Hydrodynamic Force Identification from Stochastic Vortex Induced Vibration Experiments with Slender Beams
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- Institutt for marin teknikk 
As offshore industry goes to deeper waters, vortex induced vibrations (VIV) becomes an increasingly challenging factor in riser design. However, VIV is still not fully understood. This thesis studies the hydrodynamic forces and motions of a flexible cylinder subjected to VIV in order to improve the existing empirical VIV prediction tools. The first part of the thesis presents a variety of methods to identify hydrodynamic forces by analyzing measured accelerations and/or bending strains from VIV experiments with flexible beams. Such analyses are needed since direct force measurements on slender beams are not feasible. Once the hydrodynamic forces are found, force coefficients for use in empirical models for predicting VIV can be calculated. This is discussed in the second part of the thesis. Results from analyses with the new set of coefficients show a significant improvement compared to the use of coefficients found from other sources. The last part of the thesis focuses on the phase angle between IL and CF motions at discrete positions on a flexible beam. This parameter is essential for the understanding of interaction between IL and CF response. It is shown that the phase angle is controlled by certain parameters like reduced velocity, how the actual mode is influenced by bending stiffness and tension, and also impact from standing and traveling waves on the global response.