Affiliations: Department of Mechanical Engineering, Niigata
University, 8050 Ikarashi 2, Niigata, 950-2181, Japan
Abstract: The flow field around a rotationally oscillating circular cylinder
in a uniform flow is studied by using a particle image velocimetry to
understand the mechanism of drag reduction and the corresponding suppression of
vortex shedding in the cylinder wake at low Reynolds number. Experiments are
conducted on the flow around the circular cylinder under rotational oscillation
at forcing Strouhal number 1, rotational amplitude 2 and Reynolds number 2,000.
It is found from the flow measurement by PIV that the width of the wake is
narrowed and the velocity fluctuations are reduced by the rotational
oscillation of the cylinder, which results in the drag reduction rate of 30%.
The mechanism of drag reduction is studied by phase-averaged PIV measurement,
which indicates the formation of periodic small-scale vortices from both sides
of the cylinder. It is found from the cross-correlation measurement between the
velocity fluctuations that the large-scale structure of vortex shedding is
almost removed in the cylinder wake, when the small-scale vortices are
generated at the unstable frequency of shear layer by the influence of
rotational oscillation.