Flow-induced vibration of tandem circular cylinders with selective roughness: Effect of spacing, damping and stiffness

Abstract

Abstract Flow Induced Vibrations (FIV), and particularly Vortex Induced Vibrations (VIV) and galloping, of two locally rough, rigid, circular cylinders in tandem are studied by varying the stiffness of the supporting springs, the linear viscous damping, and the cylinder spacing. The test parameters are: mass ratio set to 1.34, center-to-center spacing of 1.57, 2.0 and 2.57 diameters; spring-stiffness 400 N/m ∼ 1200 N/m, damping-ratio 0 . 02 ζ 0 . 26 , and Reynolds number 30,000 ⩽ Re ⩽ 120,000, which falls in the TrSL3 flow regime. The virtual spring-damping system Vck developed in the Marine Renewable Energy Laboratory (MRELab) enables embedded computer controlled change of viscous-damping and spring-stiffness for fast and mathematically correct oscillator realization, without including the hydrodynamic force in the closed control loop. Experimental measurements for the oscillatory response of both cylinders, are studied to reveal the interaction of upstream and downstream cylinders. All the experiments were conducted in the Low Turbulence Free Surface Water (LTFSW) Channel of the MRELab of the University of Michigan. The main findings are: (1) In the range of spacing tested, for all the values of K and ζ , the upstream cylinder’s amplitude response is enhanced due to the presence of the downstream cylinder. This effect is very strong in galloping. (2) With the exception of the initial branch of VIV, the downstream cylinder’s amplitude response is also enhanced but with higher standard deviation in two tandem oscillating cylinders. (3) Three oscillation patterns are identified in galloping: In-phase, out-of-phase, and alternating between the two in a given experiment. (4) Four interactive zones are identified.