A laminar boundary layer develops in a favourable pressure gradient where the
velocity profiles asymptote to the Falkner & Skan similarity solution. Flying-hot-wire
measurements show that the layer separates just downstream of a subsequent region of
adverse pressure gradient, leading to the formation of a thin separation bubble. In an
effort to gain insight into the nature of the instability mechanisms, a small-magnitude
impulsive disturbance is introduced through a hole in the test surface at the pressure
minimum. The facility and all operating procedures are totally automated and phase-averaged data are acquired on unprecedently large and spatially dense measurement
grids. The evolution of the disturbance is tracked all the way into the reattachment
region and beyond into the fully turbulent boundary layer. The spatial resolution of
the data provides a level of detail that is usually associated with computations.
Initially, a wave packet develops which maintains the same bounded shape and
form, while the amplitude decays exponentially with streamwise distance. Following
separation, the rate of decay diminishes and a point of minimum amplitude is reached,
where the wave packet begins to exhibit dispersive characteristics. The amplitude then
grows exponentially and there is an increase in the number of waves within the packet.
The region leading up to and including the reattachment has been measured with a
cross-wire probe and contours of spanwise vorticity in the centreline plane clearly show
that the wave packet is associated with the cat's eye pattern that is a characteristic of
Kelvin–Helmholtz instability. Further streamwise development leads to the formation
of roll-ups and contour surfaces of vorticity magnitude show that they are three-dimensional. Beyond this point, the behaviour is nonlinear and the roll-ups evolve
into a group of large-scale vortex loops in the vicinity of the reattachment. Closely
spaced cross-wire measurements are continued in the downstream turbulent boundary
layer and Taylor's hypothesis is applied to data on spanwise planes to generate three-dimensional velocity fields. The derived vorticity magnitude distribution demonstrates
that the second vortex loop, which emerges in the reattachment region, retains its
identity in the turbulent boundary layer and it persists until the end of the test section.