Wake Formation From a Pair of Circular Cylinders Traversing Between Small- and Large-Incidence Flow Regimes

Cross flow past a pair of equal-diameter staggered circular cylinders, with either one of the pair subject to forced harmonic transverse oscillation, is investigated experimentally within Reynolds numbers Re = 525–750. The center-to-center pitch ratio and stagger angle of the cylinders at their mean position are 2.5° and 21°, respectively. Results with cylinder excitation frequencies in the range 0.07 ≤ f_eD/U ≤ 1.18 (D = cylinder diameter, U = mean flow velocity) at a constant oscillation amplitude (peak-to-peak) of 0.44D are reported. Flow visualization of the wake formation region and hot-film measurements of the wake velocity are reported. Emphasis is placed on the mechanisms leading to vortex shedding. Results show that the wake undergoes considerable modification with the oscillation of either of the two cylinders; this modification depends strongly on the value of f_eD/U. The flow patterns remain essentially the same as those of the corresponding static cases for f_eD/U < 0.10. However, the flow at higher oscillation frequencies than that can no longer maintain those patterns. In particular, there are distinct regions of fundamental and superharmonic synchronizations between the dominant wake periodicities and the cylinder oscillation over the whole range of f_eD/U. Moreover, the manner in which the wake responds to the cylinder oscillation depends strongly on whether it is the upstream or downstream cylinder which is being oscillated.