Fluid-structure Coupling Responses Of Cylindrical Structures Subjected To Cross Flow

Cylindrical systems(tube arrays)are widely used in bridge engineering,ocean engineering,ship engineering and nuclear engineering.The dynamic behaviors of the cylindrical systems are closely related to the safe operations of engineering.It is extremely necessary to study the fluid-structure coupling vibrations of circular cylinder systems.In this thesis,the computational fluid dynamics(CFD)method was used to study the fluid-structure coupling vibrations of two-dimensional cylinder systems with a Reynolds number of 150.Fluent solver was used to solve the fluid dynamic equations.User-defined functions were nested to solve the dynamic equations of the structures.The two-way fluid-structure coupling was realized by combining the overlapping grid techniques.The numerical models of single circular cylinders with single-degree-of-freedom and two-degree-of-freedom were established.The numerical models of two circular cylinders in tandem arrangement were established.The fluid-structure coupling vibrations and responses of the cylindrical systems under different reduced velocities,different vibration combinations and different spacing ratios were calculated.The fluid-structure coupling vibration responses of single cylinders with 1DOF and 2DOF were calculated.The results show that the mechanism of the fluid-structure coupling vibration of a single cylinder under the action of cross flow is vortex-induced vibration.The lock-in and phase switch phenomenons of vortex-induced vibration were caught.The vibration response of a single cylinder system was represented by initial branch,lower branch and nonsynchronous region.The frequency locking occurred in a large range of reduced velocities,and the amplitudes of the lock-in area are much larger than other areas.Compared with the simulation results of 1DOF and 2DOF,the vibration along the flow direction haves a distinct effect on the vibration in the cross flow direction.The amplitude of the response gets bigger when the vibration along the flow direction is considered.The trajectories of the cylinder with2 DOF were "8" shapes.The hysteresis between the lower branch and the non-synchronous region was captured with increment of reduced velocities and decrement of reduced velocities.The wake vortex mode of single cylinder system is 2S mode.The fluid-structure coupling vibration responses of two circular cylinders in tandem arrangement were calculated.The results show that when the downstream cylinder is fixed,due to the influence of the downstream cylinder,the lock-in area of the upstream cylinder is obviously different from that of a single cylinder.The size of the lock-in area decreases as the spacing ratio increases.When the distances between the two cylinders are small,galloping occurres on the upstream cylinder,and the maximum amplitude reachs to 1D.When the distances are large,the amplitudes of the upstream cylinder are similar to that of a single cylinder.When the upstream cylinder is fixed,no frequency locking occurs on the downstream cylinder.When the distances between the two cylinders are small,the amplitude,frequency and wake vortex mode of the downstream cylinder change abruptly with the increment of reduced velocities.When the distances are large,a second frequency branch appears on the downstream cylinder where the reduced velocity is large.When the upstream and downstream cylinders vibrate at the same time,the vibrations of the downstream cylinder have no effect on the vibrations of the upstream cylinder.While the vibrations of the upstream cylinder always have effects on the downstream one.The maximum amplitudes of the downstream cylinder are greater than that of the upstream cylinder under different spacing ratios.The amplitude of the downstream cylinder reaches maximum when the free shear layer of the upstream cylinder completely passes through the gap and merges with the vortex of the downstream cylinder.