Analysis Of Vortex-Induced Vibration Response Of A Circular Cylinder Attached By Ribs

Marine risers have been serving in the marine environment for a long time,which inevitably results in biological attachment,and causes the riser wall to be roughened.The roughened riser wall will inevitably affect its vortex-induced vibration response.In this paper,the uniform roughness of the riser caused by biological attachment is simulated by the axially uniformly distributed auxiliary ribs,and its effect on the vortex-induced vibration of the riser is studied.The effects of the height and coverage of the auxiliary ribs on the vortex-induced vibration of rigid risers are mainly studied from the aspects of physical experiments and numerical simulation.The specific research and conclusions are as follows:(1)The velocity profile was measured in a small flume,and a single-degree-of rigid riser vibration device was designed and installed.The vibration displacement of the riser was captured by high-speed camera technology.The smooth riser and the auxiliary ribbed riser with different structures are placed at different inflow velocities to obtain the vibration displacement of the riser,and compared with the classical experimental results of Williamson’s three branches.It is pointed out that the difference of the comparison results lies in the quality ratio of the riser and the problem of experimental operation.(2)After validating the numerical simulation of low Reynolds number static flow and vortex-induced vibration,the two-dimensional two-degree-of vortex-induced vibration of the auxiliary ribbed riser is simulated by using k-ε model.It is found that the influence of the auxiliary rib on the vibration of the riser is related to the height and coverage of the riser.When the protrusion height of 0.05D and coverage is 20%,the vibration amplitude of the riser will be galloping,and the coverage of the riser will be 60%、100%of the auxiliary ribbed riser has similar suppression effect on the amplitude of Y direction.the vibration trajectories of auxiliary ribbed risers with protrusion height of 0.05D and coverage of 20%are transformed from regular "8" shape to multiple "8" shapes.The reason is that the boundary layer of the auxiliary ribbed riser with 20%coverage first separates and occurs at the front rib protrusion.The higher the protrusion height is,the larger the full range of vortices between adjacent protrusions develops,which leads to the increase of lift force and Y-direction amplitude of the riser,thus resulting in the riser galloping.At the same time,the Drag force of the riser increases when the coverage is 20%,and the trajectory of the riser becomes irregular when galloping occurs.Regularly,the "8" superposition transformation of butterfly wing is produced.(3)The vortex shedding mode of the auxiliary ribbed riser with 20%coverage and 0.03D and 0.05D bulge height is transformed from 2P mode to P+S mode,while that of other structures is transformed from 2S mode to 2P mode.The pressure coefficient curve will fluctuate sharply in the protruding position of the auxiliary ribs,with the coverage of 20%being the most obvious.From the point of view of the characteristics of wake flow field,the vibration trajectory of riser changes,and the vortex shedding mode changes accordingly.(4)Under the actual working conditions,the Reynolds number is high and the three-dimensional characteristics are obvious.The LES model is used to supplement and calculate the three-dimensional numerical simulation of the flow around the auxiliary ribbed riser.The average drag coefficient of the auxiliary ribbed riser is larger than that of the smooth riser,but the Strauha number is smaller than that of the smooth riser.Moreover,under the condition of high Reynolds number,the three-dimensional characteristics of the vortices are obvious,and the boundary layer of the auxiliary ribbed riser is fully developed.