High Harmonic Forces and Predicted Vibrations from Forced In-line and Cross-flow Cylinder Motions
Author(s)Dahl, Jason; Hover, Franz S.; Triantafyllou, Michael S.
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String-like ocean structures, such as deep water marine risers are susceptible to a condition of dual lock-in, where both the in-line and transverse natural frequencies are excited due to vortex shedding in the wake. This type of excitation can result in dominant, large amplitude third harmonic forces in the cross-flow direction that do not exist in conditions allowing only cross-flow motion. Forced motions of a rigid cylinder in both the in-line and cross-flow directions are performed to obtain coefficients defining the magnitude of third harmonic lift forces for given cylinder motions. In-line motion amplitude, cross-flow amplitude, phase between in-line and cross-flow motion, and reduced velocity are varied, producing a four-dimensional matrix of data points, at a Reynolds number of 8800. In free vibrations, variation of the effective added mass drives the system to specific steady-state oscillations, under lock-in conditions. These free vibration steady-state oscillations are successfully predicted with the new forced oscillation data set, using the simplifying assumption that lock-in occurs in both the in-line and cross-flow directions, and the necessary assumption that the normalized average power over one cycle must be zero for a free vibration. The new data set and our procedure will allow a more accurate strip-theory approach to marine riser VIV analysis and design.
DepartmentMassachusetts Institute of Technology. Department of Mechanical Engineering
Proceedings of the Eighteenth (2008) International Offshore and Polar Engineering Conference
International Society of Offshore and Polar Engineers (ISOPE)
Dahl, Jason, Franz S. Hover, and Michael S. Triantafyllou. "High Harmonic Forces and Predicted Vibrations from Forced In-line and Cross-flow Cylinder Motions." Proceedings of the Eighteenth (2008) International Offshore and Polar Engineering Conference, Vancouver, BC, Canada, July 6-11, 2008. © 2008 by The International Society of Offshore and Polar Engineers (ISOPE)
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