Response Characteristics And Reduction Of Flow-induced Vibration (FIV) On Multiple Flexible Cylinders

In the offshore engineering,the flow-induced vibration(FIV)of the multiple cylinders is an important factor leading to structural fatigue damage.Researches on the FIV of isolated cylinder have been widely carried out,but the characteristics of FIV on flexible multi-cylinder system have yet to be explored.In this thesis,the indoor large-scale model experiment is carried out.The corresponding Reynolds number ranges from 800-16000.By changing the spacings between the cylinders,the FIV responses of staggered cylinders under 12 different spatial arrangements is analyzed.By installing the helical strake to the multi-cylinder system in certain spatial arrangements,the influence of the flow interference on the effect of the FIV suppression device is analyzed.The experimental data analysis results show that:(1)In staggered spatial arrangements,the FIV response of cylinders with high aspect ratio is characterized with high-modality and multi-mode resonance in both CF and IL directions,and the axial distribution of displacement is in the form of combination of standing wave and traveling wave.(2)For the flexible staggered cylinders,the FIV responses of both upstream and downstream cylinders are affected by complex flow interference.The displacement and Strouhal number of the upstream cylinder are in accordance with the law of “proximity interference”.The response of the downstream cylinder is dominated by the “wake and proximity interference”.Due to the influence of wake interference,the displacement response of downstream cylinder is dominated by the "wake-induced flutter" mechanism.(3)Helical strake suppresses the FIV on staggered upstream cylinder effectively,and the suppression efficiency is barely affected by the smooth or straked downstream cylinder.The corresponding suppression efficiency on the downstream cylinder is remarkably reduced by the influence of the upstream wake.However,response resemble to WIV(wake-induced vibration)is not observed on the staggered downstream cylinder,which would occur on downstream straked cylinder in tandem arrangement.The thesis studies the response characteristics,mechanism and corresponding suppression measures of FIV on multiple flexible cylinders.It can provide theoretical basis and reference for the design and manufacture of the deep-sea flexible cylinder structure like marine riser and mooring lines,as well as the suppression device.It is of great significance for practical engineering.