Vortex-induced vibrations of a cylinder in the streamwise direction

• Main Author: Cagney, N.
• Published: University College London (University of London) 2013
• Subjects: 621
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.626186

Vortex-induced vibration (VIV) of a circular cylinder has been the focus of extensive research, as it can lead to fatigue damage in a wide range of industrial applications. When the forces induced by the periodic shedding of vortices from a structure in crossflow coincide with one of its natural frequencies, the structure can exhibit large amplitude vibrations. The majority of the work performed in this area has focused exclusively on transverse vibration, while relatively little is known about VIV acting in the streamwise (flow) direction, although this is known to have a strong effect on the overall response of structures with multiple degrees-of-freedom (DOFs). This work aims to characterise the behaviour of the wake and the structural response of a cylinder throughout the streamwise VIV response regime, which is crucial if the wealth of information on the transverse-only case is to be extended to the more practical and complex case of multi-DOF structures. Experiments were performed on a cylinder free to move in the streamwise direction for a range of reduced velocities in a closed-loop water tunnel. Particle-Image Velocimetry (PIV) was used to simultaneously measure the cylinder displacement and the velocity field in the wake, in the Reynolds number range 400 - 5500. The response regime was characterised by two branches, separated by a region of low amplitude vibration, as reported in the literature. Five distinct regions were identified, each of which was discussed in terms of the dominant wake mode, structural response characteristics, velocity profiles and estimates of the strength and trajectories of the shed vortices. In the first branch the wake was found to switch intermittently between the symmetric S-I mode (in which two vortices were shed simultaneously from either side of the cylinder) and the alternate A-II mode (which is similar to the von Karman vortex street observed behind stationary bodies). A criterion was developed which could determine which mode was dominant in a given instantaneous PIV field, and the effect of both modes on the cylinder response and wake characteristics was examined. Multi-modal behaviour was also observed in the second branch. At one value of reduced velocity, the wake could exhibit one of three modes; the A-II, the SA (similar to the A-II mode, with the vortices forming closer to the cylinder base) or the A-IV mode (which was characterised by the shedding of two pairs of counter-rotating vortices). Each mode was associated with a different cylinder response amplitude. The stability of the cylinder response while each mode dominated was examined using phase-portraits, which indicated that the system behaved as a hard oscillator. The forces acting on the cylinder were estimated using two methods, based on the measurements of the cylinder displacement signal and the flow field, respectively. The results found using both methods were in agreement, and the accuracy of the estimates was discussed. It was found that the amplitude of the unsteady drag force was very low between the two response branches, which was thought to be the cause of the reduction in the cylinder vibrations in this region. Finally, the effect of the various wake modes on the amplitude of the fluid forces throughout the response regime was examined. The results presented in this study provide a comprehensive description of the behaviour of the wake and the associated fluid forces throughout the streamwise response regime. The work reveals the inherent differences between the extensively studied case of transverse-only VIV and the streamwise-only case, which is crucial if the wealth of information available on transverse VIV is to be extended to the more practical multi-DOF case.