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An Experimental Investigation on VIV of Cantilevered Flexible Cylinders with Orthotropic Bending Stiffness
Wagner Antonio Defensor Filho

Last modified: 2018-04-24

Abstract


A previous work by Fujarra et al. (2001) revealed a ‘high speed mode branch’ in cantilevered flexible cylinders subjected to VIV. That work studied a flexible cylinder with orthotropic bending stiffness in a recirculating water channel, by setting the ratio between the first natural frequencies, in the in-line and cross-wise directions to the flow, very close to four (fx1/fy1= 4.08). Such a new stable response branch, triggered by upstream hydrodynamic perturbations, was characterized by amplitudes of order of one diameter and by high frequencies components in the energy spectrum, so that it might be also called a “high energy response branch”. In that experiment, the ‘high energy response branch’ extended from reduced velocity circa 12 up to 20, being extinguished by the onset of the second vibration mode lock-in. Aiming at investigating further this intriguing experimental finding, a new experimental campaign has been planned and performed. Four flexible cylinders with distinct orthotropic bending ratios were designed and built. The flexible cylinders were vertically clamped to a 3D load cell. A series of experimental runs were carried out, under ascending and descending flow speeds. The flexible cylinder vibration was captured trough an underwater optical system, placed downstream, by tracking 3D Cartesian coordinates of reflective strips placed all along the span, at spaced cross sections. Besides standard spectral analysis techniques, modal decomposition in time domain and the Hilbert-Huang Transform have been recursively used together to determine amplitude and dominant response frequencies. ‘High energy modal response branches’ have been revealed. This paper aims at presenting some preliminary results and discussing the effect of the bending stiffness ratio on this peculiar vibration response.


Keywords


Flexible Cylinder; VIV; Experiments; Orthotropic Stiffness

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