An experimental investigation of flow-induced vibration of high-side-ratio rectangular cylinders

Abstract

This study reports an experimental investigation on the flow-induced vibration (FIV) of elastically mounted rectangular cylinders with high-side-ratio in free-stream flow. The side ratio (σ), defined as the ratio of the cross-flow side width (h) to the streamwise side width (b) of the cylinder, namely σ=h∕b, was varied from 2.0 to 5.0. The fluid–structure system was modelled using a low-friction air-bearing system in conjunction with a free-surface water channel facility. The structural vibration was characterised over the reduced velocity range of 2⩽U∗=U∕(fnwh)⩽16, where U is the free stream velocity and fnw is the natural frequency of the system in quiescent water. The corresponding Reynolds number varied in the range of 940⩽Re=Uh∕ν⩽8200, where ν is the fluid kinematic viscosity. The mass ratio, defined as the ratio of the oscillating mass to the displaced fluid mass, varied from 6.56 to 12.18, depending on the cylinder models. It was found that the vibration response was dominated by vortex-induced vibration (VIV) response for U∗≲8.4 for all the σ cases tested. In the VIV lock-in regime, the local peak amplitude response was found to increase with σ (i.e. from A∕h≃1.05 for σ=2.0 to A∕h≃1.69 for σ=5.0). Interestingly, beyond the VIV lock-in regime, while a galloping response was observed for the cases of σ⩽4.0, where the vibration amplitude increased linearly with U∗, the σ=5.0 case exhibited a bounded galloping regime with the amplitude increasing up to A∕h≃1.44 at U∗=10.8, prior to an abrupt drop to A∕h≈0.35 of a desynchronisation regime for higher U∗ values. The results suggest that this unexpected collapse of galloping response could be due to exceeding the relative incidence angle threshold for possible galloping.