A boundary layer maintained as close as possible to separation over an exte
nded distance was produced, in accordance with the concept of Stratford. Th
e resulting layer was two-dimensional in the mean, had nearly a constant sh
ape factor of 2.5 and approximately linear streamwise growth of its integra
l length scales. The flow exhibited a definite non-equilibrium character, i
ndicated by the different scales required for collapse of the mean velocity
and turbulence intensity profiles. It was also very sensitive to the thick
ness of the upstream boundary layer. External excitation was imposed for di
agnostic purposes and as a tool for delaying separation. The oscillatory mo
mentum level of c(mu) approximate to 0.1% was tested for its ability to inc
rease the skin friction c(f) at the prescribed geometry. Various frequencie
s, corresponding to the Strouhal number 0.008 < f<theta>(0)/U-ref < 0.064,
were used for the free stream reference velocity of U-ref = 15 ms(-1) and f
or two different inflow conditions. Notable increase (close to 60%) in c(f)
was observed at higher frequencies that did not undergo maximum amplificat
ion. The increase in c(f) was accompanied by a reduction in the boundary la
yer thickness and in the shape factor H. The latter decreased in one case f
rom 2.5 to 2.1. The overall turbulence level in the boundary layer decrease
d due to the addition of plane external perturbations.