Flow-induced vibration of a trapezoidal cylinder placed at typical flow orientations

Abstract

This paper reports the numerical results of the flow-induced vibration (FIV) of a trapezoidal cylinder with base length ratio of 0.3 placed at various flow orientations. Three typical attack angles (θ) of 0° (shorter base facing flow), 90° and 180° are examined in the computations that carried out for a reduced velocity range of Ur=2–20 at a low Reynolds number of 150. The vortex-induced vibration (VIV)-desynchronization regime is observed in the trapezoidal cylinders with θ=0° and θ=90°. In contrast, instead of the VIV lower and desynchronization branches, galloping emerges at Ur > 4 for the trapezoidal cylinder at θ=180° with a continuous growth of response amplitude, presenting the full interaction between VIV and galloping. The response regime is associated with the energy transferred and the vortex shedding mode as well as the added mass coefficient and the phase lag between the cross-flow displacement and the lift coefficient. In the VIV branch, more energy is extracted from the ambient flow by the trapezoidal cylinder in comparison with the square cylinder, and it grows with the attack angle. The transferred energy is further increased when the cylinder undergoes galloping response, while it is significantly reduced when the response enters into the desynchronization branch. Although the typical 2S vortex shedding mode is observed at θ=0°, the transition from the primary vortex street to the secondary vortex street emerges in the wake. The P* and P*+S modes occur in the VIV branch of 90°-and 180°-oriented trapezoidal cylinders, respectively, where P* denotes the reborn pair of vortices that is merged from a single vortex and a pair of ones. In the galloping branch, P*+S, 2P+2S, 4P+2S and 6P+2S modes successively occur with increasing Ur, due mainly to the elongation of oscillation journey.