An experimental study to investigate the validity of the independence principle for vortex-induced vibration of a flexible cylinder over a range of angles of inclination

Abstract

Vortex-induced vibration of an inclined flexible cylinder placed at different angles of inclination with respect to the incoming flow is studied experimentally. The tests were conducted in a re-circulating water tunnel and the angles of inclination were varied from 0° to 60° with increments of 15°. Dynamic response of the system in terms of the amplitude and frequency of oscillations is studied in the reduced velocity range of 2.3–13.9 and the Reynolds number range of Re=320–1610. The validity of the Independence Principle (IP), which states that the behavior of an inclined cylinder is essentially driven by the normal component of the incoming flow velocity, is investigated for this range of reduced velocities and angles of inclination. Based on the onset of oscillations and the magnitudes of modal weight contributions, it is shown that the dynamic response of the system is different from that of a completely vertical cylinder for all angles of inclination larger than 15°. Even for the small angle of inclination of 15°, for the normal reduced velocity range of 2 to 6, where the onset of oscillations and the magnitude of modal weight contributions are similar to those observed for a vertical cylinder, the frequency response and the excited structural modes are different. It is concluded that the onset of oscillations and the magnitudes for the modal weight contributions are not sufficient indicators of the validity of the IP, and that the modal response in terms of the excited structural modes and the frequencies should also be considered. In general, it is shown that the IP is not valid even at reduced velocities where the modal weight contributions are similar for the inclined and vertical cylinders.