Experimental investigation of vortex shedding from a plate: effect of external velocity perturbation

Vortex shedding behind a plate with a square trailing edge was investigated in a water tunnel. Both the case where the shedding occurs naturally and the case where the shedding is "forced" by an applied transverse velocity field were studied. The perturbation velocity at the trailing edge of the plate was calibrated by accurately measuring the velocity at this point using laser doppler anemometry. The spanwise cross-correlation of the fluctuating velocity was determined based on velocity measurements taken at two different spanwise separations; hot film sensors were used to measure the velocities. It was found that high spanwise correlation of the vortex shedding could be induced by applying a low level perturbation at the natural shedding frequency. It was found that there was a threshold below which the vortex shedding was not well correlated. To prove that the correlations were due to an ordering of the vortices as they were shed, rather than being mainly due to the perturbation field, validation tests were conducted. The phenomenon was also studied by flow visualization using hydrogen bubbles illuminated by a laser. In natural shedding, spanwise waviness of the vortices 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.