Open-Loop Control of Disk Wakes

Open-loop control in the near wake of a disk in subsonic air and water flows was investigated using particle image velocimetry at a Reynolds number based on the disk diameters of 67,000 and 20,000, respectively. The air model had tabs along the edge that were driven radially by electromagnetic actuators, and the water model had slots connected to a piston and cylinder external to the flow. In both systems, the frequency of actuation could be varied, and in air, the spatial extent of the forcing could be controlled. Symmetrical actuation produced a reduction in the recirculation-region length of 10% for both air and water. Additionally, a helical excitation at the fundamental natural vortex-shedding frequency in air caused a reduction of 15% and caused the wake to oscillate strongly in phase with the excitation. The most effective symmetrical actuation frequencies correspond to twice the natural frequency of the asymmetric mode in the unforced wake. The effects of actuation on the instantaneous and mean velocity fields are presented and discussed in detail.