EXPERIMENTAL INVESTIGATION OF VORTEX SHEDDING FROM A PLATE - EFFECT OF EXTERNAL VELOCITY PERTURBATION

Vortex shedding behind a plate with a square trailing edge was investi gated in a water tunnel. Both the case where the shedding occurs natur ally and the case where the shedding is ''forced'' by an applied trans verse velocity field were studied. The perturbation velocity at the tr ailing edge of the plate was calibrated by accurately measuring the ve locity at this point using laser doppler anemometry. The spanwise cros s-correlation of the fluctuating velocity was determined based on velo city measurements taken at two different spanwise separations; hot fil m sensors were used to measure the velocities. it was found that high spanwise correlation of the vortex shedding could be induced by applyi ng a low level perturbation at the natural shedding frequency. It was found that there was a threshold below which the vortex shedding was n ot well correlated. To prove that the correlations were due to an orde ring of the vortices as they were shed, rather than being mainly due t o the perturbation field, validation tests were conducted. The phenome non was also studied by flow visualization using hydrogen bubbles illu minated by a laser. In natural shedding, spanwise waviness of the vort ices could be seen clearly in the cross-stream view. This explains the limited spanwise correlation length for this case. On the application of a low-level perturbation this waviness disappeared, corresponding to the high degree of spanwise correlation measured by the hot films.