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

Abstract

A boundary layer maintained as close as possible to separation over an extended distance was produced, in accordance with the concept of Stratford. The resulting layer was two-dimensional in the mean, had nearly a constant shape factor of 2.5 and approximately linear streamwise growth of its integral length scales. The flow exhibited a definite non-equilibrium character, indicated by the different scales required for collapse of the mean velocity and turbulence intensity profiles. It was also very sensitive to the thickness of the upstream boundary layer. External excitation was imposed for diagnostic purposes and as a tool for delaying separation. The oscillatory momentum level of cμ ≈ 0.1% was tested for its ability to increase the skin friction cf at the prescribed geometry. Various frequencies, corresponding to the Strouhal number 0.008 < fθ0/Uref < 0.064, were used for the free stream reference velocity of Uref = 15 m s−1 and for two different inflow conditions. Notable increase (close to 60%) in cf was observed at higher frequencies that did not undergo maximum amplification. The increase in cf was accompanied by a reduction in the boundary layer thickness and in the shape factor H. The latter decreased in one case from 2.5 to 2.1. The overall turbulence level in the boundary layer decreased due to the addition of plane external perturbations.