Study On Two Degrees Of Freedom Vortex-induced Vibration Of Circular Cylinder

In the field of ocean engineering, many pivotal components have similar circular cylinder shape,and according to the fluid mechanics theory,those ocean structures likely suffer from the structure vibration and dynamic loads caused by viscous vortex, especially when the natural vibration frequency of structures is equal to the frequency of vortex shedding, the vibration of the whole ocean structures will become more intense, which results in destruction of structures eventually.Based on detailed reviews and summary of the domestic and foreign research methods and the latest achievements concerning vortex-induced vibration of ocean riser, firstly, the Navier-Stokes equations of incompressible viscous fluid is solved with CFD software FLUENT, and two degrees of freedom forced oscillation and self-excited vibration of cylinder are analyzed with self-complied UDF (User Defined Function) code and dynamic mesh technology; the double wake oscillator model is established based on Van der Pol equation,and then a new double coupling oscillator model is introduced with combing the structural oscillator model. The paper research works are mainly summarized as follows:1. A two-dimensional numerical model of the flow field around a fixed cylinder is established in order to validate the reliability of the numerical method and computational model used in this paper, which lays foundation for the following numerical calculation of forced oscillation and self-excited vibration of circular cylinder.2. In order to study the subsequent vortex-induced vibration under combined uniform flow and oscillatory flow, firstly, the transverse and two degrees of freedom forced oscillation of circular cylinder under uniform flow at low Reynolds number is respectively studied by altering the vibration frequency and amplitude, specifically analyzing the lift and drag coefficients,frequency of vortex shedding and vortex shedding modes in the wake; the vibration response of forced oscillation under combined uniform flow and oscillatory flow is analyzed,and it is found that oscillatory flow can change vortex shedding modes,hydrodynamic coefficients and the range of lock-in.3. The two degrees of freedom vortex-induced vibration of an elastically mounted circular cylinder is researched, which obtains motion response characteristics and vortex shedding modes, including vortex shedding frequency in the wake, time history curve of vibration displacement in cross-flow and in-line, motion trajectory of the system, time history curve of hydrodynamic coefficient; the different mass ratio and damping ratio affecting two degrees of freedom VIV of elastically mounted circular cylinder are studied; the self-excited vibration of circular cylinder under combined uniform flow and oscillatory flow is studied,and the results show that the range of lock-in is expanded, and in-line amplitude increases greatly, even exceeding the peak of cross-flow amplitude.4. The double wake oscillator model is introduced based on Van der Pol equation, and then VIV response of experimental examples is accurately predicted by the new coupling model; the optimal coupling parameters are determined according to model parameters discussion; it is found that a new motion trajectory consisted of two "8" shape under the high natural vibration frequency ratio between in-line and cross-flow is formed.