On Mode Competition During VIVs of Flexible SFT's Flexible Cylindrical Body Experiencing Lineally Sheared Current

Most of the floating tunnel and supporting tendon or cable of submerged floating tunnel (SFT) are essentially cylindrical body. Multi-mode vortex-induced vibration (VIV) of these flexible bodies frequently happens in non-uniform flow due to structural flexibility and non-uniform distribution of fluid velocity. One of the challenging issues of multi-mode VIV is about mode competition. And the mechanism and its quantitively measurement of mode competition, in terms of excitation region and length of potentially participating modes along with modal weights, become more complicated than single-mode VIV.In this study, mode competition and multi-mode VIV of flexible body in lineally sheared current is explored based on our numerical simulations which combine finite element approach with a hydrodynamic model so as to carry out nonlinearly simultaneously dynamic response in time domain. Our numerical results show that multi-mode VIV may occur both in non-uniform and uniform fluid profiles. In sheared current, as the towing speed (or the maximum speed) increases, because more modes with higher modal order number participate into the dynamic response, the average RMS displacement just change a little while the average RMS stress progressively rises. Moreover, there are different dominant frequencies distributing along cylinder span, and the length of the first dominant frequency gets smaller due to larger shearing parameter along with more intense competition between all participating modes.