Flow-induced vibrations of a wake cylinder at a low mass-damping ratio

This study experimentally investigates flow-induced vibrations of an elastically mounted cylinder (diameter D) in a wake, at a low mass-damping ratio of m∗ζ = 0.163. This work focuses on the influence of reduced velocity U_r (= 1.51 − 16.82), streamwise spacing ratio L/D (= 2.0 − 6.0), and transverse spacing ratio T/D (= 0 − 3.0) between the cylinders on vibration responses, frequency responses, forces, added mass, and phase lag between lift and displacement. Five distinct vibration branches − initial branch (IB), upper branch (UB), lower branch (LB), desynchronized branch (DB), and galloping branch (GB) − are identified and discussed with respect to T/D, L/D, and U_r space. The cylinder undergoes combined vortex-induced vibration (VIV) and galloping for T/D < 0.37 − 0.75 (depending on L/D). At 0.37 − 0.75 < T/D < 1.75, where two wakes of different sizes are formed, galloping is absent, leaving strong VIV only in the vibrations. The added mass associated with the cylinder oscillation declines progressively from IB to GB with a rapid decrease in IB. The added mass is positive for IB and UB, but negative for LB and GB. The connection between vibration responses and flow structures around fixed cylinders is established.