**Flow-induced vibration of two mechanically coupled pivoted circular**

cylinders

Experiments were carried out with 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 gap between the two cylinders is varied from 0 to 4.9D (D being the diameter of the cylinder) and the center of gravity (*cg*) is varied from -2.5D to +2.5D from the pivot point. The hydrodynamic forces on the cylinders are measured with a load cell and the surrounding flow is measured simultaneously using particle image velocimetry (PIV). The Reynolds number is varied in the range 500<Re<2.4x10^{4} during the test to find the maximum possible displacement amplitude for each configuration. The spectral analysis of the measured velocity field was also performed to have information about the vortex shedding frequency of the unsteady flow in the gap region and cylinder wake. Four mechanisms of vibration are identified. The cylinders experience flutter if both located on the upstream of the pivot and the gap between them is zero. Vortex excitation is observed when both cylinders are located on the downstream of the pivot while the gap is zero or when the center of gravity is on the pivot and the gap between the cylinders exceeds 3.9D. If one cylinder is located on the pivot and the other cylinder is on the downstream, wake induced vibration (WIV) takes place. It is observed that gap switching induced vibration (GSIV) is the dominant mechanism of vibration if *cg* is on the pivot point and the gap is smaller than 2.5D. However, in cases where *cg* is not close to the pivot, the drag and lift force have a strong effect on the vibration. The setup and schematics are shown in Figure 2.1 and 2.2:

Figure 2.4 shows all possible responses in different configurations. The responses are categorized based on amplitude and frequency of the vibration:

### PIV results

- Both cylinders on the downstream side of the pivot:

- Both cylinders on the upstream side of the pivot:

- Each cylinder on an equal distance from the pivot point with large gap:

- One cylinder on the pivot point (stationary) and another at the upstream:

- A cylinder on the pivot point(stationary) and another at the downstream:

- Each cylinder on an equal distance from the pivot point with small gap: