Experimental and Numerical Analysis of Forced Motion of a Circular Cylinder

2011 ◽  
Author(s):  
Decao Yin ◽  
Carl M. Larsen
Keyword(s):  
Influence Factor ◽  
Amplitude Ratio ◽  
Cross Flow ◽  
Flexible Beam ◽  
Hydrodynamic Coefficients ◽  
Forced Motion ◽  
Vortex Pattern ◽  
Severe Fatigue ◽  
Vortex Induced Vibrations ◽  
Uniform Current

Vortex induced vibrations (VIV) of long, slender marine structures may cause severe fatigue damage. However, VIV is still not fully understood, which calls for further research on this topic. This paper discusses results from experimental and numerical investigations of forces on rigid cylinders subjected to combined in-line (IL) and cross-flow (CF) motions, and it aims at improving the understanding of the interaction between IL and CF response components. Model tests with a long flexible beam were conducted at MARINTEK for the Norwegian Deepwater Programme (NDP). The model was 38 m long and it was towed horizontally so that both uniform and linear sheared current profiles could be obtained. Orbits for cross section motions at selected positions along the beam were identified in these tests. Forced motion experiments using these orbits were later carried out in the Marine Cybernetic Laboratory at Norwegian University of Science and Technology (NTNU). A 2 m long rigid cylinder was towed horizontally and forced to follow the measured orbits with identical amplitude ratio, non-dimensional frequency and Reynolds number as for the flexible beam tests. Parts of the results from these tests were published by Yin & Larsen in 2010. In this paper results from an investigation of trajectories for six positions along the beam in a uniform current condition will be shown. Three orbits have nearly the same CF amplitude ratio at the primary CF frequency, and the other three have similar IL amplitude ratio at the primary IL frequency, which is twice the CF frequency. Hydrodynamic coefficients have been found from experiments and numerical computations were carried out to find vortex shedding patterns for these cases. The main conclusions are that the IL motion component is a significant influence factor, and that higher order displacement components are more pronounced in IL direction than CF. Significant displacements in IL direction at 6 times the primary CF frequency were also observed, the ‘2T’ vortex pattern was captured when strong IL motion components were present. It is also seen that hydrodynamic coefficients should be found for combined CF and IL orbits and thereby improve the empirical models for prediction of VIV.

On Determination of VIV Coefficients Under Shear Flow Condition

2010 ◽  
Author(s):  
Decao Yin ◽  
Carl M. Larsen
Keyword(s):  
Amplitude Ratio ◽  
Cross Flow ◽  
Flexible Beam ◽  
Hydrodynamic Coefficients ◽  
Periodic Patterns ◽  
Research Activity ◽  
Severe Fatigue ◽  
Vortex Induced Vibrations ◽  
Load Effects ◽  
Offshore Oil Industry

Long marine risers exposed to ocean currents will experience vortex induced vibrations (VIV), which may cause severe fatigue damage. VIV is, however, generally less understood than other load effects. The offshore oil industry has therefore supported an intensive research activity on VIV during the last two decades. High mode VIV model tests with long flexible riser models were initiated by the Norwegian Deepwater Programme (NDP). A 38 m horizontally towed instrumented riser was tested in uniform and linearly sheared current profiles with varying towing speed. A second series of experiments has been conducted with a motion-controlled rigid cylinder in order to find the hydrodynamic coefficients for realistic cross-section trajectories. The pipe was forced to follow periodic patterns found from the NDP tests with flexible beam. The Reynolds’ number and the non-dimensional frequency, as well the amplitude ratio was kept identical for both types of tests, ensuring that the flow conditions for these two experiments remain the same. The hydrodynamic coefficients calculated from natural trajectories show a general agreement with pure harmonic forced motion tests. A slight change of excitation regions was, however, found for cross-flow response. Another observation is that in-line excitation force coefficients have much higher values than found from pure in-line test.

