Forced Oscillation Experiments for VIV of Circular Cylinders and Behaviors of VIV and Lock-In Phenomenon

This paper describes a model test to examine characteristics of vortex-induced vibrations (VIV) of tubing in water, e.g. a marine riser, and gives consideration of VIV characteristics from experimental results. Rigid circular cylinders are used for the model experiment because the authors assume that the model is a part of a marine riser. The test models are suspended into still water through a flat spring, and hence VIV can be found with hydroelasticity in spite of a rigid circular cylinder. This is a forced oscillation test in still water. The circular cylinders for the model test have drafts of 30cm, 60cm and 80cm, and diameter of 5cm and 8cm. The models are oscillated with periods of 0.5s to 4.6s and with about 7cm amplitude of the oscillation. In uniform flow, there is just one velocity of which flow causes the VIV lock-in. However, it is confirmed that there are several cases to experimental models, cases in which the VIV lock-in occurs in oscillatory flow. In addition, it is found that power spectra of VIV can be approximately classified into four patterns according to the number of spectral peaks and its property. Power spectra of VIV behavior from the model tests are compared with two-dimensional CFD computations.