Numerical simulations of flow-induced vibrations of two rigidly coupled cylinders with uneven diameters in the upper transition Reynolds number regime

Abstract

Two degree-of-freedom (2-DoF) flow-induced vibrations (FIV) of two rigidly coupled cylinders are numerically investigated at the Reynolds number of 3.6 × 106. Two- dimensional (2D) Unsteady Reynolds-Averaged Navier–Stokes (URANS) simulations are performed combined with the k − ω SST turbulence model. A low mass-damping system is considered with a mass ratio of 2 and a damping ratio of zero. The diameter ratio is set to 0.25. The influence of the reduced velocity (Ur ), the position angle (α) of the small cylinder relative to the large cylinder and the gap ratio (G/D) between the cylinders on the FIV response of the system are analyzed. The simulations are performed for 2 ≤ Ur ≤ 12. To analyze the effect of α, three values are considered, [α = 0◦, 90◦, 180◦] with a constant value of G/D = 0.1. The effect of G/D is studied for the α = 90◦ configuration in which the G/D = 0.25 and G/D = 0.5 configurations are analyzed in addition to the G/D = 0.1 configuration. It is found that the lock-in regime extends beyond Ur = 12 for the α = 0◦, 180◦ and α = 90◦ at G/D = 0.1 and 0.25 configurations compared with the single cylinder configuration. Extended lock-in range is observed when the small cylinder is placed at α = 90◦, with G/D = 0.1, and at α = 180◦. In addition, for the α = 90◦ configuration, the results obtained for C D, C L and Ay,max/D converge to those of the single cylinder with the increase of G/D. A 2T shedding mode is observed for the single cylinder at 6.5 ≤ Ur ≤ 10 and for the α = 0◦ configuration when Ur ≥ 8. For the α = 90◦ and 180◦ configurations, the wake patterns are irregular. With the increase of G/D, a longer vortex street behind the small cylinder is observed, which creates many vortical structures in the wake of the cylinders.