Numerical investigation of flow-induced torsional vibration and galloping of a triangular prism

Torsional vibration and galloping of a triangular prism (TP) in steady flow is investigated numerically at mass ratio 2.5, low Reynolds number 150, three angles of attack, and reduced velocities up to 40. The vibration of the TP is torsional galloping characterised by monotonic increase of the angular amplitude with the increase of reduced velocity. The angular displacement and amplitude are non-dimensionalised by 2π/3, which is the geometrical period in the rotation direction. The response of the TP is well correlated to the direction of the fluid moment coefficient on a stationary TP with a constant rotation angle. The rotation angles are consistently divided into excitation and damping ranges where the directions of the mean fluid moment of a stationary TP and the rotational angle are the same, and opposite to each other, respectively. When the reduced velocity is less than a critical value, the vibration amplitude falls into a damping range, and it increases with the increase of reduced velocity. When the reduced velocity is greater than this critical value, the galloping of the TP is strong and very aperiodic. The vibration amplitude switches very frequently between multiple amplitudes. Every identified amplitude is very close to the upper boundary of a damping range. Multiple-amplitude torsional galloping is a distinct feature that was not found in transverse galloping in the crossflow direction.