Flow around nonparallel tandem cylinders

Flow around two nonparallel tandem cylinders is investigated experimentally to understand the associated fluid dynamics at a Reynolds number Re = 5.6 × 10⁴. Two cylinders of identical diameter D are oppositely inclined by 7.5° measured from the normal to the free stream direction, which leads to an included angle of 15° between the cylinders. Strouhal number (St) and time-mean and instantaneous flow field measurements for L^∗ (= L/D = 1-4.05, where L is the cylinder center-to-center spacing) leads to identification of three distinct flows: 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). Regime I is further subdivided into regimes IA and IB contingent on shear layer reattachment and its influence on quasi-steady vortex in the gap and wake. 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. The sporadic presence of reattachment and coshedding flows results in a jump in St at regime II. In contrast to parallel cylinders, nonparallel cylinders experience another jump in St associated with the coshedding flow at L^∗ = 2.5 in regime II. A wake-flow bifurcation at L^∗ = 2.5 is responsible for the jump, separating the wake flow turning towards the small L^∗ and towards the large L^∗.