The alternate shedding of vortices in the near wake of bluff bodies results in fluctuating forces that may weaken the structural integrity of buildings, smokestacks, or oil rig supports and ultimately can lead to catastrophic failure. Accordingly, much attention has been paid to these flows due to their relations with problems of engineering interest. Researchers often attempt to gain insight into the complex wake structures generated by bluff bodies by studying the flow past circular cylinders. Our studies have investigated the mutual interaction of vortex wakes behind a pair of counter-rotating cylinders to gain insight into the effect that counter rotation can have on the control of unsteadiness in the wake of the cylinder pair. It is known that unsteadiness can be suppressed in the wake of a single rotating cylinder when the cylinder is rotated above a critical speed, but little is known about the interaction of wakes from a pair of rotating cylinders placed normal to the freestream flow direction.

            We employ digital particle image velocimetry to study the flow past a pair of counter rotating cylinder placed normal to the freestream flow direction. To ensure a 2-D flowfield, the study is restricted to Reynolds number ranging from 100 to 200 and the gap-to-diameter ratio, g*, is varied  (g* = 1, 3, and 5) to gain further insight as to how the wakes were interacting. An unsteady wake consisting of a pair of von Karman vortex street is observed when the cylinders are rotated below a critical rotational speed. Above this critical value, counter rotation of the cylinders suppresses vortex formation.  The critical rotational speed varies only slightly with Reynolds number but exhibits a strong dependence on the gap-to-diameter ratio.  As the gap-to-diameter ratio increases, the critical rotational speed approaches values expected to suppress vortex formation for a single rotating cylinder, indicating that the wakes of the cylinder pair have more interaction for small gap-to-diameter ratios.  At sufficiently high rotational speeds the streamlines around the cylinder pair resemble a doublet potential flow, shown in the figure below.

            This work was done in collaboration with Antony Jameson’s group from Stanford University who performed a computational simulation that was in excellent agreement with the experiments.