The Influence Of Gap Flow On The Characteristics Of Flow-around And Flow-induced Vibration For Two PTC Circular Cylinders

The Flow-Induced-Motion(FIM)of the multi-bluff bodies are common phenomena in marine engineering,wind engineering,and aerospace engineering.On the one hand,security problems in many engineering fields are related to FIM because it can make structural fatigue damage to chimneys,cables,bridges,and offshore structures,etc.On the other hand,the device which extracts hydrokinetic power from FIM of elastically mounted rigid cylinder was recently developed as an effective means to harvest renewable clean energy in the ocean/river/wind.In order to improve the output power density,it is crucial to find out the driving mechanism of FIM and to enhance FIM of the cylinder intentionally.Thus,the measures to strengthen the FIM of cylinders were obtained,such as Passive Turbulence Control(PTC).With the rapid development of fluid engineering in recent years,the FIM of the multi-bluff bodies attract a lot of attention,there are still many problems need to be further studied.The study on the vibration of two-cylinders,one of the simplest multi-cylinders forms,is the basis of the research on the vibration of multi-cylinders.However,previous studies focused most on smooth cylinders,the studies on PTC cylinders with elastic support are barely known.The response characteristics and effect mechanisms of the FIM of two PTC cylinders at different spacing ratios need to be further studied.In this thesis,flow around two fixed PTC cylinders in tandem are investigated by2-D URANS simulations based on OpenFOAM.Drag coefficient,lift coefficient,Strouhal number and near-wake structures of the two cylinders are discussed.Furthermore,FIM of two staggered PTC cylinders with elastic support are investigated by numerical approach.The main conclusions are as follows.Firstly,the dynamic characteristics of flow past two fixed PTC cylinders with different spacing d in tandem are investigated numerically in detail and the critical spacing ratios are obtained.With the increase of the spacing d,the average drag coefficient of upstream cylinder is always positive,the upstream cylinder is subjected to the thrust that points to the downstream cylinder.The average drag coefficient of downstream cylinder has negative values.At this time,the downstream cylinder is subjected to the suction that points to the upstream cylinder.The lift coefficients,drag coefficients and Strouhal numbers of two cylinders increase dramatically while the spacing d of the two cylinders is equal to the critical spacing d_c.The critical spacing of the two cylinders is d_c=3D When Re=2000,and the critical spacing of the two cylinders turns into d_c=2.5D When Re=6000~14000.Furthermore,FIM responses of two staggered PTC cylinders with elastic support are studied.The distance between two cylinders along the cross-flow direction is T.The vibration modes of two cylinders are periodic vibration that exists in small reduced-velocity,double-periodic vibration that exists in medium reduced-velocity and small spacing,multiple-periodic vibration that exists in medium reduced-velocity and great spacing,and quasi-periodic vibration that exists in great reduced-velocity.For two staggered cylinders,the beating phenomenon is not observed in periodic vibration mode.Though the staggered spacing T changes,the amplitude of the upstream cylinder barely changes,and the maximum amplitude can be obtained at U*=6.There is a critical reduced-velocity U_c*=7,which changes the magnitude relationship between two cylinders'amplitude.The amplitude of the downstream cylinder is larger than the amplitude of the upstream cylinder while U*>U_c*.Finally,the influence of the gap flow on the flow around the cylinder and the flow-induced vibration are deeply explored by analyzing the wake structure.For the first case,the wake of two tandem cylinder shows the single elongated structure,alternating phenomenon,unstable region and dual-vortex structure when Re=2000.The single elongated structure is not observed when Re=6000~14000.There is no gap flow occurs when the spacing d is less than the critical spacing d_c.The shear layer which is separated from the upstream cylinder directly envelopes the downstream cylinder,and the vortex shedding of two cylinders is suppressed.When the spacing d is greater than the critical spacing d_c,there is a gap flow.As a result,the vortex shedding of two cylinders is motivated.With the increase of the spacing d,the wake structure of the upstream cylinder seems like the wake structure of a single cylinder,and the influence of the gap flow on the downstream cylinder is weakened.For the second case,the gap flow becomes the main factor affecting the vibration of the two cylinders when T>0.6D and U*<8,and it can be divided into the gap flow dominant region.The gap flow attaches to the downstream cylinder,which promotes the boundary layer separation of the downstream cylinder and enhances the vibration of the downstream cylinder.