Unsteady flow field behind levitated short-finite circular cylinder with angle of attack

Abstract

Flow field in the near wake of a short-finite circular cylinder at L/D=1.0 with an angle of attack between 0∘–15∘, where the transition from the non-reattaching flow to the reattaching flow appears, is investigated in wind tunnel tests with a supportless condition. Stereo particle image velocimetry measurements were applied to the experiments at the Reynolds number of 3.46×10^4, and velocity fields in the near wake were obtained. The data was mainly analysed using spectral proper orthogonal decomposition. Characteristic large-scale wake structures of recirculation bubble pumping and large-scale vortex shedding were identified in the near wake of the cylinder regardless of the angle of attack. The phase difference of expansion and contraction of the recirculation flow appears in the recirculation bubble pumping at α≠0∘. On the other hand, the eigenfunctions of velocity fluctuations at the vortex shedding frequency show a similar spatial pattern regardless of α. Frequency analyses of wake position calculated from the reconstructed velocity field clarified that peak frequency is different between two in-plane directions when α≠0∘. In addition, three vortex shedding patterns (anticlockwise/clockwise circular and flapping) are identified not only at α=0∘ but also α≠0∘. The feature of wake position in the radial direction for each pattern is observed regardless of the angle of attack. The relationship between the recirculation bubble pumping and the wake position in the radial direction is apparent in the non-reattaching flow but is weaker with α in the reattaching flow.journal articl