Cross-flow Vortex-induced Vibrations Research Articles

Reduction of VIV using suppression devices—An empirical approach

Helical strakes are known to reduce and even eliminate the oscillation amplitude of vortex-induced vibrations (VIV). This reduction will increase the fatigue life. The optimum length and position of the helical strakes for a given riser will vary with the current profile. The purpose of the present paper is to describe how data from VIV experiments with suppressing devices like fairings and strakes can be implemented into a theoretical VIV model. The computer program is based on an empirical model for calculation of VIV. Suppression devices can be accounted for by using user-defined data for hydrodynamic coefficients, i.e. lift and damping coefficients, for the selected segments. The effect of strakes on fatigue damage due to cross flow VIV is illustrated for a vertical riser exposed to sheared and uniform current. Comparison of measured and calculated fatigue life is performed for a model riser equipped with helical strakes. A systematic study of length of a section with strakes for a set of current profiles is done and the results are also presented.

The relationship between in-line and cross-flow vortex-induced vibration of cylinders

Cable strumming experiments were conducted at Castine, Maine in 1981 and from an icebreaker in 1983. The purpose was to study the vibration characteristics of long flexible cylinders subjected to vortex-induced oscillation. Particular emphasis was placed on the investigation of the relationship between in-line and cross-flow vibration. Under non-lock-in, random vibration conditions, linear spectral analysis indicated that in-line and cross-flow response were linearly independent of each other, while the results of modal analysis showed that the moving average vibration energies in these two directions were strongly related. A higher order spectral analysis was performed to demonstrate a non-linear correlation between in-line and cross-flow vibration of flexible cylinders excited by vortex shedding in both uniform flow and sheared flow conditions. The results of bispectral analysis demonstrated the existence of a quadratic relationship between in-line and cross-flow motion under both lock-in and non-lock-in conditions. The well-known frequency doubling phenomena in in-line response was proven to be the result of such a quadratic correlation.