Control of flow past a circular cylinder via a spanwise surface wire: effect of the wire scale |
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Authors: | Alis Ekmekci Donald Rockwell |
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Institution: | (1) University of Toronto Institute for Aerospace Studies, Toronto, ON, M3H 5T6, Canada;(2) Department of Mechanical Engineering, Lehigh University, Bethlehem, PA 18015, USA |
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Abstract: | Flow phenomena induced by a single spanwise wire on the surface of a circular cylinder are investigated via a cinema technique
of particle image velocimetry (PIV). The primary aim of this investigation is to assess the effect of the wire scale. To this
end, consideration is given to wires with different diameters that are 0.5, 1.2, and 2.9% of the cylinder diameter. The Reynolds
number has a subcritical value of 10,000. Compared to the thickness of the unperturbed boundary layer developing around the
cylinder between 5° and 75° from the forward stagnation point, the former two wires have smaller scales and the latter has
a larger scale. Two angular locations of the wire, defined with respect to the forward stagnation point of the cylinder, are
found to be critical. When the wire is located at these critical angles, either the most significant extension or the contraction
of the time-mean separation bubble occurs in the near wake. These critical angles depend on the wire scale: the smaller the
wire, the larger the critical angle. The small-scale and large-scale wires that have diameters of 1.2 and 2.9% of the cylinder
diameter induce bistable shear-layer oscillations between different separation modes when placed at their respective critical
angles corresponding to maximum extension of the near-wake bubble. These oscillations have irregular time intervals that are
much longer than the time scale associated with the classical Kármán instability. Moreover, the large-scale wire can either
significantly attenuate or intensify the Kármán mode of vortex shedding at the critical states; in contrast, the small-scale
wires do not notably alter the strength of the Kármán instability. |
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