Improved In-Line Vortex-Induced Vibrations Prediction for Combined In-Line and Cross-Flow Vortex-Induced Vibrations Responses

2017 ◽  
Vol 140 (3) ◽  
Author(s):  
Decao Yin ◽  
Elizabeth Passano ◽  
Carl M. Larsen
Keyword(s):  
Cross Flow ◽  
Hydrodynamic Coefficients ◽  
Marine Structures ◽  
Flexible Pipe ◽  
Flexible Beams ◽  
Forced Motion ◽  
Pipe Model ◽  
Vortex Induced Vibrations ◽  
Flow Vortex ◽  
Semi Empirical

Slender marine structures are subjected to ocean currents, which can cause vortex-induced vibrations (VIV). Accumulated damage due to VIV can shorten the fatigue life of marine structures, so it needs to be considered in the design and operation phase. Semi-empirical VIV prediction tools are based on hydrodynamic coefficients. The hydrodynamic coefficients can either be calculated from experiments on flexible beams by using inverse analysis or theoretical methods, or obtained from forced motion experiments on a circular cylinder. Most of the forced motion experiments apply harmonic motions in either in-line (IL) or crossflow (CF) direction. Combined IL and CF forced motion experiments are also reported. However, measured motions from flexible pipe VIV tests contain higher order harmonic components, which have not yet been extensively studied. This paper presents results from conventional forced motion VIV experiments, but using measured motions taken from a flexible pipe undergoing VIV. The IL excitation coefficients were used by semi-empirical VIV prediction software vivana to perform combined IL and CF VIV calculation. The key IL results are compared with Norwegian Deepwater Programme (NDP) flexible pipe model test results. By using present IL excitation coefficients, the prediction of IL responses for combined IL and CF VIV responses is improved.

Importance of Added Mass for the Interaction Between IL and CF Vibrations of Free Spanning Pipelines

2011 ◽  
Author(s):  
Ida M. Aglen ◽  
Carl M. Larsen
Keyword(s):  
Cross Sections ◽  
Amplitude Ratio ◽  
Large Diameter ◽  
Cross Flow ◽  
Added Mass ◽  
Flexible Beam ◽  
Hydrodynamic Coefficients ◽  
Cross Sectional ◽  
Mass Coefficient ◽  
The Stability

The importance of cross-flow (CF) response generated by vortex induced vibrations (VIV) of free spanning pipelines has long been recognised. The significance of in-line (IL) vibrations has recently been understood and hence also been subjected to research. The combined effect of CF and IL vibrations is, however, still not fully described. This paper highlights the CF-IL interaction with focus on the transition zone from pure IL to CF dominated response, giving special attention to how the added mass influences the interaction. Results from extensive flexible beam tests connected to the Ormen Lange (OL) development have been used as a basis for this study. Trajectories for cross sectional motions from the flexible beam test were identified, and then used as forced motions of a large diameter rigid cylinder exposed to uniform flow. Non-dimensional parameters like Reynolds number (Re), amplitude ratio and reduced frequency were identical for the two tests. Hence, forces found from the forced motion test could be used to find hydrodynamic coefficients valid for the flexible beam case. This paper discusses the results from the flexible beam tests with a relatively short length to diameter ratio (L/D) of 145. Modal analyses by Nielsen et al. (2002) show that the first mode dominates in both directions for these particular tests, even though the IL response frequency is twice the CF frequency. In this paper the added mass variations along the OL flexible beam is studied. Forces acting on 4 different cross sections along the beam are measured for 7 different prototype velocities. For each test the hydrodynamic coefficients are calculated, and the results show how the added mass changes along the beam for increasing velocities, and thereby creates resonance for both IL and CF response. The stability of the added mass coefficient throughout the time series is also evaluated.

