Hydrodynamics of a flexible cylinder under modulated vortex-induced vibrations |
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| Institution: | 1. State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China;2. Collaborative Innovation Centre for Advanced Ship and Deep-Sea Exploration, Shanghai, 200240, China;3. Department of Mechanical Engineering, University of California at Berkeley, Berkeley, CA, 94720, USA;1. State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong University, Shanghai, China;2. Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration, Shanghai Jiao Tong University, Shanghai, China;3. Department of Mechanical and Structural Engineering and Materials Science, University of Stavanger, Stavanger, Norway;4. SINTEF Ocean, Trondheim, Norway;5. Department of Marine Technology, Centre for Ships and Ocean Structures, Norwegian University of Science and Technology, Trondheim, Norway;6. Statoil, Trondheim, Norway;1. State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China;2. Institute of Polar and Ocean Technology, Institute of Marine Equipment, Shanghai Jiao Tong University, Shanghai, 200240, China;3. Sun Yat-Sen University, Zhuhai, 528478, China;4. China National Offshore Oil Corporation (CNOOC) Research Institute, Chaoyang District, Beijing, 10010, China;1. Department of Mechanical Engineering, Universitat Rovira i Virgili (URV), 43007 Tarragona, Spain;2. Naval Architecture Department, Technical University of Madrid (UPM), 28040 Madrid, Spain;1. State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong University, Shanghai, China;2. Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration, Shanghai 200240, China;3. Marintek, Trondheim, Norway;4. Mechanical Engineering School of Engineering, University of Kansas, 1450 Jayhawk Blvd, Lawrence, KS 66045, USA |
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| Abstract: | Both amplitude modulation and frequency modulation of Vortex-induced Vibration (VIV) are observed in a recent model test of a flexible cylinder under oscillatory flow, but its hydrodynamics has not yet been broached in detail. This paper employs the Forgetting Factor Least Squares (FF-LS) method for identification of time-varying hydrodynamics of a flexible cylinder under modulated VIV. The FF-LS method’s applicability to accurately identify time-varying hydrodynamic coefficients is demonstrated through an elastically mounted rigid cylinder under flow with a given modulated motion. Furthermore, we propose a framework to predict instantaneous amplitude (envelope) and frequency using time-varying hydrodynamic coefficients to establish their analytical relationship. This prediction method is further extended to a highly tensioned flexible cylinder through Fourier series expansion in the spatial domain. By performing the identification procedure for all sampled data of a flexible cylinder undergoing oscillatory flow, we obtain the corresponding time-varying hydrodynamics in the cross-flow direction considering the amplitude and frequency modulation. The results show that, under modulated VIV, hydrodynamic coefficients of the flexible cylinder also show time-varying characteristics. We further investigate differences between identified hydrodynamic coefficients and those obtained from the database of a cylinder with modulated motion under flow. Prediction results using these identified time-varying coefficients reveal that the time-varying excitation coefficients mainly influence the amplitude modulation, and the time-varying added-mass coefficients contain the major information of frequency modulation. These results further suggest including the temporal derivative of the instantaneous amplitude as one determining parameter in building databases to improve the prediction of modulated VIV. |
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| Keywords: | Modulated vortex-induced vibration Time-varying hydrodynamics Forgetting Factor Least Squares method Amplitude modulation Frequency modulation |
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