Axial-flow-induced structural vibration in a 5×5 cylinder cluster

Abstract

This work aims to experimentally study the incident turbulence intensity T_u effect on the flow-induced vibration of an elastic cylinder positioned at the center of a 9- or 25-cylinder cluster subjected to an axial flow. T_u is examined at 0.71% – 0.80% and 2.30% – 2.91%. The pitch-to-diameter ratio P* is 1.36 ∼ 1.64. Lateral vibrations along two orthogonal directions are simultaneously measured with the interstitial flow of the cylinder bundle. Two mechanisms are identified behind the elastic-cylinder vibration at low and high T_u. One is the presence of a varying velocity gradient within the cylinder bundle, and the other is incident flow fluctuations. At low T_u (0.71% – 0.80%), the root-mean-square vibration amplitude A_rms* of the elastic cylinder exhibits strong dependence on the P* and cylinder number N. Increasing velocity gradient with decreasing P* or increasing N plays a key role in destabilizing the shear layers surrounding the elastic cylinder, inducing eddies to separate from the cylinder-wall and actively interact with those near the neighboring cylinder. Therefore, the near-wall velocity fluctuation u_rms* and A_rms* are increased. At high T_u (2.3% – 2.91%), A_rms* is weakly dependent on P* compared with that at low T_u. It is found that the shear-layer instability surrounding the elastic cylinder is mainly intensified by the incident flow fluctuations with a higher T_u, accounting for the enhanced eddy activities, while the velocity-gradient effect associated with a change in P* is of less importance.