An experimental study on axial-flow-induced vibration of confined flexible rods with sequenced transverse rib roughness

Experiments were conducted with the aim of studying the dynamic behavior of vertical flexible silicon rods with clamped ends and different sequenced transverse rib roughness along their length. The rods were subjected to confined axial flow. In general, the rods have similar characteristics regarding their spatial oscillation with varying dimensionless flow velocities. Dynamic instability via high-mode flutter developed at a high flow velocity, which is characterized as a clockwise whirling motion of the first to the fourth modes. The transition from lower flutter mode to higher flutter mode was accompanied by a wave that traveled along the rods. Finally, high flow velocities led to a chaotic motion in all the rods. In this article, the onset of rod instability for the rib-roughness rods is considered the beginning of the first-mode clockwise whirling motion, unlike the onset divergence that appears on a smooth- surfaced rod with no ribs. The experimental results revealed that there is a non-negligible dependence between the ribs’ pitch ratio and the critical flow velocity value at the onset of rod instability, which the rod destabilizes by increasing the friction factor according to the rib optimum pitch geometric at p/e ≅ 8.