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Research Papers

Experimental Comparison of Two Degrees-of-Freedom Vortex-Induced Vibration on High and Low Aspect Ratio Cylinders with Small Mass Ratio

[+] Author and Article Information
Rodolfo T. Gonçalves

e-mail: rodolfo_tg@tpn.usp.br

Guilherme F. Rosetti

e-mail: guilherme.feitosa@tpn.usp.br

André L. C. Fujarra

e-mail: afujarra@usp.br
TPN – Numerical Offshore Tank,
Department of Naval Architecture
and Ocean Engineering,
Escola Politécnica – University of São Paulo,
Avenue Professor Mello Moraes, 2231,
Cidade Universitária,
São Paulo, SP, 05508-900, Brazil

Guilherme R. Franzini

e-mail: gfranzini@usp.br

César M. Freire

e-mail: cesar.freire@usp.br

Julio R. Meneghini

e-mail: jmeneg@usp.br
NDF – Fluid & Dynamics Research Group,
Department of Mechanical Engineering,
Escola Politécnica – University of São Paulo,
São Paulo, SP, 05508-900, Brazil

1Corresponding author.

Contributed by the Design Engineering Division of ASME for publication in the JOURNAL OF VIBRATION AND ACOUSTICS. Manuscript received March 4, 2011; final manuscript received December 13, 2011; published online October 29, 2012. Assoc. Editor: Massimo Ruzzene.

J. Vib. Acoust 134(6), 061009 (Oct 29, 2012) (7 pages) doi:10.1115/1.4006755 History: Received March 04, 2011; Revised December 13, 2011

Vortex-induced motion (VIM) is a specific way for naming the vortex-induced vibration (VIV) acting on floating units. The VIM phenomenon can occur in monocolumn production, storage and offloading system (MPSO) and spar platforms, structures presenting aspect ratio lower than 4 and unity mass ratio, i.e., structural mass equal to the displaced fluid mass. These platforms can experience motion amplitudes of approximately their characteristic diameters, and therefore, the fatigue life of mooring lines and risers can be greatly affected. Two degrees-of-freedom VIV model tests based on cylinders with low aspect ratio and small mass ratio have been carried out at the recirculating water channel facility available at NDF-EPUSP in order to better understand this hydro-elastic phenomenon. The tests have considered three circular cylinders of mass ratio equal to one and different aspect ratios, respectively L/D = 1.0, 1.7, and 2.0, as well as a fourth cylinder of mass ratio equal to 2.62 and aspect ratio of 2.0. The Reynolds number covered the range from 10 000 to 50 000, corresponding to reduced velocities from 1 to approximately 12. The results of amplitude and frequency in the transverse and in-line directions were analyzed by means of the Hilbert-Huang transform method (HHT) and then compared to those obtained from works found in the literature. The comparisons have shown similar maxima amplitudes for all aspect ratios and small mass ratio, featuring a decrease as the aspect ratio decreases. Moreover, some changes in the Strouhal number have been indirectly observed as a consequence of the decrease in the aspect ratio. In conclusion, it is shown that comparing results of small-scale platforms with those from bare cylinders, all of them presenting low aspect ratio and small mass ratio, the laboratory experiments may well be used in practical investigation, including those concerning the VIM phenomenon acting on platforms.

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References

Figures

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Fig. 1

Example of different apparatuses for VIV study. Source: Flemming & Williamson [21].

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Fig. 2

Definitions of two degrees-of-freedom experiments, where X is the flow, U, direction (in-line) and Y is the transverse direction (cross-flow). Source: Jauvtis & Williamson [14].

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Fig. 3

Main characteristics of the experimental apparatus, on the left, rigid cylinder mounted in a pendulum, and on the right, rigid cylinder mounted in a cantilevered beam

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Fig. 4

Nondimensional results for motions in the transverse direction of two degrees-of-freedom VIV on cylinders with low aspect ratio (L/D≤2.0) and small mass ratio (m*≤3.0)

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Fig. 5

Nondimensional results for motions in the in-line direction of two degrees-of-freedom VIV on cylinders with low aspect ratio (L/D≤2.0) and small mass ratio (m*≤3.0)

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Fig. 6

Nondimensional results of transverse oscillation frequency and natural transverse frequency in still water for two degrees-of-freedom VIV on cylinders with low aspect ratio (L/D≤2.0) and small mass ratio (m*≤3.0)

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Fig. 7

Nondimensional results of in-line oscillation frequency and transverse one for two degrees-of-freedom VIV on cylinders with low aspect ratio (L/D≤2.0) and small mass ratio (m*≤3.0)

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Fig. 8

Comparison between nondimensional results for motions in the transverse direction of two degrees-of-freedom VIV on cylinders with mass ratio approximately equal to the unity (m*=1.00)

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Fig. 9

Comparison between nondimensional results for motions in the in-line direction of two degrees-of-freedom VIV on cylinders with mass ratio approximately equal to the unity (m*=1.00)

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Fig. 10

Comparison between nondimensional results of transverse oscillation frequency and transverse natural frequency in still water for two degrees-of-freedom VIV on cylinders with mass ratio approximately equal to the unity (m*=1.00)

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Fig. 11

Comparison between nondimensional results for motions in the transverse direction of two degrees-of-freedom VIV on cylinders with small mass ratio (m*≤3.0)

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Fig. 12

Comparison between nondimensional results for motions in the in-line direction of two degrees-of-freedom VIV on cylinders with small mass ratio (m*≤3.0)

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Fig. 13

Comparison between nondimensional results of transverse oscillation frequency and transverse natural frequency in still water for two degrees-of-freedom VIV on cylinders with small mass ratio (m*≤3.0)

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