Effect of the spacing ratio on vortex-induced vibration of three rigidly coupled cylinders in an equilateral-triangular arrangement

Equilateral-triangular arrangements of three-cylinder systems and rigid connection measures are common in engineering. In this paper, three cylinders in three equilateral-triangle arrangements with spacing ratios L/D = 2, 4, and 6 at a Reynolds number Re = 200 are studied through a numerical simulation. The characteristics of vortex-induced vibration (VIV), aerodynamic force, and flow field are analyzed. The results show that the variation trends of the VIV responses in relation to the spacing ratio of the three rigidly coupled cylinders vary depending on different cases (case A, case B, and case C). The lock-in regions of d/D = 2 are significantly larger than those for the other spacing ratios. At this spacing ratio, the vibration amplitude in case A is smaller than that in other cases, which may be caused by the bifurcation between the lock-in region. The lock-in region for frequency corresponds to or is slightly less than the vibration region. In the late lock-in region, the vortex shedding frequency is several times higher than the natural vibration frequency for d/D = 2 in cases B and C. The wake changes obviously with the increase in spacing ratio. The wake appears as merging vortexes in the transverse directions at large reduced velocities for d/D = 2. The wake parallel vortex in the lock-in region excites large vibrations for d/D = 4 and 6 in cases B and C.