Flow-induced vibrations of two mechanically coupled pivoted circular cylinders: Characteristics of vibration

Abstract

This paper presents the results of an experimental investigation on the vibration mechanisms of two connected circular cylinders that are free to rotate around a pivot in different arrangements including both cylinders on the downstream, both on the upstream and a cylinder on each side of the pivot point. The Reynolds number is varied during the test to find the maximum possible displacement amplitude for each configuration. Four main mechanisms of vibration are identified. The cylinders experience galloping if both located on the upstream of the pivot and the gap between them is zero. Vortex excitation (VE) is observed in two configurations and referred to as VEfSt and VEfN. VEfSt occurs when both cylinders are located on the downstream of the pivot while the gap is zero. The frequency responses lock into the Strouhal frequency in this case. VEfN occurs when the center of gravity (cg) is on the pivot and the gap ratio (G=gap∕cylinderdiameter) between the cylinders is G > 3.9. In this case, the frequency response locks into the natural frequency of the system. If one cylinder is located on the center of the rotation and the other cylinder is on the downstream, wake-induced vibration (WIV) takes place. While for G < 1.4 the response is a typical wake galloping, for G > 1.4 two vibration modes are recognizable as ‘combined vortex resonance and galloping’. For all configurations with G> 0, gap-switching-induced vibration (GSIV) is observed especially for 1.9 < G < 2.4. However, GSIV is the dominant mechanism of vibration if cg is on the pivot point. In cases where cg is not close to the pivot, the drag force may enhance the vibration if the Reynolds number is not large enough to suppress the motion.