J. M. (2017). Observed mode shape effects on the vortex-induced vibration of bending dominated flexible cylinders simply supported at both ends. Abstract 8The structural mode excitation of bending-dominated flexible cylinders undergoing vortex-induced vibrations was investigated using multivariate analysis of the excited empirical modes. The response of the bending-dominated cylinders was compared with the response of a tension-dominated cylinder using the same analysis technique. Experiments were conducted in a recirculating flow channel with a uniform free stream with Reynolds numbers between 650 and 5500. Three bending-dominated cylinders were tested with varying stiffness in the cross-flow and in-line directions of the cylinder in order to produce varying structural mode shapes associated with a fixed 2:1 (in-line:cross-flow) natural frequency ratio. A fourth cylinder with natural frequency characteristics determined through applied axial tension was also tested. The spanwise in-line and cross-flow responses of the flexible cylinders were measured through motion tracking with high-speed cameras. Global smooth-orthogonal decomposition was applied to the spatio-temporal response for mode identification. Measured responses are compared with the analytic response of a beam subjected to a uniform periodic loading. Both the analytic and experimental results show that for excitation of low mode numbers, the cylinder is unlikely to oscillate with an even mode shape in the in-line direction due to the symmetric drag loading, even when the system is tuned to have an even mode at the expected frequency of vortex shedding. In addition, no mode shape changes were observed in the in-line direction unless a mode change occurs in the cross-flow direction, implying that the in-line response is a forced response dependent on the cross-flow response. An even mode oscillation (i.e. second mode) in the in-line direction is observed to be excited in the tensioned cylinder, however this is only observed in a hysteretic response region, resulting in a pedaling mode response. The results confirm observations from previous field and laboratory experiments, while demonstrating how structural mode shape can affect vortex-induced vibrations.

No abstract

No abstract

No abstract