Flow-induced vibrations of an equilateral triangular prism at various angles of attack

Abstract

Flow-induced vibrations (FIV) of an equilateral triangular prism are numerically studied using the immersed boundary method in a parametric space of α=0°–60° and U∗=U∕fnD=1–20, with α=0°(60°) representing the configuration of one vertex facing upstream (downstream). The Reynolds number based on the effective edge length perpendicular to the incoming flow is Re = 200 and the mass ratio is m∗=4m∕3ρD2=2.0. In above, U is the incoming flow velocity, fn is the natural frequency of the prism, D is the edge length of the prism, m is the prism mass and ρ is the fluid density. Three different vibration regimes, i.e., VIV at α=0°–25°, combined VIV and galloping at α=30°–40°, and galloping at α=45°–60°, are confirmed. In the VIV regime, the vibration amplitude is relatively small and the vibration frequency linearly increases with U∗, signifying the non-locked response. In the combined VIV and galloping regime, the amplitude is determined by the competition of the VIV and galloping modes, with large amplitude appearing when the galloping mode dominates. In the galloping regime, the vibration amplitude monotonically increases with U∗, with a lower-than-unity vibration frequency. Moreover, seven wake modes are observed in the examined α and U∗ ranges: 2S mode, 2S(P) mode, P+S mode, P+S/2S mode, the competition mode (CM), and the galloping modes (miS+niS and mS+nS). In CM, the VIV and galloping modes compete each other. In the galloping modes, the vortices are arranged in a ‘V’ shape. The effects of the streamwise vibration on the transverse responses is significant only in the combined VIV and galloping regime.