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

2012 ◽  
Vol 134 (6) ◽  
Author(s):  
Rodolfo T. Gonçalves ◽  
Guilherme F. Rosetti ◽  
André L. C. Fujarra ◽  
Guilherme R. Franzini ◽  
César M. Freire ◽  
...  
Keyword(s):  
Aspect Ratio ◽  
Degrees Of Freedom ◽  
Small Mass ◽  
Experimental Comparison ◽  
Small Scale ◽  
Mass Ratio ◽  
Vortex Induced Vibration ◽  
Two Degrees Of Freedom ◽  
Low Aspect Ratio ◽  
Aspect Ratios

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.

Experimental Comparisons to Assure the Similarity Between VIM (Vortex-Induced Motion) and VIV (Vortex-Induced Vibration) Phenomena

2011 ◽  
Author(s):  
Rodolfo T. Gonc¸alves ◽  
Ce´sar M. Freire ◽  
Guilherme F. Rosetti ◽  
Guilherme R. Franzini ◽  
Andre´ L. C. Fujarra ◽  
...  
Keyword(s):  
Aspect Ratio ◽  
Water Channel ◽  
Analytical Models ◽  
Mass Ratio ◽  
Water Displacement ◽  
Transform Method ◽  
Number Range ◽  
Vortex Induced Vibration ◽  
Low Aspect Ratio ◽  
Induced Motion

Vortex-Induced Motion (VIM) is another way to denominate the Vortex-Induced Vibration (VIV) in floating units. The main characteristics of VIM in such structures are the low aspect ratio (L/D < 4.0) and the unity mass ratio (m* = 1.0, i.e. structural mass equal water displacement). The VIM can occur in MPSO (Monocolumn Production, Storage and Offloading System) and spar platforms. These platforms can experience motion amplitudes of around their characteristic diameters. In such cases, the fatigue life of mooring and riser systems can be greatly reduced. Typically, the VIM model testing campaigns are carried out in the Reynolds range between 200,000 and 400,000. VIV model tests with low aspect ratio cylinders (L/D = 1.0, 1.7 and 2.0) and unity mass ratio (m* = 1.0) have been carried out at the Circulating Water Channel facility available at NDF/EPUSP. The Reynolds number range covered in the experiments was between 10,000 and 50,000. The characteristic motions (in the transverse and in-line direction) were obtained using the Hilbert-Huang Transform method (HHT) and then compared with results obtained in experiments found in the literature. The aim of this investigation is to definitely establish the similarity between the VIM and VIV phenomena, making possible to increase the understanding of both and, at same time, allowing some analytical models developed for VIV to be applied to the VIM scenario on spar and monocolumn platforms, logically under some adaption.

Experimental Study on Vortex-Induced Vibration of Floating Circular Cylinders With Low Aspect Ratio

2014 ◽  
Author(s):  
Rodolfo T. Gonçalves ◽  
André L. C. Fujarra
Keyword(s):  
Experimental Study ◽  
Aspect Ratio ◽  
Water Channel ◽  
Circular Cylinders ◽  
Structural Damping ◽  
End Effects ◽  
Vortex Induced Vibration ◽  
Low Aspect Ratio ◽  
Aspect Ratios ◽  
Ratio Range

Experiments regarding vortex-induced vibration on floating circular cylinders with low aspect ratio were carried out in a recirculation water channel. The floating circular cylinders were elastic supported by a set of linear springs to provide low structural damping on the system. Eight different aspect ratios were tested, namely L/D = 0.2, 0.3, 0.4, 0.5, 0.75, 1.0, 1.5 and 2.0. These aspect ratios were selected to cover the aspect ratio range of the main offshore circular platforms, such as spar and monocolumn. The aims were understanding the VIM of such platforms; due to this, the cylinders were floating, or m* = 1. The range of Reynolds number covered 2,800 < Re < 55,400. The amplitude results showed a decrease in amplitude with decreasing aspect ratio in both directions. The frequency results confirm a different behavior for cylinders with L/D ≤ 0.5; in these cases, the cylinder free-end effects were predominant. The resonant behaviour was no longer observed for L/D ≤ 0.2. The decrease in Strouhal number with decreasing aspect ratio is also verified. All the results presented here complement the work presented previously for stationary circular cylinder with low aspect ratio presented by Gonçalves et al. (2013), Experimental Study on Flow around Circular Cylinders with Low Aspect Ratio, OMAE2013-10454.

