Dynamic response of elliptical cylinders undergoing transverse flow-induced vibration

Abstract

While the flow-induced vibration (FIV) of a circular cylinder has been characterised in considerable detail, the FIV of elliptical cylinders has not received similar attention. This study investigates the dynamic response of elastically-mounted elliptical cylinders with various cross-sectional aspect ratios (or elliptical ratios) in a free stream. The elliptical ratio is defined by ϵ=b∕a, where a and b are the streamwise and cross-flow dimensions, respectively, of the cross section of a cylinder placed at zero incidence angle. The elliptical ratios tested were in the restricted range of 0.67⩽ϵ⩽1.50, which was designed to achieve some but not too large a geometric deviation from a circular cylinder. The fluid–structure system was modelled using a low-friction air-bearing rig in conjunction with a free-surface recirculating water channel facility. This experimental set-up yielded very low mass and damping ratios. The FIV response was characterised as a function of reduced flow velocity. The results showed that the ellipses exhibited different response regimes as ϵ varied. Surprisingly, for the lowest elliptical ratio ϵ=0.67, there existed two separated lock-in regimes. However, for ϵ⩾0.80, lock-in occurred over only a single reduced velocity range that was characterised by different response regimes. As ϵ was increased above unity, lock-in tended to occur at considerably lower reduced velocities than for the circular cylinder case. Moreover, the peak vibration amplitude increased with the elliptical ratio. This means that the body vibration is enhanced, rather than attenuated, as the afterbody is reduced for an ellipse.