Leijian Song

Hydrodynamic Forces and Coefficients on Flexible Risers Undergoing Vortex-Induced Vibrations in Uniform Flow

AbstractIn this study, the distribution and variation of the hydrodynamic forces and hydrodynamic coefficients in both inline (IL) and cross-flow (CF) directions on flexible risers experiencing vortex-induced vibration (VIV) in a uniform flow were investigated. The hydrodynamic forces in the CF and IL directions were theoretically calculated by the Euler-Bernoulli beam vibration equation. With the least-squares method, the hydrodynamic coefficients in both CF and IL directions were further investigated. The results indicate that the hydrodynamic force and vibration displacement almost simultaneously reach their maximums. The excitation coefficients obtained in this paper do not always agree with those obtained by the forced-oscillation tests: some excitation coefficients are even negative within the usual defined exciting nondimensional frequency regime and are related with not only the nondimensional frequency and amplitude but also phase angles of the CF and IL displacements. The added-mass coefficient ...

Experimental Investigation on the Suppression Device of VIV of a Flexible Riser

Experimental investigations were conducted on a flexible riser with and without helical strakes. A uniform current was obtained by towing a riser model in a tank, and the vortex-induced vibration (VIV) suppression of strakes with different heights and pitches was studied. The results of the bare riser show that the characteristics of the synchronization of the VIV for a flexible riser have many orders, and the excited mode jumps from one to another abruptly. During the high order synchronization regime, the VIV response decreases with the increased order of the synchronization. The experimental results also indicate that the response characteristics of a bare riser can be quite distinct from those of a riser with helical strakes, and the suppression performance depends on the geometry of the helical strakes. The fatigue damage in the CF direction is of the same order as that in the IL direction for the bare riser. However, for the riser fitted with helical strakes, the fatigue damage in the CF direction is much smaller than that in the IL direction. The experimental results also confirmed that strake height has a greater influence on the VIV response than the strake pitch, and the drag exerted on the riser increases with strake pitch and height.Copyright

Nonlinear Hydrodynamics of a Floating Cylinder in Oscillatory Flow Alone and Combined with a Current

AbstractThis paper experimentally investigates the nonlinear hydrodynamic forces on a semisubmerged floating collar under full-scale Reynolds number (R) and Keulegan-Carpenter number (KC) oscillatory flow alone and combined with a current. The collar was forced to oscillate harmonically in the direction of the current within a wide range of oscillation amplitudes and frequencies. The first five orders of the horizontal hydrodynamic forces are obtained using Fourier transformation techniques. The influence of the KC, the frequency parameter (β), and the reduced velocity (Vr) on the high-order components of the hydrodynamic force were investigated. The results show that, under oscillatory flows, the first five harmonics of the hydrodynamic force coefficients increase with an increase of KC and β. As the reduced velocity (Vr) increases, the first- and second-order components are greatly affected. These first two order components contribute greatly to the overall hydrodynamic force in the combined flow condit...