Numerical Simulation Of Mass Ratio’s Effect On Vortex-induced Vibrations Of Slender Circular Cylinder

With the decrease of oil and gas resources on the land, the development of thedeep water oil resources becomes a consensus among all countries in the world. Withthe increase of the depth of the water, the displacement of the deep water riser isbecoming bigger and bigger and the effect of fluid-solid coupling is becoming moreand more intense. Therefore, the vortex-induced vibration research of the deep waterrisers is being paid more attention to. In order to study the mass ratio’s effect on thecylinders’ vortex-induced vibration, the two-degrees-of-freedom vortex-inducedvibrations of the cylinders with large and small mass ratio are simulated with thesoftware ANSYS-CFX. It can be found that at the same velocity and reduced velocity,the vibration frequency, the amplitude and the motion trace are completely different,the main conclusions are as follows:When the reduced velocity is from1to10, the frequency of the drag force on thecylinder with small mass ratio is twice of that of the lift force, the frequency of thein-line VIV is also twice of that of the cross-flow VIV. The in-line VIV frequency ofthe cylinder is equal to that of the drag force and the cross-flow VIV frequency of thecylinder is equal to that of the lift force. This is consist with the traditional theorywhich is the response frequency is equal to the load frequency. The VIV trace of thecylinder is mainly a figure “8” under the different reduced velocities. When thereduced velocity is1,2and3, the mode of vortex shedding is mainly “2S”; when thereduced velocity is4,5,6and7, the mode of vortex shedding is mainly “2P”; whenthe reduced velocity is8,9and10, we do not find the clear mode of vortex shedding.When the reduced velocity is1,2and3，the frequency of the drag force on thecylinder with large mass ratio is twice of that of the lift force, the frequency of thein-line VIV is also twice of that of the cross-flow VIV. The in-line VIV frequency ofthe cylinder is equal to that of the drag force and the cross-flow VIV frequency of thecylinder is equal to that of the lift force. The VIV trace of the cylinder is mainly a figure “8”. When the reduced velocity is5, the frequency of the in-line VIV is equalto that of the cross-flow VIV, which is equal to that of the lift force. This is obviouslydifferent from the traditional theory. The VIV trace of the cylinder is the oval. Whenthe reduced velocity is4,6and7, the frequency of the in-line VIV isn’t not only equalto that of the cross-flow VIV, but also isn’t twice of that of the cross-flow VIV. Butthe cross-flow VIV frequency of the cylinder is equal to that of the lift force. The VIVtrace of the cylinder is the mixture of the “8” and the oval which is very chaotic.When the reduced velocity is8,9and10, the frequency of the in-line VIV is the sameas that of the cross-flow VIV which is close to the inherent frequency of the cylinderand is different from the frequency of the drag force or lift force. The VIV trace of thecylinder is the oval. When the reduced velocity is from1to10, the mode of vortexshedding is mainly “2S”.The innovation of this paper is that at the same fluid velocity and reducedvelocity, the VIV characteristic of the cylinders with the small mass ratio is comparedwith that of the cylinder with the large mass ratio. We find the new theoryphenomenon which is that the frequency of the in-line VIV of the cylinder is equal tothat of the cross-flow VIV and the VIV trace is the oval. This provides the certaintheory basis for the further research of the vortex-induced vibration.