Hydrodynamic Coefficients for Vortex Induced Vibrations of Slender Beams

2009 ◽  
Author(s):  
Prashant K. Soni ◽  
Carl M. Larsen ◽  
Jie Wu
Keyword(s):  
Cross Section ◽  
Amplitude Ratio ◽  
Higher Order ◽  
Reliable Data ◽  
Flexible Beam ◽  
Hydrodynamic Coefficients ◽  
Rigid Cylinder ◽  
Vortex Induced Vibrations ◽  
Slender Beams ◽  
Frequency Components

Empirical codes for prediction of vortex induced vibrations need reliable data for hydrodynamic coefficients. Such data are almost exclusively based on measured forces on rigid cylinders that have forced harmonic motions in cross-flow (CF) or in-line (IL) directions. This type of experiment is not able to capture all effects that could be important for realistic cross section motions of slender beams due to two reasons: 1. Slender beams will normally have combined IL and CF oscillations. 2. Higher order frequency components will normally be present for vibrating beams. It is difficult to measure local forces on short segments of flexible beams in laboratory tests due to the small diameters. The most convenient instrumentation is to use a large number of strain gauges or accelerometers along the beam. Proper data processing will then give reliable data for the motions, which means that the trajectory of cross sections can be found. Hence, the following set of experiments can be carried out in order to find hydrodynamic coefficients under realistic VIV conditions: 1. Experiments with a slender flexible beam and processing of recorded strains or accelerometers to identify cross section trajectories. 2. Measurement of forces on a rigid cylinder section with forced motions. Reynolds number, amplitude ratio, orbit shape and non-dimensional frequency must be identical in the flexible beam and rigid cylinder tests. Such experiments have been carried out, and the results are presented in terms of hydrodynamic coefficients for combined CF and IL oscillations. Coefficients are found for the primary CF and IL frequencies, but also for higher order frequency components. Results are presented and discussed in relation to well known results from pure CF and IL oscillations. One way of verifying that the coefficients have been correctly identified, is to apply the coefficients in an empirical response model and compare analysis results to the observation. This step has, however, not been carried out so far.

Forced Motion Experiments With Measured Motions From Flexible Beam Tests Under Uniform and Sheared Flows

2012 ◽  
Author(s):  
Decao Yin ◽  
Carl M. Larsen
Keyword(s):  
Cross Section ◽  
Cross Sections ◽  
Amplitude Modulation ◽  
Cross Flow ◽  
Hydrodynamic Forces ◽  
Flexible Beam ◽  
Forced Motion ◽  
Vortex Induced Vibrations ◽  
Time Histories ◽  
Orbit Types

Hydrodynamic forces on a cylinder under realistic combinations of in-line (IL) and cross-flow (CF) vortex induced vibrations (VIV) have been investigated. Signals of strain gauges and accelerometers from the Norwegian Deepwater Programme (NDP) tests with a long, slender beam were used to identify cross section orbits. 19 cross sections almost evenly distributed along the pipe were selected, and their motions were applied in controlled motion experiments with a rigid cylinder. Dimensionless parameters like Reynolds number and non-dimensional frequency were identical for the two sets of experiments. Comparison between hydrodynamic coefficients found from forced motion tests with observed motion time histories and periodic approximations are presented. Force histories are also investigated in detail. Orbit types for combined IL and CF VIV are categorized based on relative amplitude and phase, and it is shown that IL motions exhibit chaotic character more easily than CF. Amplitude modulation is observed frequently. Cases where cross section motions are close to periodic have similar hydrodynamic forces as for periodic motion, implying that periodic forced motion tests are relevant to get valid force information. Many cases have amplitude modulated IL motions, while CF motions are quasi-steady. In such cases, IL amplitude modulation can cause abrupt change of IL forces and also 3rd order CF forces, which can accumulate large fatigue damage. When both IL and CF motions are chaotic, the force-motion relationship is impossible to describe by constant coefficients.

Vortex Pattern Comparison for Periodic and Harmonic Combined Cross-Flow and In-Line Forced Oscillations

2009 ◽  
Author(s):  
Prashant K. Soni ◽  
Carl M. Larsen
Keyword(s):  
Vortex Shedding ◽  
Cross Flow ◽  
Periodic Trajectory ◽  
Empirical Models ◽  
Forced Oscillations ◽  
Flexible Beam ◽  
Hydrodynamic Coefficients ◽  
Cylinder Wake ◽  
Rigid Cylinder ◽  
Vortex Pattern

