Transverse displacement phase analysis on 2DOF FIV of rigid cylinders in tandem configurations
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ORIGINAL ARTICLE
Transverse displacement phase analysis on 2DOF FIV of rigid cylinders in tandem configurations P. H. Todesco Pereira1 · F. Cenci1 · A. L. C. Fujarra1 Received: 21 February 2019 / Accepted: 8 February 2020 © The Japan Society of Naval Architects and Ocean Engineers (JASNAOE) 2020
Abstract With the development of offshore oil production and the resulting application of riser pipes, it is necessary to get a better understanding of the flow-induced vibrations (FIV) phenomenon involving multiple cylindrical structures. In this scenario, little is available concerning the phase difference between the transverse displacements of two long rigid cylinders (aspect ratio of 30) in tandem and free to vibrate due to the flow incidence. Aiming to fill this gap, experiments were carried out at the towing tank of the Technological Research Institute (IPT) for two similar rigid cylinders in a tandem arrangement. Both cylinders were elastically supported with two degrees of freedom (2DOF), the in-line and transverse direction. Different tandem distances were tested ( T = 2D , 2.5D, 3D, 4D and 6D) throughout the complete excitation of the lock-in range for the upstream cylinder, i.e., reduced velocities ranged from 3 to 13, which corresponds to Reynolds numbers from 3 × 103 to 3 × 104 . This work found the upstream cylinder having a qualitatively similar response compared with the isolated one. The downstream cylinder exhibited large response amplitudes compared with the upstream one in the lower branch of vibration. Lastly, the phase difference between transverse displacements of both cylinders was classified according to five phase patterns, two of them apparently related to the initial and upper branches of vibration in all cases. Keywords Flow-induced vibration · Long rigid cylinders · Tandem · Two degrees of freedom · Towing tank list of symbols Ax Response amplitude of displacements in the in-line direction Ay Response amplitude of displacements in the transverse direction D External diameter of the cylinders fdy Non-dimensional frequency of displacements in the transverse direction fny Natural frequency of displacements in the transverse direction fx Response frequency of displacements in the in-line direction fy Response frequency of displacements in the transverse direction
* A. L. C. Fujarra [email protected]
L Immersed length of the cylinders m∗ Mass ratio or reduced mass ms Structural mass in oscillation md Mass of the displaced fluid Re Reynolds number rms Root mean square value of a time-dependent quantity T Gap between cylinders in tandem U Flow velocity U ∗ Reduced velocity x1 and x2 In-line displacements of the upstream and downstream cylinders y1 and y2 Transverse displacements of the upstream and downstream cylinders 𝜃y1,y2 Instantaneous phase angle between transverse displacements 𝜃peak Prevalent phase angle 𝜁 Structural damping coefficient
P. H. Todesco Pereira [email protected] F. Cenci [email protected] 1
Federal University of Santa Catarina, Flor
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