Study On The Flow-induced Vibration Mechanism And Energy Capture Of Three Tandem Cylinders

Flow-induced vibration(FIV)is a complex physical phenomenon involving fluid mechanics and structural dynamics,which is widely present in nature and engineering.Cylinder structures are typical of industrial and engineering building structures such as chemical towers,offshore structures,chimneys,heat transfer bundles,ocean risers,etc.Meanwhile,with the need of socio-economic development and environmental protection,energy harvesting devices based on FIV have gained wide attention.Therefore,it is important to carry out the research of multi-cylinder FIV and investigate the mechanism of vibration,which has important academic value and practical engineering significance.In this paper,the mechanism of coupled nonlinear vibrations of multi-cylinder in turbulent flow and the FIV energy harvesting characteristics is investigated through wind tunnel experiments and numerical simulations.A low-turbulence wind tunnel experimental system is built,and the flow-induced vibration characteristics of tandem flexible multi-cylinders in turbulent flow are studied.Depending on the spacing ratio L/D,multi-cylinder FIV are classified into four vibration regimes: Regime I(L/D≤1.5),the upstream cylinders vibrate divergently wake-induced galloping(WIG)response;Regime Ⅱ(1.5<L/D≤2.1),the upstream cylinder vibrates vortex-induced vibration(VIV)response,and vibration of the downstream cylinder is suppressed;Regime ⅡI(2.1<L/D≤4.0),the downstream cylinders are under WIG response;Regime Ⅳ(4.0<L/D≤6.5),the cylinders vibrate like the single cylinder.Meanwhile,the effect of the initial state of the cylinder on the multi-cylinder FIV is investigated,indicating that the initial state of the downstream cylinder has less effect on the upstream cylinder,while the vibration of the downstream cylinder is more violent when the upstream cylinder is fixed.To further analyze the coupled vibration mechanism of the three cylinders,based on the numerical calculation of two-way SST-SAS,the vibration response,wake structure and nonlinear characteristics of the three cylinders in turbulent flow are obtained.The coupling effect between the cylinders is elucidated by the analysis of the three cylindrical vibration coherence and phase.Meanwhile,according to the difference of coherence and phase,three vibration transition mechanisms are proposed: separated state,continuous state and partially separated state.At the same time,it can provide technical support for the next study of multi-cylinder FIV energy harvesting.Based on the flow-solid-electric coupling FIV energy harvesting technology,an elastic support energy harvesting model is designed to investigate the multi-cylinder FIV energy harvesting characteristics,and the influence of spacing ratio L/D and wind speed U on the output voltage,output power and efficiency,and output stability of the energy harvesting device is systematically analyzed.The results indicate that the small spacing ratio(L/D≤1.5)has obvious advantages in energy output size,output stability and effective output wind speed range.At large spacing ratios,large energy output performance can be produced only in the lower wind speed VIV region and the larger wind speed WIG region.Meanwhile,the energy harvesting characteristics of two cylinders with different diameters are investigated,the influence of diameter ratio d/D and flow velocity U on the energy harvesting of the downstream cylinder is systematically analyzed,which indicates that the presence of upstream cylinders significantly improves the energy harvesting capacity of downstream cylinders,and the influence of different diameter ratios on their energy harvesting characteristics is also different.At low flow speeds,the energy harvesting dominance is more obvious for the d/D=0.8,but at high flow speeds,the energy harvesting advantage is more obvious for the d/D=0.6.The results of this paper reveal the mechanism of coupled FIV in multi-cylinder structures and demonstrate the energy harvesting characteristics,which provide a theoretical basis for improving structural safety in engineering and the development of green energy such as wind energy.