Flow around two non-parallel tandem cylinders

Abstract

An experimental investigation at a Reynolds number (Re) of 5.6 × 104 is carried out to understand fluid dynamics associated with two nonparallel cylinders in a tandem configuration. The upstream and downstream cylinders of identical diameter D are oppositely rotated by 7.5° measured from the normal to free stream direction, which leads to an included angle of 15° between the cylinders. Measurements of Strouhal number (St) and time-mean and instantaneous flow fields are conducted to explore the flow structure variation in the spanwise direction at different spacing ratio L* (=L/D = 1–4.05, where L is the cylinder center-to-center spacing). Three distinct flows are identified, namely, alternating reattachment flow (regime I, 1 ≤ L* < 2.15), bi-stable flow (regime II, 2.15 ≤ L* ≤ 3.1), and coshedding flow (regime III, 3.1 < L* ≤ 4.05). Based on shear layer reattachment and its influence on quasi-steady vortex in the gap and wake regions, regime I is further divided into two subregimes, IA and IB. The three flow regimes are totally different from those for parallel cylinders. A spiral vortex forming in the gap that varies along the cylinder span is responsible for making the difference. Due to the intermittent presence of reattachment and coshedding flows, jump in St prevails for 3.9 < L* < 4.1 for the parallel cylinders and 2.15 ≤ L* ≤ 3.1 for nonparallel cylinders, the range being remarkably long for the latter. Besides this, another jump in St associated with the coshedding flow at L* = 2.5 in regime II for nonparallel cylinders is revealed, which does not occur for parallel cylinders. A wake-flow bifurcation at L* = 2.5 is responsible for the jump, separating the wake flow turning up (L* < 2.5) and turning down (L* > 2.5). The independence principle for nonparallel cylinders is not valid for the flows in regimes I and II but is valid for the flow in regime III, while that for parallel cylinders is valid for all regimes.