Effect of yaw angle on vibration mode transition and wake structure of a near-wall flexible cylinder

Abstract

The multi-mode transition and vortex structures in the vortex-induced vibration (VIV) of a near-wall flexible cylinder under different yaw angles are investigated through three-dimensional direct numerical simulation. Yaw angles α = 0°–60°, gap ratio G/D = 0.8, and Re = 500 are adopted. With the increase in α, the dominated vibration mode decreases from the 6th to 1st mode in the in-line (IL) direction and the 3rd to 2nd mode in the cross-flow (CF) direction. For the IL vibration, no mode transition occurs at α = 0°, whereas frequently mode transition is observed at α > 0°, due to the intermittent participation and spanwise competition of different modes, thus showing an intensified traveling-wave characteristic. For the CF vibration, mode transition is not excited at any α case even with spanwise mode competitions, due to the significant weight of the dominated mode, thus showing a strong standing-wave characteristic. The asymmetrical distributions of vibration displacements and force coefficients are established because of irregular energy transfer along the span. The spanwise vortex tubes at α = 0°–30° are separated into several cells associated with the dominated vibration mode, showing a locally parallel vortex shedding. However, positively yawed and negatively yawed vortex shedding are observed at α = 45° and 60°, respectively. The vortex strengths vary along the cylinder, where large-scale and small-scale vortices are observed at the CF anti-node and node planes, respectively. The independence principle is only valid at α < 15° for predicting the multi-mode vibrations and hydrodynamics, significantly reduced from that of α < 45° in the wall-free case or the mono-mode VIV case.