Flow-induced vibration of two mechanically coupled pivoted circular cylinders: Vorticity dynamics

Abstract

Herein, 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 while the other cylinder is at 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 of 500<Re<2.4×104 during the test to find the maximum possible displacement amplitude for each configuration. Then, an spectral analysis is performed on the measured velocity field to have information about the vortex shedding frequency of the unsteady flow in the gap region and cylinder wake. When the gap between the cylinders (g) is zero, the cylinders behave like a single bluff body (SBB) in cross-flow. However, depending on the arrangement, the wake patterns are different. If both cylinders are at the downstream side of the pivot, the cylinders act as a single body with an extended surface and alternating vortexes shed behind the cylinders in each cycle forming a single vortex street on the wake side (SBBV). Sliding both cylinders to the upstream side shifts the vibration mechanism to the single body galloping region (SBBG). The shear layer behind the two cylinders is stretched along the fluid flow and vortexes are formed at a greater distance downstream comparing to the SBBV.Having one cylinder on the pivot changes the vibration to a typical wake induced vibration if the other cylinder is at the downstream. If the other cylinder is at the upstream, the vibration is similar to a single cylinder system but the cylinder on the pivot disturbs the wake and changes the stagnation point on the upstream cylinder.When both cylinders are at an equal distance from the pivot point, three major flow fields are observed. For larger gaps (g>3.9D), there is no reattachment of the shear layers downstream and two synchronized vortex streets form behind the two cylinders. By decreasing the gap, the downstream cylinder starts to interact with the vortexes shed from the upstream cylinder. If 2.5D < g < 3.9D, the gap flow pattern is split-gap where the vortexes split inside the gap before reaching the downstream cylinder. As the gap decreases, vortex excitation (VE) is less effective and gap-switching induced vibration (GSIV) dominates the vibration mechanism. Reducing the gap to G<0.9 turns the split-gap flow to an entire-gap flow pattern that forms a sharp velocity gradient inside the gap region and increases the total lift force dramatically on the system which maintains a larger vibration amplitude.