Effect of Mass Ratio on the Vortex-Induced Vibrations of a Long Tensioned Beam in Shear Flow

The flow past a cylindrical tensioned beam of aspect ratio 200 is predicted by direct numerical simulation of the three-dimensional Navier-Stokes equations. The beam is free to oscillate in inline and crossflow directions and submitted to a linearly sheared oncoming flow. The ratio between high and low inflow velocities is 3.67, with a maximum Reynolds number of 330. Two structure/fluid mass ratios are considered, 6 and 3. Structure vortex-induced vibrations are characterized by mixed standing-traveling wave patterns. A reduction of mass ratio from 6 to 3 leads to purer, more pronounced traveling wave responses and larger amplitude vibrations in both directions. While multifrequency structure vibrations are observed at m = 6, case m = 3 exhibits monofrequency responses. A large zone of synchronization between vortex shedding and structure vibration (lock-in) is identified in the high velocity region. The topology of fluid-structure energy exchanges shows that the flow can excite the structure at lock-in and damps its vibrations in non-lock-in region. Inline/crossflow motion synchronization is monitored. Similar zigzagging patterns of inline/crossflow motion phase difference are put forward for both mass ratios, highlighting a predominant character of counterclockwise orbits in the excitation region.