Experimental Study On The Effect Of Axial-flow Turbulence Intensity On The Vibration Of Cylinder Cluster

With the increasing consumption of traditional fossil energy sources and the negative impact of burning fossil fuels on the human environment,nuclear power is receiving more and more attention as an efficient,clean,safe and economical energy source.As the advanced nuclear reactor technology develops rapidly,it is particularly important to ensure the stable operation of nuclear reactors.Axial-flow-induced vibration of fuel rods and control rods is the main cause of fuel rod wear,fatigue and even nuclear leakage.Thus,research on the axial-flow-induced vibration of fuel rods has a high engineering application value for the development of miniaturized nuclear reactor technology,and is also of great theoretical significance for the study of fluid-structure coupling problems associated with a group of cylinderical structures.The systematic experimental studies and theoretical achivements on the axial-flow-induced vibration of elastic cylinder are scare,and the understanding of the mechansim of fluid-structure coupling betweent elastic-cylinder vibration and the surrounding turbulent flow field is still to be improved.The present work aims to provide a systematic exper-imental study on the complex turbulent flow and vibration characteristics of the elastic cylinder in a cylinder cluster,and further gaining a deeper understanding of the physical mechanism of the coulping between the cylinder vibration and flow structures.The elas-tic cylinder in this experiment is made of silicone rubber（RTV-4230-E）,while the rigid cylinder is made of FEP（Fluoro-Ethylene Polymer）tube,whose optical refractive index is close to the water.The range of the incoming flow velocity in water tunnel is 0.20-2.12m/s,which corresponds to a turbulence intensityof 0.71-0.80%.The range of gap width^*（=/,whereandis center to center cylinder gap width and the cylinder diamater,respectively）is 1.21-1.71.The turbulence generator was fixed upstream of the test section to produce a stable and isotropic flow field with a high turbulence intensity（2.30%-2.91%）.In this project,the cylinder vibration and turbulent flow field are measured simultaneously through a Doppler vibrometer and a particle image velocimeter PIV,and a detailed analysics of the vibration characteristics and turbulent flow parameters in the cylinder bundle is conducted.The experimental results show that the free-stream flow velocity and turbulence intensity have a significant effct on the axial flow-induced vibration of the elastic cylinder.As flow velocity increases,the large eddy structures near the elastic cylinder wall break up and separate away from the cylinder wall,enhancing the the shear layer instabilities of the elastic cylinder and exciting the cylinder.At high（2.30%-2.91%）,the high turbulence fluctuations in the incident flow interacts with the shear layer around the cylinder,promoting instabilities in the shear layer around the elastic cylinder.As a result,the wall pressure fluctuations and near-wall fluctuating velocity are amplified,casuing larger vibration amplitude than its counterpart at low（0.71%-0.8%）.However,the dominant vibration frequency remains unaffcted by an increases in.For a tandem cylinder bundle with one elastic-cylinder and two rigid-cylinders,two regimes,i.e.,RI（^*≥1.57）and RII（^*<1.57）are identified based on their distrinct behaviors in the cylinder vibration and flow strcutures.In RI,a drop in^*lead a significantly decrease in the maximum axial flow velocity across the gap between the cylinders,which suppress the flow instability in the shear layer around the elastic cylinder,thus accounting for the contracted vibration amplitude.While in RII,an amout of large eddy structures are observed to separate from the wall,developing towards the center of gap and interacting with the eddy sructures on the opposite cylinder wall,thus resulting into the amplified instabilities of shear layer around the elastic cylinder,which is responsible for the sharp rise in vibration amplitude.While on the other side that not containing rigid cylinders,the broken and separation of near-wall eddy structures contributes to the thickened and more unstable shear layer,while the high momentum fluid flows more easily inward the cylinder wall,thus enhancing the fluid momentum transfer near the cylinder wall.As a result,the elastic cylinder receives sufficient energy from the flow fied to vibrate substantially.In a one-elastic and multiple-rigid cylinder cluster,at low（0.71%-0.80%）,an increase in the rigid cylinder number or a decrease in the gap width would produce a high velocity gradient within the cylinder bundle,causing the eddy structures to separate from the wall and actively interact with the eddy structures near the neighbouring cylinder.As a result,the instability of shear layer around the elastic cylinder is excited,which causes the cylinder to vibrate substantially.Whenincreases to 2.30%-2.91%,the influence of gap width on the elastic-cylinder vibration is weakened,and the incident flow fluctuations plays a key role in the flow-structure interaction of elastic cylinder.The instability of shear layer around the elastic cylinder is amplified by the incident flow fluctuations of a high,enhancing the eddy activities around the elastic cylinder wall,and thus resulting into the large vibrtion amplitude of elastic-cylinder,while the velocity-gradient effect associated with a change in^*is of less importance.Moreover,the interventions of the incident flow turbulence weakens the synchronization between the velocity fluctuation in the gap and the elastic-cylinder vibration.In summary,a systematic experimental study on the axial-flow-induced vibration of elastic cylinder in a cylinder cluster is presented.The effect of turbulence intensity on the fluid-structure interaction of elastic cylinder subjected to an axial flow is revealed by the simultaneous measurements of the cylinder vibration and the flow field around the cylinder.Two regimes are identified based on the distinct behaviours in cylinder vibration and flow strucutres,and the coupling mechanism between the cylinder vibration and near-wall flow structures is revealed.The present research has further deepened the understanding of the mechanism of the axial-flow-induced viration and fluid-sructure interaction,which is of important guiding significance for the design and safe operation of nuclear reactors,and also lays a theoretical foundation for the development of advanced nuclear reactor technology.