Flow-induced vibrations of three and four long flexible cylinders in tandem arrangement: An experimental study

Multiple flexible cylindrical structures arranged in tandem are commonly used in various fields of engineering. Up to now, limited information is available about the flow-induced vibrations (FIVs) of more than two tandem flexible cylinders. Although some FIV characteristics of multiple tandem flexible cylinders can be studied by considering a tandem flexible cylinder pair, there are other characteristics remain unrevealed which require further investigations. This paper presents an experimental study on the FIVs of three and four flexible cylinders in tandem arrangement. The flexible cylinder models are identical with a mass ratio of 1.9 and a length-to-diameter ratio of 350. The centre-to-centre spacings between the cylinders are kept constant as 6D. The towing speed ranges from 0.05 m/s to 1 m/s and the corresponding Reynolds number lies in the range of 800-16 000. It is found that the presence and vibrations of the downstream cylinders have almost negligible effects on the responses of the most upstream cylinder. Wake-induced vibration (WIV) type responses are observed at high reduced velocities for the downstream cylinders. Due to the shielding effects of the upstream cylinder/cylinders and the influence of the second WIV regime, the transitions between adjacent frequency synchronisation regions and neighbouring vibration modes of the downstream cylinders are delayed to higher reduced velocities. Most of the cylinder responses are dual resonant except for the most downstream cylinder in the three-cylinder system which has identical dominant frequencies in the in-line and cross-flow directions. Combined standing wave and travelling wave responses and standing wave dominated responses are identified for three and four tandem flexible cylinders undergoing FIVs.