Frequency lock-in is caused by coupled-mode flutter |
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| Institution: | 1. IRPHE, Marseille, France.;2. \"Cornell University, Ithaca, NY, USA;1. School of Mathematics, Physics & Information Science, Zhejiang Ocean University, Zhoushan 316000, China;2. School of Computing, Engineering and Mathematics, University of Western Sydney, Locked Bag 1797, Penrith 2751, NSW, Australia;3. Institute for Infrastructure Engineering, University of Western Sydney, Locked Bag 1797, Penrith 2751, NSW, Australia;4. State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology, Dalian 116023, China;5. School of Civil, Environmental and Mining Engineering, The University of Western Australia, 35 Stirling Highway, Crawley 6009, WA, Australia;1. Highway College, Chang’an University, Xi’an, Shaanxi, 710064, China;2. Department of Bridge Engineering, Tongji University, Shanghai, 200092, China;3. State Key Laboratory of Disaster Reduction in Civil Engineering, Tongji University, Shanghai, 200092, China;4. Key Laboratory of Transport Industry of Bridge Wind Resistance Technology, Tongji University, Shanghai, 200092, China;1. Department of Mechanics, Huazhong University of Science and Technology, Wuhan 430074, China;2. Hubei Key Laboratory for Engineering Structural Analysis and Safety Assessment, Wuhan 430074, China |
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| Abstract: | The mechanism underlying the lock-in of frequencies in flow-induced vibrations is analysed using elementary linear dynamics. Considering the case of lock-in in vortex-induced vibrations (VIV), we use a standard wake oscillator model, as in previous studies, but in its simplest form where all nonlinear terms and all dissipative terms are neglected. The stability of the resulting linear system is analysed, and a range of coupled-mode flutter is found. In this range, the frequency of the most unstable mode is found to deviate from the Strouhal law when the frequency of the wake oscillator approaches that of the free cylinder motion. Simultaneously the growth rate resulting from coupled-mode flutter increases, which would lead to higher vibration amplitudes. The extent of the range of lock-in is then compared with experimental data, showing a good agreement. It is therefore stated that the lock-in phenomenon, such as in VIV, is a particular case of linear coupled-mode flutter. |
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