Flow-induced oscillations of two cylinders in tandem and staggered arrangements

A stabilized finite element formulation is employed to study flow-induced oscillations of a pair of equal-sized cylinders in tandem and staggered arrangement placed in uniform incompressible flow. Computations, restricted to 2-D, are carried out for Reynolds numbers 100 for various values of the structural frequency of the oscillator. The cylinders are separated by 5.5 times the cylinder diameter in the streamwise direction. For the staggered arrangement, the cross-flow spacing between the two cylinders is 0.7 times the cylinder diameter. In this arrangement, the downstream cylinder lies in the wake of the upstream one and therefore experiences an unsteady in-flow. Since the spacing between the two cylinders is beyond the critical value for proximity interference, it is expected that the upstream cylinder behaves like an isolated single cylinder, while the downstream one experiences wake-induced flutter. The Re=100 flow leads to a very organized wake and large amplitude motion is observed for the downstream cylinder. The trajectory of the upstream cylinder resembles a figure-of-eight. The downstream cylinder shows a similar behaviour for the tandem arrangement. However, for the staggered arrangement, the trajectory of the rear cylinder resembles a tilted oval. Soft lock-in is observed in almost all the cases