For prediction of vortex-induced vibrations (VIV) the empirical models apply hydrodynamic coefficients to represent the fluid forces on the slender structures. The coefficients are in most cases found by measuring forces on a rigid cylinder under harmonic pure in-line (IL) or pure cross-flow (CF) forced motions, and are generally presented as functions of non-dimensional motion amplitude and frequency. The objective of the present work has been to find hydrodynamic coefficients for realistic combinations of CF and IL motions. Such trajectories were found from measured VIV of a flexible beam, and then used as forced motions of a rigid cylinder in uniform flow. Hydrodynamic forces were measured and used for calculation of hydrodynamic coefficients. The diameter of the rigid cylinder was larger than for the flexible beam in order to obtain optimum conditions for both experiments. However, both Reynolds number and non-dimensional frequency were identical for the two test types. The flexible beam oscillations were not perfectly periodic, but close to. More than one periodic trajectory could hence be identified as representative for the observed response, and these were used as forced motions in order to study the variability of the hydrodynamic coefficients. Alternative harmonic loops were also constructed in order to investigate the potential for using coefficients from harmonic tests as basis for empirical models. The vortex shedding process behind the cylinder has been mapped using Particle Image Velocimetry (PIV). PIV planes can picture the difference in cylinder wake for these trajectories and thus help to understand the process. The vorticity patterns at instantaneous positions for both periodic and harmonic trajectories are obtained. The vortices were mapped and the forces were measured simultaneously. Higher order harmonic components of the force are seen for all types of trajectories, and a correlation between these components and the vortex shedding pattern is observed.

Response of Pipelines Under VIV: An Experimental Investigation

2010 ◽  
Author(s):  
Prashant K. Soni ◽  
Carl M. Larsen
Keyword(s):  
Response Pattern ◽  
Dynamic Properties ◽  
Cross Flow ◽  
Flexible Beam ◽  
Hydrodynamic Coefficients ◽  
Response Model ◽  
Flexible Pipe ◽  
Damping Force ◽  
Robust Response ◽  
A Current

Pipelines laid on an uneven bottom often have free spans. For cases with long spans, one may have several modes and eigenfrequencies that can be excited by vortex shedding. Furthermore, due to the sag effect of a long free-span, the dynamic properties are different in vertical and in horizontal directions. This causes a complex response pattern in the cross-flow (CF) and in-line (IL) directions. From previous research we know that pure IL response at relatively low current velocities may significantly contribute to fatigue damage. This response type must be studied in addition to the combined IL and CF response. The objective of this paper is to present experimental results from flexible beam experiments where both response types are studied, as well as to present results from an empirical response model for the same cases. The empirical model is based on two types of experiments. The first set of experiments were conducted with a flexible pipe for both single and double span configuration. Pure IL and combined IL and CF motions were observed. In the second set of experiments, forces on a rigid cylinder were measured under forced motions in a current. The motions were found from measurements of cross section in the flexible pipe tests. Hydrodynamic coefficients such as drag, added mass, excitation and damping force coefficients were found and then applied in the empirical response model. In the present paper the results from the flexible beam experiments are presented and also compared with the results from the empirical response model. The results so far are encouraging, but further work and more data are needed in order to have a general and robust response model for combined CF and IL VIV.

Hydrodynamic Coefficients for Riser In-Line VIV in Sheared Currents

2007 ◽  
Author(s):  
Djoni E. Sidarta ◽  
Kostas F. Lambrakos ◽  
Carl M. Larsen
Keyword(s):  
Fatigue Damage ◽  
Laboratory Tests ◽  
Ad Hoc ◽  
Science And Technology ◽  
Cross Flow ◽  
Hydrodynamic Coefficients ◽  
Simply Supported ◽  
Hydrodynamic Damping ◽  
Uniform Current ◽  
Mode Response

