  Transonic buffet is commonly associated with self-sustained flow unsteadiness
involving shock-wave/boundary-layer interaction over aerofoils and wings. The
phenomenon has been classified as either laminar or turbulent based on the
state of the boundary layer immediately upstream of the shock foot and distinct
mechanisms for the two types have been suggested. The turbulent case is known
to be associated with a global linear instability. Herein, large-eddy
simulations are used for the first time to make direct comparisons of the two
types by examining free- and forced-transition conditions. Corresponding
simulations based on the Reynolds-averaged Navier--Stokes equations for the
forced-transition case are also performed for comparison with the
scale-resolving approach and for linking the findings with existing literature.
Coherent flow features are scrutinised using both data-based spectral proper
orthogonal decomposition of the time-marched results and operator-based global
linear stability and resolvent analyses within the Reynolds-averaged
Navier--Stokes framework. It is demonstrated that the essential dynamic
features remain the same for the two buffet types (and for the two levels of
the aerodynamic modelling hierarchy), suggesting that both types arise due to
the same fundamental mechanism.
