Flow-induced vibrations (FIVs) of two elastically mounted circular cylinders in staggered arrangement were experimentally investigated. The Reynolds number range for all experiments (2.5 × 104 < Re < 1.2 × 105) was in the transition in shear layer 3 (TrSL3) flow regime. The oscillator parameters selected were: mass ratio m* = 1.343 (ratio of oscillating mass to displaced fluid mass), spring stiffness K = 250 N/m, and damping ratio ζ = 0.02. The experiments were conducted in the low turbulence free surface water (LTFSW) channel in the MRELab of the University of Michigan. A closed-loop, virtual spring–damper system (Vck) was used to facilitate quick and accurate parameter setting. Based on the characteristics of the displacement response, five vibration patterns were identified and their corresponding regions in the parametric plane of the in-flow spacing (1.57 < L/D < 4.57) and transverse cylinder spacing (0 < T/D < 2) were defined. The hydrodynamic forces and frequency characteristics of the vibration response are also discussed.

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