Experimental Study Of Two Tandem Cylinders In Crossflow: Flow-induced Vibration

Flow-induced vibration of cylindrical structures is of both fundamental and practical significances.Industrial and engineering architectures are often the combinations of multiple cylindrical structures,such as cables of cable-stayed bridges,groups of chimney stacks,tubes in heat exchangers,transmission line bundles,masts,chemical reaction towers,offshore platforms,risers,undersea pipelines,adjacent skyscrapers,etc.Naturally,it is of practical importance to understand the proximity effect on flow-induced vibration associated with a group of cylindrical structures.The interaction between two tandem cylinders is very strong involving shear layer reattachment,shear layer interference,quasi-periodic vortices,vortex impingement,recirculation,etc.As such,two tandem circular cylinders is an excellent model to understand the fluid-structure interactions involved in multiple structures.Most of previous investigations have been performed on two fix-supported rigid circular cylinders.However,engineering structures are,strictly speaking,not rigid;as a matter of fact,they are elastic and can be associated with violent flow-induced vibrations.Despite previous investigations,our understanding has yet to be greatly improved on the fluid-structure interactions involved in flow-induced vibration of two tandem cylinders in crossflow.Thus this work aims to study experimentally the flow-induced vibration of two tandem cylinders.Extensive measurements are conducted to capture the cylinder vibration and frequency responses,surface pressure,shedding frequencies,and flow fields using laser vibrometer,hotwire,pressure scanner and particle image velocimetry techiniques.A novel experimental setup to measure cylinder vibration and induced forces is designed that will facilitate estimating added damping,added mass,and energy transfer between the fluid and vibrating cylinder,which may succeed to dig out the physics and insight into the flow-induced vibration.A systematic study of the cross-flow induced vibration on a spring-supported circular cylinder in the wake of a fix-supported cylinder of smaller diameter is presented.First,six distinct flow regimes are identified.It has been found that a violent vibration may erupt for the spring-supported cylinder and its dependence on diameter ratio and distance ratio is documented.Then,a careful examination and analysis of the flow structure,along with the simultaneously captured pressure distribution around and vibration of the downstream cylinder,cast light upon the mechanisms behind this vibration and its sustainability.Finally,how the structural vibration is initiated is unveiled.The roles of added mass,flow-induced damping and physical aspects in the process of initiating the vibration are discussed in detail.The transition or growth process is characterized by three stages: pre-initial transition stage,initial transition stage and the late transition stage.It has been found that natural vortex shedding initiates the cylinder vibration in the pre-initial transition,though both vortex shedding and the gap shear layer switching are responsible for a rapid growth in vibration in the initial transition.Yet,it is the gap shear layer switching that accounts for the sustainable violent vibration in the late transition.When the upstream cylinder is elasctically-mounted and thus is able to vibrate,the vibration characteristics of both cylinders are more complicated,the physics of which is not clear.Hence,a systematic experimental investigation on the flow-induced vibrations of two identical elasctically-mounted tandem circular cylinders is presented.Both cylinders are allowed to vibrate only laterally.First,four vibration regimes are identified based on the characteristics and generation mechanisms of the cylinder galloping vibrations.Then,several findings are made on the mechanisms of vibration generation and sustainability.The gap vortices around the base surface of the upstream cylinder contribute to the positive work on the cylinder,sustaining the upstream cylinder vibration.Besides,the reattachment,detachment,and switch of the gap shear layers result in largely positive work on the downstream cylinder,playing an important role in sustaining its vibration.The alternate reattachment,detachment,rolling up and shedding of the upper and lower gap shear layers in the gap get involved alternately in one cycle of vibrations.Also,it has been found that the neighbored cylinder of different initial conditions(vibrating or fixed)or natural frequency may have a pronounced impact on the vibration of the other.In summary,the flow induced vibration of two tandem cylinders in crossflow are studied experimentally.The vibration response,vortex shedding frequency,pressure distribution and lift force,and flow structures in the gap and wake of the cylinders are systematically measured.The flow regimes are identified.In addition,the flow physics or mechanisms behind the violent vibration and its sustainability are elucidated.Moreover,the initiation of the vibration is illuminated clearly.The results will help us thoroughly understand the intristic mechanism of flow induced vibration of tandem cylinders and also design control scheme to suppress the vibration.Furthermore,the results will benefit the design of other multiple cylindrical structures,such as cables of cable-stayed bridges,groups of chimney stacks,tubes in heat exchangers,adjacent skyscrapers,marine risers etc.