Experimental And Numerical Investigation On Flow Past Four Cylinders With And Without Vortex-induced Vibration At Different Incident Angles

With the development of deepwater oil and gas exploration,multiple cylinders are widely applied in offshore and ocean engineering,such as platform,riser,pipeline and mooring system etc.When the cylinder bundles are placed in a close arrangement,the risk of fatigue failure for mutiple cylinders will be increased due to the collision effect under the combined interactions among the flow,wave and force oscillation of FPSO(Floating Production Storage and Offloading)platforms.Moreover,due to the complex sea conditions in the deep water environment,the multi-cylinder system is always suffered the flow interactions from different directions.The purpose of this thesis is to investigate the effect of incident angle on flow past four cylinders with and without vortex-induced vibration,the details are as followed:1.Particle image velocimetry technique(PIV)is used to obtain the instantaneous and time-averaged flow field of four stationary cylinders in a square configuration with different incident angles at L/D=1.5 and 2.0.The instantaneous and time-averaged velocity vectors,vorticity contours,streamline topology and vortex shedding frequency of four cylinders are analyzed.The experimental result shows that the incident angle has a significant effect on the wake regime of the four cylinders in a square arrangement.Depending on the incident angle,the wake patterns of four cylinders can be divided into three types:extended body regime(a=0°),reattachment regime(a=15°?30°),co-shedding regime(a=45°).With increasing of the incident angle,the variation shape of the mean streamwise velocity with y/D behind the four cylinders is changed from "W" to "U".2.Numerical simulations on the vortex-induced vibration of two tandem circular cylinders and four cylinders in a square arrangement at different spacing ratios are carried out at Re=50.The dynamic response,force coefficients,and wake regime for each cylinder are presented.The results show that for two tandem cylinders,with increasing of the spacing ratio,the oscillation amplitude of the upstream cylinder is gradually consistent with a single cylinder,while the oscillation amplitude of the downstream cylinder is still maintained at a large value,which indicates that with increasing of the spacing ratio,the effect of the downstream cylinders on the upstream cylinders is gradually weakened,while the downstream cylinders are still affected by the upstream wake even at large spacing ratio.The maximum value CD of the upstream cylinder is reached at Vr=5,while the downstream cylinder reaches the maximum value CD at Vr=7.At Vr=7,The wake of the downstream cylinder is single-row configuration at L/D?3 and evolves into a double-row configuration at L/D=4.0?5.0.For four cylinders,both the maximum inline and transverse amplitudes of the upstream cylinders are occurred at Vr=5,which is called double resonance phenomenon.The inline amplitudes of upstream cylinders at Vr=5 in L/D=3.0 are larger than those of L/D=4.0 and 5.0.The maximum transverse amplitudes of downstream cylinders are occurred at Vr=7 for all spacing ratios.At L/D=3.0,wake modes are antisymmetric at Vr=3 and Vr=13,whilethe two upstream cylinders and two downstream cylinders move together in the same direction and the wake vortex near the downstream cylinders is complex at Vr=9.At L/D=4.0?5.0,The wake vortex street of the downstream cylinders is double-row configuration at Vr =3 and becomes a single-row configuration at Vr =7?13.3.Numerical simulations of two-degree-of-freedom vortex-induced vibration have been performed for four circular cylinders in a square arrangement at Re=150.The effects of incident angles and reduced velocities on the dynamic responses and vortex shedding modes of the four cylinders are investigated.Due to the complex interferences among the four cylinders,the variations of the dynamic responses with reduced velocities appear to be more complicated than that of an isolated cylinder.The incident angle is found to have a significant effect on the dynamic responses of the downstream cylinders,rather than the upstream cylinders.With increasing of a,the maximum transverse and streamwise amplitudes of cylinder 2 are approximately to be larger than the other three cylinders.For all the examined incident angles a,the mean drag coefficients of four cylinders generally increase with reduced velocity and then slightly decrease.The mean lift coefficients of four cylinders behave differently with increasing of a,especially for Vr<8.The motion trajectories and vortex shedding patterns of four cylinders are found to be not only depended on the reduced velocities but also the incidence angles.With increasing of incident angle a,the motion trajectory and vortex shedding pattern for each cylinder behave differently.