Three-dimensional transition in the wake of two tandem rotating cylinders

Three-dimensional (3-D) transition in the wake of two tandem co-rotating cylinders is studied numerically for Reynolds numbers 180 <= Re <= 500. Infinite cylinders of equal diameter (D) and span (8D) are placed at streamwise gaps Lx = 2.5D and 7.5D, and rotated at rotation rates alpha = 0, 0.5, 1 and 2. For alpha = 0 and Lx = 7.5D, 3-D transition in the wake of the upstream cylinder resembles that for an isolated cylinder with the formation of Mode-A and Mode-B instabilities, whereas for the downstream cylinder, only Mode-A instability occurs in both the Lx = 2.5D and 7.5D cases with alpha = 0. When the cylinders rotate at alpha = 1 for Lx = 7.5D, staggered transition takes place with the appearance of only Mode-B and only Mode-A instabilities in the near-wake regions of the upstream and downstream bodies, respectively. In the case Lx = 2.5D, the wake of the rotating cylinders transitions via Mode-C and Mode-D instabilities for alpha = 0.5, Mode-D and Mode-D' for alpha = 1, and Mode-C and Mode-A for alpha = 2. A sudden decrease in near-wake fluctuations, accompanied by a downward jump in the force coefficients, is observed as the 3-D instability shifts from one mode to another, for alpha >= 1 and Lx = 2.5D. This discontinuity is caused by the formation of a pair of opposite-signed recirculation zones that mitigate each other's effects in the near-wake region.