wing section, appearing in the
following order of increasing frequency: (a) a low-frequency transonic buffet
mode, (b) an intermediate-frequency separation bubble mode, and (c)
high-frequency wake modes associated with vortex shedding. Simulations are run
over a range of Reynolds and Mach numbers to connect the lower frequency modes
from moderate to high Reynolds numbers and from pre-buffet to established
buffet conditions. The intermediate frequency mode is found to be more
sensitive to Reynolds-number effects compared to those of Mach number, which is
the opposite trend to that observed for transonic buffet. Spectral proper
orthogonal decomposition is used to extract the spatial structure of the modes.
The buffet mode involves coherent oscillations of the suction-side shock
structure, consistent with previous studies including global mode analysis. The
laminar separation-bubble mode at intermediate frequency is fundamentally
different, with a phase relationship between separation and reattachment that
does not correspond to a simple `breathing' mode and is not at the same
Strouhal number observed for shock-induced separation bubbles. Instead, a
Strouhal number based on separation bubble length and reverse flow magnitude is
found to be independent of Reynolds number within the range of cases studied.
