Study On VIV Of Two Cylinders In Tandem Arrangement Under Subcritical Reynolds Numbers

Vortex-Induced Vibration（VIV）,which occurs in many fields of engineering,has been studied extensively,especially in the past fifty years,due to its significant implications in different engineering applications including offshore structures,cooling towers,bridges,heat exchangers and electrical transmission lines.In recent years,due to energy problems,the research on the use of eddy vibration mechanism for power generation has important reference value.The vortex induced vibration of two circular cylinders in tandem arrangement is studied experimentally and numerically in this paper.The two cylinders are rigid,with the upstream one being fixed and the downstream one being able to oscillate only in cross flow direction.The Reynolds number ranges from 5000～40000.The spacing ratio is S/D=3～10（S is the center to center distance,D is the diameter of the cylinders）.The experiments were carried out in the circulating water tank.The response of VIV of the downstream cylinder was studied at different spacing ratios.The amplitude of the downstream cylinder was measured by laser displacement sensor.The variation of amplitude ratio of the downstream cylinder A/D（A is the cylinder vibration amplitude,D is the cylinder diameter）and the frequency ratio f_osc/f_n(f_osc is the cylinder vibration frequency,f_n is the natural frequency of the cylinder)with the reduction speed U^*were analyzed.Meanwhile,the influence of the upstream cylindrical wake on the vibration characteristics of the downstream cylinder were discussed.In the numerical model,the commercial software Starccm+is used to conduct two-dimensional numerical simulation,in which all the constraints and experimental parameters of single cylinder and tandem double cylinder are consistent with the experimental process,which aims to explain the influence of upstream cylinder on downstream cylinder from the perspective of flow field characteristics through the flow field pressure and vortex volume distribution between upstream and downstream cylinders.The results demonstrate that:during the experiment,lock in range becomes significantly wider compared to the VIV（vortex induced vibration）of a single cylinder.Especially for S/D being 3,4 or 5,the amplitude response demonstrate galloping-like behaviour.In the numerical results,similar results were obtained to the experimental data.Dynamic lift force on the cylinders is monitored and analyzed and vorticity contours are observed in the numerical simulations in order to explain the phenomena occurred in the oscillation of the downstream cylinder.Furthermore the oscillation of the training cylinder at large reduced velocity up to 22are simulated,and we found that the galloping response disappeared for S/D=5,however the amplitude continuously increases with increasing reduced velocity for S/D=3 and 4.Through a series of experiments and numerical simulations,the internal mechanism of eddy vibration in series double cylindrical cylinders is clarified,and the conclusions obtained in this paper can lay a corresponding theoretical foundation for the development and utilization of ocean current energy generation device VIVACE（Vortex-Induced Vibration Aquatic Clean Energy）.