Vortex-induced vibration of two rigidly coupled cylinders in staggered arrangement at low Reynolds number

The flow characteristics of two rigidly coupled staggered circular cylinders undergoing vortex-induced vibration are investigated numerically at a fixed Reynolds number (Re=150) in the range of reduced velocity (U*) of 3.0–15.0. The staggered cases are compared to the corresponding tandem cases at a constant mass ratio (m*) of 2.0. The center-to-center distance varies in the streamwise and transverse direction from 2.0 to 4.0. The tandem cases exhibit a greater maximum vibration amplitude and a broader region of significant amplitude than their corresponding staggered cases, except at L=2.0. In comparison with tandem cases, staggered cases display a broader synchronization region. The downstream cylinder primarily influences the wake characteristics, which significantly affect overall lift and phase. Three response branches (initial, upper, and lower) are observed for all the arrangements. For the tandem case where L=2.0, three vortex modes are identified: overshoot mode at transition from initial to lower branch, alternate reattachment (AR) at initial branch, and continuous reattachment at lower branch. In contrast at L=4.0, only AR mode is found. Both AR and co-shedding modes are present at L=3.0. The staggered cases exhibit either asymmetric vortices with the downstream cylinder shear layer being more elongated than that of the upstream cylinder, or symmetric in-phase 2S vortex streets.