Experimental study of perforated plate wakes upon starting motion

We study the flow and drag generated by a perforated plate set in motion in a tow tank during the initial phases of its flow development over times less than that required to yield a fully developed wake. The effect of open area fraction on the vortex dynamics is investigated and found to cause a slow transition between a solid plate’s tight vortex-rollup behaviour and a highly perforated plate’s weak shear-layer behaviour. This change corresponds to suppression of the initial drag peak and subsequent drag undershoot of a solid plate. Paradoxically, this results in a region of time where adding perforation increases drag. We use our experimental data during the short acceleration period to close Inoue’s porous plate vortex model by leveraging a modification of small-time self-similar vortex dynamical theory introduced by Rott. We show that the results from Inoue’s model are useful in explaining many of the general aspects of the flow behaviour across all perforation levels tested, including the effect of perforation on force, circulation and vortex motion during and after the plate’s acceleration from rest.