Date of Award
Master of Science in Ocean Engineering
Jason M. Dahl
Vortex-induced vibration (VIV) is a key issue in ocean engineering design. Numerous deep water structures can experience VIV when exposed to marine currents due to the shedding of vortices downstream of the structure, causing vibration in both the in-line and cross flow directions. This thesis presents an experimental approach that models the motions association with the excitation of a marine riser by considering the combined in-line and cross flow motions excitation of a flexible cylinder.
Under the lock-in phenomena, where the vortex shedding frequency approaches the effective natural frequency, large motion amplitudes can be observed. The vibration of a flexible structure due to VIV is related to the natural frequency of the structure (time dependence) and mode shapes (spatial frequency dependent) associated with the natural frequency. Beams, designed to become excited at the first, second and third mode in the in-line direction, were molded into a flexible cylinder with a length of 43.18 cm. High speed cameras captured image sequences over the period of 12 seconds at 250 frames per second. Using motion tracking software, both the in-line and transverse amplitudes can be displayed by plotting orbital patterns. The assumption that mode shape can influence the motions of a flexible cylinder undergoing VIV is examined.
The fundamental theme throughout this work has been the effect of three different mode shapes in the in-line direction on a flexible cylinder in a free stream. The design of a symmetric mode (first and third) in the in-line direction produced regular and periodic patterns along the entire length of the cylinder for each flow speed tested. However the observations of the cylinder with the 1-2 mode ratio phenomena produced a series of unique trajectories including figure-eight, crescent, and teardrop orbital trajectories. It is shown that for the low modenumber beams tested there are three primary responses possible for the cylinder when trying to excite second mode in the in-line direction and the first mode in the cross-flow direction. This study shows that when a symmetric mode shape is designed, regular patterns are produced. When an asymmetric mode shape is designed to be excited, three unique patters are seen (a) repeatable patterns with teardrop shape,(b) repeatable patterns with figure-eight shape, and (c) random unrepeatable patterns. It was observed that the first mode was being excited in the in the in-line and cross-flow directions and the effective frequencies would remain close to each other. During the transition period between orbital patterns, a 1:2 ratio as seen in the first and third cylinders was witnessed. Random and unrepeatable patterns only occurred when the second mode was excited in the cross-flow direction.
Zimmer, Harrison, "MODAL EXCITATION OF A FLEXIBLE CYLINDER UNDERGOING VORTEX-INDUCED VIBRATIONS" (2014). Open Access Master's Theses. Paper 323.