Experimental Study of Perforated Plate Wakes Upon Starting Motion

The flow and drag generated by a perforated plate set in motion up to a formation time of T=8 is studied using a tow-tank. The effect of open area fraction on the vortex dynamics is investigated and found to mediate a slow transition between a solid plate's tight vortex-rollup behavior and a highly perforated plate's weak shear-layer behavior. 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 holes increases drag. We use our experimental data to close Inoue's porous plate vortex model by leveraging a modification of small-time vortex dynamical theory introduced by Rott. We show that the results from the closed Inoue model are useful in explaining most of the flow behavior across all perforation levels tested.