THE EFFECTS OF EXCITATION ON THE COHERENT AND RANDOM MOTION IN A PLANE WALL-JET

Three components of velocity fluctuations were measured in a plane tur bulent wall jet which was modulated periodically by a sinusoidal press ure fluctuation in its settling chamber. The experiment was carried ou t in a closed-loop wind tunnel in the absence of an external stream at Reynolds number Re-j = U(j)b/nu = 6900 and Strouhal number St(j) = fb /U-j = 9.5 x 10(-3), where b is the width of the slot from which the j et emerges at an efflux velocity U-j. A detailed comparison is provide d with similar measurements made in a natural, unexcited turbulent wal l jet. One of the purposes of this experiment was to establish the kin etic energy transfers which take place in the wall jet under controlle d 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 dec omposition of the equations of motion as suggested by Hussain (1983) a nd quickly observed that the usefulness of this decomposition depends on the definition of coherent motion, which is ambiguous in the presen ce of phase jitter. Two such definitions were considered and the resul ts are discussed in the experimental case-study provided. An attempt i s made to define quantitatively the intensities of the coherent motion in externally excited, wall-bounded flows. It is a case-study and not a parametric investigation aimed at maximizing the effects of period oscillations on the wall jet.