| Flow-induced vibration (FIV) of circular cylinders exists in many engineering, such as mechanical engineering, marine engineering and so on. After decades of studying on this problems by many researchers, there are still many open questions due to its extreme complexity and a fully understanding are far from reachable. However, the ability of accurately predicting the responses of cylinders are not only important from the academic point of view, but also helpful to improve the energy transform coefficient of the VIVACE-a novel ocean current energy harvesting device.In this thesis, numerical simulations of FIV of circular cylinders were carried out to systematically investigate the interaction phenomena and the mechanisms hidden behind. After reviewed the latest research progress on FIV of circular cylinders, the author carried out a series of numerical simulations which covers several hot-spot issues, including VIV of an isolated circular cylinder, FIV of two side-by-side and tandem arranged circular cylinders and FIV of four squared-arranged circular cylinders. The vibration responses and the wake patterns were systematically investigated, together with the effects of spacing ratio on FIV of circular cylinders, and the coupling mechanisms of cylinder responses were discussed. Several new phenomena observed in FIV of circular cylinders and their physical nature and mechanisms were deeply investigated. The valuable findings are summarized as follow:1. The effects of the in-line vibration on the cross-flow vibration of an isolated circular cylinder are only predominant in the regime of lock-in. The mean drag force on a circular cylinder undergoing VIV is lower that of flow around a circular cylinder when the reduced velocity is larger than 8.0. This can be attributed to two factors. One is related with the fact that the cylinder vibration, although its amplitude may be very small, makes the vortices shed from the cylinder closer to the base of cylinder and the other is associated with the 180 degree jump of phase between the cylinder displacement and the lift force.2. An asymmetric vibration and a new hysteresis phenomena were firstly found in FIV of two side-by-side arranged circular cylinders with the spacing ratio s/D= 2.5, but disappear when the blockage ratio B?0.08.3. Eight kinds of near-wake patterns were observed in FIV of two side-by-side circular cylinders with s/D= 2.5 and s/D= 5.0, and they are the irregular pattern (IR), in-phase-synchronized pattern (IS), anti-phase-synchronized pattern (AS), biased anti- phase-synchronized pattern (BAS), in-phase flip-flopping pattern (IFF), long-period in-phase flip-flopping pattern (LIFF), out-of-phase flip-flopping pattern (OFF) and combined pattern (CB).4. The mechanism of FIV of two tandem arranged circular cylinders with s/D= 1.5 is different from that of the cases with s/D= 3.0 and s/D= 5.0. Response of the upstream cylinder is similar to that of VIV of an isolated circular cylinder when the spacing ratio s/D?3.0. No hysteresis exists in the upstream cylinder response curve between lower branch and non-synchronization regime and the effects of the downstream cylinder on the upstream cylinder are more significant than that in the conventional point of view. The hysteresis can be observed in the cases with s/D= 1.5 and s/D= 5.0, but not observed in the case with s/D= 3.0.5. In FIV of four squared-arranged square circular cylinders, the maximum amplitude of the two upstream cylinders is Ymax/D=0.56 which is close to that of VIV of an isolated circular cylinder. The maximum amplitudes of the two downstream cylinders are different with each other, i.e., Ymax/D=1.01 and Ymsx/D=0.997, which are much larger than that of VIV of an isolated circular cylinder.6. There are three different regions of asymmetric vibration in FIV of four squared-arranged square circular cylinders, i.e., Ur=4.7?6.07, Ur=6.9?7.2 and Ur=10.5?50.0. In these regions, the wake pattern is similar to that of FIV of two side-by-side arranged circular cylinders, i.e., the narrow-wide wake pattern. With the decreasing of the amplitude difference, the difference on the wake width decreases.In conclusion, the basic characteristics of VIV of an isolated circular cylinder was investigated firstly in this thesis, and then investigations on FIV of two side-by-side and tandem arranged circular cylinders were carried out. Effects of key parameters on the cylinder responses and their mechanisms were analyzed. Finally, the simulation of FIV of four squared-arranged circular cylinders was carried out, and the responses, hydrodynamics features and wake patterns were investigated, the interaction mechanisms of FIV of four squared-arranged circular cylinders was presented. The simulation results presented in this thesis will greatly contribute to a deeper understanding on the phenomenon of FIV of circular cylinders and the mechanisms hidden behind, and therefore will be helpful for the future research work in this area. |
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