An experimental study of a boundary layer that is maintained on the verge of separation

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.