Numerical investigation on flow-induced vibration of a triangular cylinder at a low Reynolds number

Flow-induced vibration (FIV) of a triangular cylinder is numerically investigated at a Reynolds number of Re = 100. The four-step fractional finite element method is employed to solve the two-dimensional (2D) incompressible Navier-Stokes equations. The cylinder is endowed with a two-degree-of-freedom motion with the reduced mass ratio of M-r = 2. Three typical flow incidence angles, alpha = 0 degrees, 30 degrees and 60 degrees, are examined to identify the effect of incidence angle on the vibration characteristics of the cylinder. For each alpha, computations are conducted in a wide range of reduced velocities 2U(r) <= 18. The numerical results show that at alpha = 0 degrees and 30 degrees, the responses of the cylinder are dominated by vortex-induced vibration which resembles that of a circular cylinder. At alpha = 0 degrees, the peak amplitude of transverse vibration is the smallest among the three investigated alpha, and most of the cylinder motions exhibit a regular figure-eight trajectory. Some single-loop trajectories are observed at alpha = 30 degrees, where the vibration frequency in the in-line direction is always identical to that in the transverse direction. At alpha = 60 degrees, the triangular cylinder undergoes a typical transverse galloping with large amplitude and low frequency, and the vibration trajectories appear to be regular or irregular figure-eight patterns, which are strongly affected by the reduced velocity.