Coriolis effect on the vibration of flat rotating low aspect ratio cantilever plates

1981 ◽  
Vol 16 (2) ◽  
pp. 97-106 ◽  
Author(s):  
S Sreenivasamurthy ◽  
V Ramamurti
Keyword(s):  
Aspect Ratio ◽  
Degrees Of Freedom ◽  
Mode Frequency ◽  
Coriolis Effect ◽  
Skew Angle ◽  
Noticeable Effect ◽  
Torsional Mode ◽  
Low Aspect Ratio ◽  
Aspect Ratios ◽  
Skew Angles

The Coriolis effect on the first bending and first torsional frequencies of flat rotating low aspect ratio cantilever plates has been investigated using finite element method. The cantilever plate has been modelled using plane triangular shell elements with three nodes and eighteen degrees of freedom. Three typical skew angles (0, 45, and 90 degrees) and two aspect ratios (1 and 2) are considered in the analysis. In addition to the Coriolis effect other effects, namely the geometric stiffness and the supplementary stiffness, have been considered. The mass and stiffness matrices have been derived using area coordinates. It has been found that the effect of including Coriolis effect is to lower the first two frequencies. This effect is negligible when the skew angle is 90 degrees. In the other two cases, skew of 0 and 45 degrees, there is a noticeable effect on the first torsional mode frequency when the aspect ratio is unity and on the first bending mode frequency when the aspect ratio is 2. An increase in the Coriolis effect is observed when the aspect ratio is increased from 1 to 2, with the skew angles of 0 and 45 degrees and a decrease when the skew angle is 90 degrees. The difference between the two frequencies (with and without Coriolis effect) becomes more and more noticeable as the rotational speed increases.

Influence of Stiffness Ratio on Vortex-Induced Vibration of Cylinder With Low Aspect Ratio

2018 ◽  
Author(s):  
Dennis M. Gambarine ◽  
Luiz E. B. Minioli ◽  
Rodolfo T. Gonçalves ◽  
André M. Kogishi ◽  
André L. C. Fujarra
Keyword(s):  
Aspect Ratio ◽  
Degrees Of Freedom ◽  
Offshore Structures ◽  
Elastic Base ◽  
The Other ◽  
Special Focus ◽  
Stiffness Ratio ◽  
Force Coefficient ◽  
Vortex Induced Vibration ◽  
Low Aspect Ratio

Concern over the Vortex-induced Motions (VIM) acting on offshore structures, with special focus on monocolumn and spar platforms, mooring systems have crucial importance on system movements; the system has thus been transformed into a concept study herein. A floating and rigid circular cylinder with low aspect ratio (L/D = 2) was used in the experiments carried out to investigate the influence of stiffness ratio (kx/ky) on Vortex-Induced Vibration (VIV). The cylinder was mounted in an elastic base composed of four springs with differences in in-line and transverse stiffness, defining: kx/ky ≅ 0.3, 0.5, 1.0, 2.0 and 3.0. The Reynolds number analysed belongs to a range between 0.2 · 104 and 2 · 104. Some good qualitative and quantitative agreements are found for in-line amplitudes, and higher kx/ky systems demonstrate significant oscillation for low relative velocities. This variation can be seen and justified when the XY-plane trajectories were plotted. When kx/ky is defined as 2 and 3, the traditional VIV 8-shape is illustrated for reduced velocities between 3 and 6. In contrast, the other stiffness systems do not show significant movements and, consequently, a negligible XY shape. Roll and pitch degrees of freedom have shown the motions coupled with the transverse and the in-line motions respectively. Moreover, the yaw motion did not present considerable angles. kx/ky = 2 has presented the highest lift force coefficients, without a great difference from the other aspects ratios, though. The drag force coefficient showed constant values for kx/ky = 2 and 3, the smallest results were observed for the system kx/ky = 3.