A methodology for analyzing risers for in-line VIV fatigue damage has been developed that is based on the code SHEAR7, and laboratory in-line VIV coefficients. The in-line VIV fatigue in many instances governs the design of the riser since in-line VIV starts at a reduced velocity of about 1 whereas the threshold reduced velocity for cross flow VIV is about 4. The methodology can treat sheared currents on the basis of the cross flow VIV modeling in SHEAR7. Through the SHEAR7 modeling, the methodology removes conservatism implicit in the present ad hoc procedures for calculating riser in-line VIV response on the basis of the DNV-RP-F105 code. The reduction in conservatism is due to accounting properly for the power-in region in the VIV excitation, the hydrodynamic damping, and competing modal excitation (multiple mode response). The inline VIV coefficients have been derived from laboratory tests at the Norwegian University of Science and Technology (NTNU). The paper presents the in-line VIV coefficients, and examples to demonstrate the methodology for riser in sheared currents. The coefficients derived from the NTNU tests are functions of both the in-line VIV response amplitude and the reduced velocity. The coefficients presented in the paper are scaled test coefficients. The scaling of the NTNU coefficients assures that the methodology calculates in-line VIV amplitudes that are consistent with the response amplitudes in DNV-RP-F105 for the case of a simply supported riser in uniform current. This DNV code, although written for pipelines, has been extended to risers in sheared currents on the basis of conservative approaches.

Reynolds Number Effects on Hydrodynamic Coefficients for Pure In-Line and Pure Cross-Flow Forced Vortex Induced Vibrations

2009 ◽  
Author(s):  
Jamison L. Szwalek ◽  
Carl M. Larsen
Keyword(s):  
Reynolds Number ◽  
Prediction Models ◽  
Current Knowledge ◽  
Cross Flow ◽  
Reynolds Numbers ◽  
Added Mass ◽  
Hydrodynamic Coefficients ◽  
Vortex Induced Vibrations ◽  
Reynolds Number Effects ◽  
Sinusoidal Oscillations

In-line vibrations have been noted to have an important contribution to the fatigue of free spanning pipelines. Still, in-line contributions are not usually accounted for in current VIV prediction models. The present study seeks to broaden the current knowledge regarding in-line vibrations by expanding the work of Aronsen (2007) to include possible Reynolds number effects on pure in-line forced, sinusoidal oscillations for four Reynolds numbers ranging from 9,000 to 36,200. Similar tests were performed for pure cross-flow forced motion as well, mostly to confirm findings from previous research. When experimental uncertainties are accounted for, there appears to be little dependence on Reynolds number for all three hydrodynamic coefficients considered: the force in phase with velocity, the force in phase with acceleration, and the mean drag coefficient. However, trends can still be observed for the in-line added mass in the first instability region and for the transition between the two instability regions for in-line oscillations, and also between the low and high cross-flow added mass regimes. For Re = 9,000, the hydrodynamic coefficients do not appear to be stable and can be regarded as highly Reynolds number dependent.

The Numerical Research of Two-Degrees-of-Freedom Vortex-Induced Vibrations of Circular Cylinders

2012 ◽  
Vol 204-208 ◽  
pp. 4598-4601
Author(s):  
Jie Li Fan ◽  
Wei Ping Huang
Keyword(s):  
Degrees Of Freedom ◽  
Amplitude Ratio ◽  
Flow Direction ◽  
Cross Flow ◽  
Circular Cylinders ◽  
Two Degrees Of Freedom ◽  
Line Frequency ◽  
Ansys Cfx ◽  
Vortex Induced Vibrations ◽  
Lock In

The two-degrees-of-freedom of vortex-induced vibration of circular cylinders is numerically simulated with the software ANSYS/CFX. The VIV characteristic, in the two different conditions (A/D=0.07 and A/D=1.0), is analyzed. When A/D is around 0.07, the amplitude ratio of the cylinder’s VIV between in-line and cross-flow direction in the lock-in is lower than that in the lock-out. The in-line frequency is twice of that in cross-flow direction in the lock-out, but in the lock-in, it is the same as that in cross-flow direction and the same as that of lift force. When A/D is around 1.0, the amplitude ratio of the VIV between in-line and cross-flow in the lock-in is obviously larger than that in the lock-out. Both in the lock-in and in the lock-out, the in-line frequency is twice of that in cross-flow direction.

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