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Three components of velocity fluctuations were measured in a plane turbulent wall jet which was modulated periodically by a sinusoidal pressure fluctuation in its settling chamber. The experiment was carried out in a closed-loop wind tunnel in the absence of an external stream at Reynolds number Rej = Ujb/v = 6900 and Strouhal number Stj = fb/Uj = 9.5 × 10−3, where b is the width of the slot from which the jet emerges at an efflux velocity Uj. A detailed comparison is provided with similar measurements made in a natural, unexcited turbulent wall jet. One of the purposes of this experiment was to establish the kinetic energy transfers which take place in the wall jet under controlled perturbations. More specifically, we were interested in determining the interactions occurring between the steady mean flow, the coherent eddies and the ‘random’ turbulent fluctuations. We used the triple decomposition of the equations of motion as suggested by Hussain (1983) and quickly observed that the usefulness of this decomposition depends on the definition of coherent motion, which is ambiguous in the presence of phase jitter. Two such definitions were considered and the results are discussed in the experimental case-study provided. An attempt is made to define quantitatively the intensities of the coherent motion in externally excited, wallbounded flows. It is a case-study and not a parametric investigation aimed at maximizing the effects of period oscillations on the wall jet.

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.

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