Experimentally investigated vortex-induced vibration of a high aspect ratio and small mass ratio circular cylinder oscillating in low reduced velocity flows

2021 ◽  
Vol 238 ◽  
pp. 109735
Author(s):  
Hassan el Sheshtawy ◽  
Simon Tödter ◽  
Ould el Moctar ◽  
Jens Neugebauer ◽  
Thomas E. Schellin
Keyword(s):  
Aspect Ratio ◽  
Circular Cylinder ◽  
High Aspect Ratio ◽  
Small Mass ◽  
Mass Ratio ◽  
Vortex Induced Vibration

Two Degrees of Freedom Vortex-Induced Vibration Responses With Zero Mass and Damping at Low Reynolds Number

2013 ◽  
Author(s):  
Kintak Raymond Yu ◽  
Alexander Hay ◽  
Dominique Pelletier ◽  
Simon Corbeil-Létourneau ◽  
Shahin Ghasemi ◽  
...  
Keyword(s):  
Reynolds Number ◽  
Parameter Space ◽  
Degrees Of Freedom ◽  
Low Reynolds Number ◽  
Mass Ratio ◽  
Vortex Induced Vibration ◽  
Two Degrees Of Freedom ◽  
Zero Mass ◽  
Vortex Induced Vibrations ◽  
Low Reynolds

Vortex-induced vibration is an important phenomenon for offshore engineering. For applications like the piping in the deep water oil exploration projects, the mass ratios can be of order of one [1]. Hence, there is a practical need to understand the effects of low mass ratio on vortex-induced vibrations to enhance design safety. The main purpose of this study is to numerically explore the two degrees of freedom (transverse and streamwise) responses of vortex-induced vibrations of a cylinder at low Reynolds number for the limiting case of zero mass ratio and zero damping. We aim to characterize the responses. In particular, we focus on determining the maximum amplitude values. It is a continuation from the work of Etienne and Pelletier who studied such behaviors at very low Reynolds number (Re < 50) [2]. We investigate the responses in the following parameter space: Reynolds number (75 ≤ Re ≤ 175), reduced velocity (5.0 ≤ Ur ≤ 11.0) and mass ratio (m* = {0, 0.1, 1}) with a fully coupled fluid-structure interaction numerical model based on the finite element method. Our results are generally in accordance with those from previous works for the displacement trajectories, force phase diagram, and the trends in frequency response and oscillation amplitude. The maximum transverse amplitude is found to be around 0.9 in the studied parameter space. In particular, with zero mass ratio, the maximum transverse amplitude starts to occur at values of reduced velocity higher than 6.5 for Reynolds number larger than 150. This is in contrast to the results of Etienne and Pelletier [2] who found that the maximum transverse amplitude always occurs at the reduced velocity of 6.5 for Reynolds number less than 50. Furthermore, with zero mass ratio, the maximum transverse amplitude increases when the Reynolds number increases. This behavior differs from what was suggested by Williamson and Govardhan [3] for a cylinder oscillating only in the transverse direction at Reynolds numbers in the range of 85 to 200. They found that the Reynolds number has no influence on the maximum transverse amplitude. We do not notice any response branching in this parameter space. However, the results in the present work clearly consist of two distinct characteristics. This indicates that the investigated mass ratio values encompass the critical mass ratio; whose value is estimated to be around 0.1 to 0.2.

