Experimental Investigation Of Vortex-induced Vibration Of Flexible Cylinder Under Flow Interference

The riser in the actual deep-sea oil and gas exploration project often appears in the form of a group.When the risers are close to each other,the flow interference occurs,and the response characteristic is different from that of the single riser.The traditional theory of vortexinduced vibration is mainly based on the study of a single cylinder.The research results of vortex-induced vibration of flexible cylinder under flow interference are only reported in a few literatures.In this paper,the physical model experiments are carried out to study the vortexinduced vibration of the slender flexible cylinder under flow interference.By analyzing the displacement response,frequency response and trajectory of the flexible cylinder systematically,the effects of the tension and spacing on the flow interference at different reduction velocities are studied.The physical mechanism of vortex-induced vibration of flexible cylinder under flow interference is discussed.The experimental investigation in this paper contains three parts,as follows:The first part is the experimental investigation of vortex-induced vibration of a single flexible cylinder.The single flexible cylinder is vertically installed in the water flume and one third of the cylinder is submerged in water.The results show that a distinct lock-in region is obtained for each tension,revealing the initial branch,upper branch,lower branch and other important physical phenomena of vortex-induced vibration.At different reduced velocities,the vibration in the direction of IL is always controlled by the two-order mode,while the vibration in the direction of CF is controlled by the first-order mode.The control frequency increases linearly with the increase of the reduced velocity,and after the cylindrical vibration reaches the complete lock state,the control frequency does not change with the change of the reduced velocity.The tension has a significant effect on the trajectory of the cylinder,and with the change of the reduced velocity,the trajectories contain "8" shape,"new moon" shape,"half moon" shape and "raindrop" shape.The second part is the experimental investigation of vortex-induced vibration of the flexible cylinder under the flow interference of the rigid cylinder.The flexible cylinder is arranged in downstream of the rigid cylinder in tandem.Seven spacing ratios are considered to study the influence of the spacing on flow interference.The results show that the vortex-induced vibration response of the downstream flexible cylinder is different from that of the single flexible cylinder.Compared with a single flexible cylinder,the locking range of the downstream flexible cylinder is significantly wider,and the peak displacement in the CF direction is increased.The spacing ratio S/D=4.0 is the critical spacing ratio.Under the conditon of S/D <4.0,the effect of spacing on flow interference is very significant.Under the conditon of S/D ? 4.0,the effect of spacing on flow interference is small.The vibration in the direction of IL is always controlled by the two-order mode,while the vibration in the CF direction is controlled only by the first-order mode.When spacing ratio is in the range of 2.5~4.0,the spacing ratio has a greater influence on dominant frequency of the downstream flexible cylinder,and when spacing ratio is in the range of 4.0~6.0,the spacing ratio has little effect on it.The third part is the experimental investigation of vortex-induced vibration of two tandem flexible cylinders under flow interference.Five spacing ratios are considered to study the influence of the spacing on flow interference.The results show that S/D=4.0 is the critical spacing ratio.Under the conditions of S/D=2.5 and 3.0,the displacement response curve of the upstream cylinder is different from that of the downstream cylinder,indicating a significant effect of flow interference.The response characteristics under the conditions of S/D=4.0 ~ 6.0 are similar,which means that the effect of the downstream cylinder on the upstream one is limited,while interference of the upstream cylinder to the downstream one is still effect.Wake galloping occurs on the downstream cylinder when the upstream cylinder is responding in the lower branch range.By analyzing the spectrum,it is found that the effect of flow interference of the upstream cylinder to the downstream one at S/D =5.0 is weaker than that of S/D=2.5.Under the conditon of S/D= 2.5,the trajectories of the upstream cylinder in initial and upper branch range are "8" shape,and the trajectories in the lower branch range are "ellipse" shape,while the trajectories of the downstream cylinder are "ellipse" shape in the entire locking range.Under the conditon of S/D= 5.0,the trajectories of the upstream cylinder contains "new moon" shape,"raindrop" shape and the asymmetric "8" shape,but with poor periodicity.However,due to the flow interference,the downstream cylinder presents an unstable aperiodic motion.The trajectory shape contains "ellipse","8","raindrop" and "new moon".