  The transonic flight regime is often dominated by transonic buffet, a highly
unsteady and complex shock-wave/boundary-layer interaction involving major
parts of the flow field. The phenomenon is associated with a large-amplitude
periodic motion of the compression shock coupled with large-scale flow
separation and intermittent re-attachment. Due to the resulting large-scale
variation of the global flow topology, also the turbulent wake of the airfoil
or wing is severely affected, and so are any aerodynamic devices downstream on
which the wake impinges. To analyze and understand the turbulent structures and
dynamics of the wake, we performed a comprehensive experimental study of the
near wake of the supercritical OAT15A airfoil in transonic buffet conditions at
a chord Reynolds number of 2 mio. Velocity field measurements reveal severe
global influences of the buffet mode on both the surface-bound flow field on
the suction side of the airfoil and the wake. The flow is intermittently
strongly separated, with a significant momentum deficit that extends far into
the wake. The buffet motion induces severe disturbances and variations of the
turbulent flow, as shown on the basis of phase-averaged turbulent quantities in
terms of Reynolds shear stress and RMS-values. The spectral nature of
downstream-convecting fluctuations and turbulent structures are analyzed using
high-speed focusing schlieren sequences. Analyses of the power spectral density
pertaining to the vortex shedding in the direct vicinity of the trailing edge
indicate dominant frequencies one order of magnitude higher than those
associated with shock buffet. It is shown that the flapping motion of the shear
layer is accompanied by the formation of a von Karman-type vortex street of
fluctuating strength. These wake structures and dynamics will impact any
downstream aerodynamic devices affected by the wake.
