Flow over two inline rough cylinders in the postcritical regime

This study investigates the flow behavior over roughened inline cylinders for postcritical flow, a parameter space with relatively little prior scrutiny. Two cylinders of the same relative surface roughness, ks/D=1.9×10−3, separated by a pitch (i.e., L, distance between the centers of two cylinders) between 1.175≤L/D≤10 are studied at Reynolds numbers from 3×105 to 6×105 using unsteady surface pressure measurements. As pitch ratio is increased from L/D=1.175, CD of the downstream cylinder increases sharply at (L/D)c=3.25. This critical pitch ratio (L/D)c is toward the lower end of the reported range for subcritical smooth cylinders. Asymmetric mean gap flow along with alternating reattachment is found for 1.5≤L/D<2.25 (i.e., two asymmetric modes in the gap, mode 1 and mode 2, that are the reflections of each other), and symmetric gap flow with a continuous reattachment is found for 2.25<L/D≤3. The gap flow is also symmetric for the closest pitch ratio tested of L/D=1.175. While the change in upstream cylinder drag coefficient with Reynolds number broadly follows that of an isolated cylinder, for the downstream cylinder, it is approximately independent. The critical separation is also insensitive to Reynolds number within 3×105≤Re≤6×105. Transitions between the reattachment and the co-shedding flow are predominantly continuous over the spanwise planes tested. On the other hand, alternating reattachment occurs in spanwise cells, where one sectional measurement exhibits the asymmetric mode 1 while a spanwise-adjacent section exhibits the asymmetric mode 2 or even symmetric flow. Previously reported maxima in the fluctuating lift and drag coefficients of the downstream cylinder at L/D≈2.4 at subcritical Reynolds numbers are absent in the current investigation.