Three-dimensional flow around two circular cylinders in tandem arrangement

The spatial evolutions of vortices and transition to three dimensionality in the wake of two tandem circular cylinders are numerically studied. The virtual body method developed from virtual boundary method is applied to model the no-slip boundary condition of the cylinders. Two different aspects of this problem are considered. Firstly, the spacing ratio L/D is varied from 1.5 to 8 and the Reynolds number is set unchanged at Re = 220. It is shown that three dimensionality appears in the wake for L/D >= 4, whereas the flow wake keeps a two-dimensional state for L/D <= 3.5. The critical spacing for the appearance of three-dimensional instability is deduced at the range of 3.5 < L/D < 4, which is similar to the critical spacing found in two-dimensional case. For L/D = 3.5, a mode of small-scale three-dimensional instability, named mode A, is observed to appear at Re = 250 and persists over the Reynolds number range of 250-270. The three-dimensional results are compared with the two-dimensional flow with the same configuration. (C) 2006 Published by The Japan Society of Fluid Mechanics and Elsevier B.V.