Vortex-induced vibration of a rising and falling cylinder

2010 ◽  
Vol 662 ◽  
pp. 352-383 ◽  
Author(s):  
M. HOROWITZ ◽  
C. H. K. WILLIAMSON
Keyword(s):  
Degrees Of Freedom ◽  
Critical Mass ◽  
Vortex Formation ◽  
Small Mass ◽  
Added Mass ◽  
The Body ◽  
Spring Stiffness ◽  
Mass Ratio ◽  
Vortex Induced Vibration ◽  
Direct Measurements

In this study, we investigate the dynamics of a freely rising and falling cylinder. This is, in essence, a vortex-induced vibration (VIV) system comprising both transverse (Y) and streamwise (X) degrees-of-freedom (d.o.f.), but with zero spring stiffness and zero damping. This problem represents a limiting case among studies in VIV, and is an extension of recent research of elastically mounted bodies having very low spring stiffness, as well as bodies with very low mass and damping. We find that if the mass ratio (where m* = cylinder mass/displaced fluid mass) is greater than a critical value, m*crit = 0.545, the body falls or rises with a rectilinear trajectory. As the mass ratio is reduced below m*crit = 0.545, the cylinder suddenly begins to vibrate vigorously and periodically, with a 2P mode of vortex formation, as reported in the preliminary study of Horowitz & Williamson (J. Fluids Struct. vol. 22, 2006, pp. 837–843). The similarity in critical mass between freely rising and elastically mounted bodies is unexpected, as it is known that the addition of streamwise vibration can markedly affect the response and vortex formation in elastically mounted systems, which would be expected to modify the critical mass. However, we show in this paper that the similarity in vortex formation mode (2P) between the freely rising body and the elastically mounted counterpart is consistent with a comparable phase of vortex dynamics, strength of vortices, amplitudes and frequencies of motion and effective added mass (CEA). All of these similarities result in comparable values of critical mass. The principal fact that the 2P mode is observed for the freely rising body is an interesting and consistent result; based on the previous VIV measurements, this is the only mode out of the known set {2S, 2P, 2T} to yield negative effective added mass (CEA < 0), which is a condition for vibration of a freely rising body. In this paper, we deduce that there exists only one possible two degree-of-freedom elastically mounted cylinder system, which can be used to predict the dynamics of freely rising bodies. Because of the symmetry of the vortex wake, this system is one for which the natural frequencies are fNX = 2fNY. Although this seems clear in retrospect, previous attempts to predict critical mass did not take this into account. Implementing such an elastic system, we are able to predict vibration amplitudes and critical mass (m*crit = 0.57) for a freely rising cylinder in reasonable agreement with direct measurements for such a rising body, and even to predict the Lissajous figures representing the streamwise–transverse vibrations for a rising body with very small mass ratios (down to m* = 0.06), unobtainable from our direct measurements.

Experimental Study on Flow-Induced Vibration of Floating Squared Section Cylinders With Low Aspect Ratio: Part I — Effects of Incidence Angle

2015 ◽  
Author(s):  
Rodolfo T. Gonçalves ◽  
Dênnis M. Gambarine ◽  
Felipe P. Figueiredo ◽  
Fábio V. Amorim ◽  
André L. C. Fujarra
Keyword(s):  
Aspect Ratio ◽  
Incidence Angle ◽  
Ocean Basin ◽  
Circular Cylinders ◽  
Structural Damping ◽  
Flow Induced Vibration ◽  
Double Frequency ◽  
Low Aspect Ratio ◽  
Aspect Ratios ◽  
Elastically Supported

Experiments regarding flow-induced vibration on floating squared section cylinders with low aspect ratio were carried out in an ocean basin with rotating-arm apparatus. The floating squared section cylinders were elastically supported by a set of linear springs to provide low structural damping to the system. Three different aspect ratios were tested, namely L/D = 1.0, 2.0 and 3.0, and two different incidence angles, namely 0 and 45 degrees. The aims were to understanding the flow-induced vibration around single columns of multi-column platforms, such as semi-submersible and TLP. VIV on circular cylinders were also carried out to compare the results. The range of Reynolds number covered was 2,000 < Re < 27,000. The in-line and transverse amplitude results showed to be higher for 45-degree incidence compared with 0-degree, but the maximum amplitudes for squared section cylinders were lower compared with the circular ones. The double frequency in the in-line motion was not verified as in circular cylinders. The yaw amplitudes cannot be neglected for squared section cylinders, maximum yaw amplitudes around 10 degrees were observed for reduced velocities up to 15.

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