Cross Flow and In-Line Damping Measurements From Forced Excitations of a Flexible Cylinder in a Uniform Flow
Tensioned flexible cylindrical structures are important in many ocean engineering applications such as moorings for buoys and platforms, marine risers and towing cables. Modeling the vibration of these structures is complicated because these are complex three-dimensional, unsymmetrical, fluid structure interaction problems. Damping is an important, but poorly understood, component of the response prediction models developed for modeling such systems. In particular, there is a scarcity of good data on damping of flexible cylinders vibrating in uniform and non-uniform external flow. This is, in part, due to the difficulty of measuring fluid damping on a vibrating cylinder in a flow. Results are presented here which address some of these limitations. Forced vibration tests were performed on two 13 ft long tensioned flexible cylinders (an ABS tube and a steel wire) in a current tank to determine in air and still water damping as well as cross flow and in-line damping in a uniform flow. The experimental methodology is described and results are presented for a range of reduced velocities. The results show an increase in fluid damping with increased reduced velocities for small amplitudes of vibration.