Unsteady separation processes at large finite, Reynolds number, Re, are considered,
as well as the possible relation to existing descriptions of boundary-layer separation
in the limit Re → ∞. The model problem is a fundamental vortex-driven three-dimensional flow,
believed to be relevant to bursting near the wall in a turbulent
boundary layer. Bursting is known to be associated with streamwise vortex motion,
but the vortex/wall interactions that drive the near-wall flow toward breakdown have
not yet been fully identified. Here, a simulation of symmetric counter-rotating vortices
is used to assess the influence of sustained pumping action on the development of a
viscous wall layer. The calculated solutions describe a three-dimensional flow at finite
Re that is independent of the streamwise coordinate and consists of a crossflow plane
motion, with a developing streamwise flow. The unsteady problem is constructed to
mimic a typical cycle in turbulent wall layers and numerical solutions are obtained
over a range of Re. Recirculating eddies develop rapidly in the near-wall flow, but
these eddies are eventually bisected by alleyways which open up from the external
flow region to the wall. At sufficiently high Re, an oscillation was found to develop in
the streamwise vorticity field near the alleyways with a concurrent evolution of a local
spiky behaviour in the wall shear. Above a critical value of Re, the oscillation grows
rapidly in amplitude and eventually penetrates the external flow field, suggesting
the onset of an unstable wall-layer breakdown. Local zones of severely retarded
streamwise velocity are computed which are reminiscent of the low-speed streaks
commonly observed in turbulent boundary layers. A number of other features also
bear a resemblance to observed coherent structure in the turbulent wall layer.
