The dynamics of offshore structures resemble to the dynamics of long flexible cylinders. However, the complexity of flexible body interactions have pushed industry to rely on the dynamics of rigid cylinders ignoring the effect of spatial response of such structures completely. In this work, we aim to clarify the dynamics of a flexible cylinder that undergoes vortex-induced vibrations in a systematically designed experiment to see the effect of structural mode shape on the excitations. In the experiments, we test three bending dominated flexible cylinders and compare the results with a tension dominated flexible cylinder under uniform flow conditions. We study the nonlinear modal interactions by means of analyzing the spatial response using a multivariate analysis technique called generalized smooth orthogonal decomposition. Using this technique, we show that a flexible cylinder is unable to oscillate with an even mode excitation in the in-line direction, although it has second mode frequency characteristics. It is also shown that a mode switch in the in-line direction is highly dependent on the cross-flow motion where for a possible mode switch in the in-line direction, a mode switch in the cross-flow direction is necessary. In other words, a cylinder cannot oscillate with higher modes in the in-line direction keeping the cross-flow shape constant. This shows the inherent coupled response of in-line and cross-flow motions.
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