Study On The Flow-induced Vibration And Control Mechanism Of Parallel Towers Based On Turbulent Coherent Structure

Chemical tower is the equipment to complete distillation,extraction,absorption and other processes in petrochemical,coal chemical,chemical fiber and pharmaceutical industries.Parallel tower is a new type of integrated equipment with the industry development towards refinement,high efficiency and large scale.They are closed to each other,and each tower is independent without any connecting platform and frame.The flow field around this equipment involves the complex interaction among shear layer,gap vortex and wake street,causing the classical theory of vortex-induced vibration（VIV）is invalid.And the surrounding of chemical tower is at subcritical Reynolds number,indicating the turbulence makes the coherent structure of the near-wall region and wake region unstable.The interference mechanism and coupling vibration mode of each tower are not clear,resulting in the design of parallel tower lacks of theoretical basis.Therefore,the parallel tower is simplified as multiple cylinders at first,and the flow-induced vibration（FIV）mechanism in turbulent flow is investigated through experiments,numerical simulations and theoretical derivations.The velocity threshold criteria of coupled vibrations are proposed,and the aerodynamic wake oscillator model is established,so that the vibration response of multiple cylinders is calculated.Based on the bio-inspired theory,the effect mechanism of micro strakes on the vortices near the wall and FIV are studied.Finally,the FIV mechanism,response calculation method and vibration control technology of flexible multiple cylinders such as parallel tower are obtained.An aeroelastic wind tunnel experimental system with atmospheric boundary layer is built,and the FIV characteristics and mechanism of multiple cylinders in turbulent flow are studied.Six kinds of FIV regimes for flexible multiple cylinders in real wind environment are obtained.In tandem arrangement,where the shear-layer interference and wake interference co-exist,four regimes are classified based on the nondimensional span:Regime I（l<1.6 andα=0°）,the upstream cylinders vibrate divergently like“galloping”,and vibration of the downstream cylinder is suppressed;RegimeⅡ（1.6≤l<3 andα=0°）,the downstream cylinders are under wake-induced galloping while the vibration of the upstream cylinder is totally suppressed;RegimeⅢ（3≤l≤5 andα=0°）,all the cylinders all vibrate significantly with approximately equal amplitudes;RegimeⅣ（5<l≤6.5 andα=0°）,the cylinders vibrate like the single cylinder due to the weak interference.In side-by-side arrangement,where only the shear-layer interference exist.Two regimes are classified,where the amplitudes are close to that of a single cylinder:Regime V（l<1.6 andα=90°）,the outer cylinder vibrates divergently but with a small amplitude;Regime VI（1.6≤l≤3.2 andα=90°）,the three cylinders vibrate asymmetrically with small amplitudes due to the biased flow.In staggered arrangement,it is close to the vibration regimes of tandem arrangement atα<30°,however,it approaches to the vibration of side-by-side arrangement atα≥30°.Moreover,the aerodynamic damping ratio of each cylinder are calculated through RDT and ARMA algorithm.It proves that the divergent and galloping-like vibration of tandem cylinders is a negative air damping phenomenon.Based on two-way LES and theoretical derivations,the coherent structures around the cylinders were obtained,and the FIV prediction models of tandem cylinders in strong coupling regions are established from two aspects.At the aspect of velocity,for extended-body,reattachment and impingement regimes,the lock-in regionλwas analyzed,and the critical velocities for vortex-induced vibration and galloping are defined.The velocity threshold criteria are proposed,including U_vcr<U_gcr<（1+λ）U_vcr/2where VIV and galloping are weakly coupled,U_gcr=（1+λ）U_vcr/2 where VIV and galloping are strongly coupled,（1+λ）U_vcr/2<U_gcr<λU_vcrwhere VIV predominates and U_gcr>λU_vcr,where VIV and galloping are independent,which can explain the interference mechanism of the tandem cylinders and estimate the occurrence of FIV for the multi cylinders,like the parallel tower.At the aspect of vibration responses,based on the wake lamina assumption,only one reduced van der Pol equation is applied to model the wake vortices.It is coupled with the structural motion equations,and the aerodynamic wake oscillator for tandem cylinders is established.The critical velocities,displacement-velocity curves and maximum amplitudes for the system of tandem cylinders are quantitively captured by this mathematical model.The predicted and experimental results show that a Hopf bifurcation of the original stable equilibrium exists near the critical velocity,resulting in the chaotic vibration of the tandem cylinders.And the vibration responses hold the hysteresis features.At small spans,the smaller span,the more obvious hysteresis.However,at larger span,the closer to the critical span l_c（l_c≈3）,the more obvious hysteresis.According to the non-smooth surface drag reduction technology,an elastic-supported model and micro strakes were designed,and a low-turbulence open wind tunnel experimental system is built.Combined with three-dimensional SST-SAS numerical simulation model,the vibration control regimes were studied.The differences between micro fin and spiral fin are compared.The former one mainly controls the FIV of multiple cylinders by increasing the shear stress,disturbing the aerodynamic force distribution of the surface fluid,and then changing the separation position;The latter one directly destroys the Karman vortex street in the wake to control the vibration.The effect of the vortex scale and structure on the FIV are quantitatively evaluated.The irregular small-scale vortices can significantly increase the second instability of FIV at U_r≥20;the periodic alternate oscillating vortices can magnify the wake interference so that the wake-induced galloping of the downstream cylinders is promoted;the influence of medium scale and steady vortices on the FIV of tandem cylinders is very limited.Based on the theory of equivalent particle roughness,the control mechanism of micro fin on FIV of tandem cylinder is studied,guiding the vibration reduction and anti-vibration design of parallel tower.At small span,smaller micro strakes installed on the upstream cylinder cannot affect the vortex scale but reduce the aerodynamic force of the cylinder,thus the galloping-like vibration of multi cylinder can be suppressed;At large span,it should install larger micro strakes on the downstream cylinder,effectively destroying the periodic large-scale vortex,to suppress the vibration efficiently.The present results reveal the FIV and vibration control mechanism of multiple cylinders.A basis theoretical system for vibration analysis of parallel towers,coupled with vortex-induced vibration and wake-induced galloping,is established.It provides a theoretical basis for the design and vibration suppression of parallel towers.The conclusions have great theoretical significance and engineering value for improving the design level of chemical tower,ensuring the safe and stable operation of chemical equipment,as well as the development and application of large-scale chemical plant.