NUMERICAL SIMULATION OF VORTEX-INDUCED VIBRATIONS ON A FLEXIBLE RISER IN UNIFORM CURRENTS

A numerical model in a quasi-three-dimensional fashion is developed in this paper to simulate vortex-induced vibration of a flexible riser (the aspect ratio = 250, mass ratio, m*=2.9, damping ratio, zeta=0.01) in uniform current U(infinity)=[0.06, 0.80] m/s, (Reynolds number, Re=[0.01, 1.28] x 10(4)). Finite element method are and Finite volume method are applied in the structural and fluid domains respectively. Effects of fluid-structure interaction (FSI) are reckoned in by making use of kinematic equivalence of the relative flow between fluid and the body in inertial and non-inertial frames of reference. It is found transeverse motion and streamwise motion are strongly coupled, they have same changing trend at the same reduced velocity range, the upper branch appears in the range V(rn)=U(infinity)/f(n)D approximate to 5-7 for the generated nth mode, whilst the lock-in remains in the range V(rn)approximate to 3-10, the phase angles decrease from about 90 degrees in the initial branch to less than 45 degrees in the lower branch. The RMS and envelop values of cross-flow displacements are 4 similar to 6 times those of in-line, maximum amplitudes of about 1.2 diameters at cross-flow and 0.25 diameters at in-line have been observed. Standing wave response was observed as V(rl)=6, the in-line response even contains the first and the second modes at the same time. The strouhal number, Si is about 0.17 in the present cases.