Observed mode shape effects on the vortex-induced vibration of bending dominated flexible cylinders simply supported at both ends

Abstract

J. M. (2017). Observed mode shape effects on the vortex-induced vibration of bending dominated flexible cylinders simply supported at both ends. Abstract 8The structural mode excitation of bending-dominated flexible cylinders undergoing vortex-induced vibrations was investigated using multivariate analysis of the excited empirical modes. The response of the bending-dominated cylinders was compared with the response of a tension-dominated cylinder using the same analysis technique. Experiments were conducted in a recirculating flow channel with a uniform free stream with Reynolds numbers between 650 and 5500. Three bending-dominated cylinders were tested with varying stiffness in the cross-flow and in-line directions of the cylinder in order to produce varying structural mode shapes associated with a fixed 2:1 (in-line:cross-flow) natural frequency ratio. A fourth cylinder with natural frequency characteristics determined through applied axial tension was also tested. The spanwise in-line and cross-flow responses of the flexible cylinders were measured through motion tracking with high-speed cameras. Global smooth-orthogonal decomposition was applied to the spatio-temporal response for mode identification. Measured responses are compared with the analytic response of a beam subjected to a uniform periodic loading. Both the analytic and experimental results show that for excitation of low mode numbers, the cylinder is unlikely to oscillate with an even mode shape in the in-line direction due to the symmetric drag loading, even when the system is tuned to have an even mode at the expected frequency of vortex shedding. In addition, no mode shape changes were observed in the in-line direction unless a mode change occurs in the cross-flow direction, implying that the in-line response is a forced response dependent on the cross-flow response. An even mode oscillation (i.e. second mode) in the in-line direction is observed to be excited in the tensioned cylinder, however this is only observed in a hysteretic response region, resulting in a pedaling mode response. The results confirm observations from previous field and laboratory experiments, while demonstrating how structural mode shape can affect vortex-induced vibrations.

Vortex-induced vibrations of a flexible cylinder at subcritical Reynolds number

“…Such a phenomenon was also observed at higher for symmetrical systems (Gedikli et al. 2018). At higher , both mono-frequency and multi-frequency vibrations were reported for a flexible cylinder placed in a uniform current (e.g.…”

“…In (d), the natural frequencies of the first four structural modes in vacuum ( f nat ) are indicated by plain grey lines; the corrected natural frequencies, with an added-mass coefficient of 1, are denoted by grey dash-dotted lines; and the vortex shedding frequency identified by Buffoni (2003) is indicated by a red dashed line. (Gedikli et al 2018). At higher Re, both mono-frequency and multi-frequency vibrations were reported for a flexible cylinder placed in a uniform current (e.g.…”

“…The present work aims at extending the analysis of subcritical-Re VIV to flexible cylinders. Flexible-cylinder VIV have been well documented for Re > Re c (Chaplin et al 2005;Bourguet, Karniadakis & Triantafyllou 2011;Wu, Ge & Hong 2012;Huera-Huarte, Bangash & González 2014;Gedikli, Chelidze & Dahl 2018;Seyed-Aghazadeh, Edraki & Modarres-Sadeghi 2019). They are still driven by the lock-in mechanism but the flexibility of the body and its distributed interaction with the flow may lead to an increased complexity of the system behaviour.…”

Bending oscillations of a cylinder freely falling in still fluid

“…However, studies also revealed the occurrence of time-periodic deformations of the body associated with vortex shedding, for instance in the case of a flexible plate clutched at its centre (Pfister, Marquet & Carini 2019). Also, a flexible cylinder held from its ends and placed in flowing fluid exhibits vibrations that are driven by the synchronization between body deformation and flow unsteadiness (Chaplin et al 2005; Bourguet, Karniadakis & Triantafyllou 2011; Gedikli, Chelidze & Dahl 2018; Seyed-Aghazadeh, Edraki & Modarres-Sadeghi 2019). These vibrations, often referred to as vortex-induced vibrations, may involve several structural modes, sometimes simultaneously (Bourguet, Karniadakis & Triantafyllou 2013).…”

Investigation on Vortex-Induced Vibration Experiment of a Standing Variable-Tension Deepsea Riser Based on BFBG Sensor Technology

“…Only standing waves existed under the experimental cases. At T = 19.8–98.0 N, when 0.3 m/s < U < 0.6 m/s, as top tension increased, the riser’s vibration amplitude gradually decreased; as external flow velocity increased, the riser’s vibration changed from first-order mode to second-order mode [28,29] and the vibration intensity increased. The riser’s standing wave effects across the external flow velocities and top tensions were compared.…”