Galloping of circular cylinder induced by downstream strip-plate in cruciform arrangement

Abstract

It has been reported that two longitudinal vortices are periodically shed from a cruciform two-cylinder system in a uniform flow. These depend on the gap, s, between the two cylinders and can induce a vibration called the “longitudinal vortex-induced vibration”. In previous studies, the authors showed that stationary longitudinal vortices are formed by a cruciform circular-cylinder/strip-plate (CC/SP) system when the upstream cylinder is moving along the downstream strip plate, and that one of these longitudinal vortices, the necklace vortex (NV), generates a lift force that acts on the cylinder in the direction of the motion. The “stationary” position of the NV is believed to make the system unstable, leading to fluid-elastic instability vibration. This study measured the fluid force acting on the rotating upstream circular cylinder blade in a wind turbine-type CC/SP system, and the vibration behavior of a pendulum-type CC/SP system was investigated through wind-tunnel experiments. The lift force coefficient, together with the ranges for the gap ratio (s/d) and velocity ratio (λ=V/U) needed to achieve stationary positions, were obtained (d: diameter of the upstream circular cylinder, U: flow velocity, V: rotational velocity of cylinder). It was confirmed that a cross-flow vibration of the cylinder in the pendulum-type CC/SP system was induced, and an analytical method to predict the vibration amplitude was proposed based on the quasi-steady hypothesis. The vibration amplitude of the pendulum device was well predicted by the method using the lift coefficient, obtained through the wind turbine-type CC/SP experiment. In conclusion it was verified that the vibration was caused by the stationary formation of the NV. It was classified as galloping, although it did not occur for an isolated circular cylinder because of its axisymmetric cross-sectional geometry.