Showing papers on "Vortex-induced vibration published in 2008"

Vortex-induced vibration of a circular cylinder with combined in-line and cross-flow motion

Abstract: Deep water, string-like, marine risers subject to strong ocean currents, suffer from vortex-induced vibrations (VIV), where vortex shedding interacts with the structural properties of the riser, resulting in large amplitude vibrations in both in-line and cross-flow directions. This thesis presents an experimental approach to model and quantify the motions and hydrodynamic forces associated with the excitation of a deep water marine riser by considering the combined cross-flow and in-line excitation of a rigid cylinder. For deep water risers, the excitation of the structure through vortex shedding can lead to a condition of dual resonance, where the vortex shedding frequency locks in to the effective natural frequency (adjusted for added mass effects) in both in-line and cross-flow directions. Large motion amplitudes are observed in this condition along with large magnitude third harmonic forces in lift. Flow visualization of the wake behind the cylinder in combined in-line and cross-flow motion, shows that third harmonic forces are caused by the relative motion of the cylinder with respect to a '2P' (two pairs of vortices) or '2T' (two triplets of vortices) shedding pattern, since vortices shed in these modes remain in close proximity to the cylinder over one cycle of motion. Forced motions of a cylinder with combined in-line and cross-flow motions are performed, generating a database of force coefficients to be used in riser VIV prediction. The assumption of dual resonance is used to predict the motions of an elastically mounted rigid cylinder using measurements from forced cylinder motions. Two passive vortex suppression methods are studied for eliminating combined cross-flow and in-line cylinder motions and suppressing large third harmonic forces. Thesis Supervisor: Michael S. Triantafyllou Title: Professor of Mechanical and Ocean Engineering

Vortex-induced vibration super-upper response branch boundaries

Abstract: The purpose of this study was to experimentally investigate the vortex-induced vibration response of cylindrical structures with low mass ratio. Much of our understanding of vortex-induced vibration has been established through single degree of freedom and/or higher mass ratio experiments. The empirical relationships based on these data capture only the characteristics of the upper vortex-induced vibration response branch. The corresponding super-upper response branch observed in 2 degree of freedom systems at low mass ratio is shown to present significantly larger amplitudes and more regular oscillations. Understanding the boundaries of this super-upper response has implications for offshore structure design. This study revealed that damping in addition to the mass ratio appears to be one of the governing parameters in establishing super-upper response branch vibrations. Based on the experimental evidence available, super-upper response boundaries are suggested, and the potential bearing of these on empirical design formulations not considering the super-upper response are discussed.

Two-degree-of-freedom vortex-induced vibration of a pivoted cylinder below critical mass ratio

Abstract: In this study, we investigated two-degree-of-freedom (2df) vortex-induced vibrations (VIVs) of a circular cylinder with a pinned attachment at its base; it had identical mass ratios and natural frequencies in both streamwise and transverse directions The cylinder had a mass ratio, mof 045, and a mass damping, (mCCA)z, equal to 00841 Laser- induced fluorescence flow visualization and digital particle image velocimetry experiments were conducted over a Reynolds number range, 820%Re%6050 (corresponding to the reduced velocity range, 11%U � %83) Measurements and visualization studies were made in a fixed plane at the cylinder mid-height, providing a two-dimensional picture of a highly three-dimensional system However, significant insights can be gained from these experiments and form the basis of this paper A large transverse amplitude response, Aw2 (or four diameters peak-to-peak), in the upper branch was observed The streamwise amplitude response exhibits an even higher peak amplitude, Aw2:5, which is approximately 125% of peak A � Results show that there is no lower branch for this system and the transverse upper branch exhibits asymptotic behaviour, ie a wide regime of resonance For ReO3000, the Strouhal number for the vortex shedding was 016 (G9%) Both the transverse cylinder oscillation and vortex-shedding frequencies, fOS,Y and fVS, respectively, were virtually identical throughout this range While the streamwise oscillation frequency is typically twice the transverse oscillation frequency for a 2df system, this is not the case at the lowest reduced velocities where oscillations first occur Under these conditions the streamwise and transverse oscillation frequencies were identical Finally, we observed that the cylinder wake exhibits both the PCS vortex-shedding mode and a desynchronized vortex pattern, which are uncommon for flow past a cylinder experiment Very interestingly, the wide Urange over which resonance occurs is dominated by a desynchronized vortex pattern These results clearly demonstrate the differences that arise in 2df VIV occurring below the critical mass ratio

Reduction/Suppression of VIV of Circular Cylinders Through Roughness Distribution at 8×103 < Re < 1.5×105

Abstract: Vortex Induced Vibration (VIV) of a circular cylinder in a steady flow is reduced using distributed surface roughness. VIV reduction is needed in numerous applications where VIV is destructive. Roughness is distributed to the surface of the cylinder in the form of sandpaper strips to achieve three goals: (1) Trip separation in a controlled manner so that some uncertainties are removed and the flow becomes more predictable. (2) Reduce spanwise correlation, which is strongly linked to VIV. (3) Select roughness grit size to achieve the first goal without energizing too much the boundary layer, which would induce higher vorticity and circulation, and consequently lift. Our experiments show that it is possible to reduce VIV amplitude and synchronization range. More tests are needed to achieve full suppression. Our experiments are conducted in the TrSL2 and TrSL3 flow regimes.Copyright © 2008 by ASME

Energy Extraction from a Steady Flow Using Vortex Induced Vibration

Abstract: ........................................................................................................................ xxxi Chapter 1 VIVACE: A NEW CONCEPT IN OCEAN ENERGY CONVERSION ................................................................................................................. 1 1.

Reynolds Number Dependence of Flexible Cylinder VIV Response Data

Abstract: The response amplitude and the non-dimensional frequency of flexible cylinder vortex-induced vibrations from laboratory and field experiments show significant trends with increasing Reynolds number from 103 to 2 * 105 . The analysis uses complex data from experiments with wide variations in the physical parameters of the system, including length-to-diameter ratios from 82 to 4236, tension dominated natural frequencies and bending stiffness dominated natural frequencies, sub-critical and critical Reynolds numbers, different damping coefficients, standing wave and traveling wave vibrations, mode numbers from 1 – 25th , and different mass ratios.Copyright © 2008 by ASME

Enhancement of vortex induced forces and motion through surface roughness control

Abstract: Roughness is added to the surface of a bluff body in a relative motion with respect to a fluid. The amount, size, and distribution of roughness on the body surface is controlled passively or actively to modify the flow around the body and subsequently the Vortex Induced Forces and Motion (VIFM). The added roughness, when designed and implemented appropriately, affects in a predetermined way the boundary layer, the separation of the boundary layer, the level of turbulence, the wake, the drag and lift forces, and consequently the Vortex Induced Motion (VIM), and the fluid-structure interaction. The goal of surface roughness control is to increase Vortex Induced Forces and Motion. Enhancement is needed in such applications as harnessing of clean and renewable energy from ocean/river currents using the ocean energy converter VIVACE (Vortex Induced Vibration for Aquatic Clean Energy).

Enhancement of High Damping VIV Through Roughness Distribution for Energy Harnessing at 8×103 < Re < 1.5×105

Abstract: Vortex Induced Vibration (VIV) of a circular cylinder in a steady flow is enhanced using distributed surface roughness. VIV enhancement is needed in harnessing clean and renewable energy from ocean/river currents using the VIVACE (VIV for Aquatic Clean Energy) Converter (Bernitsas et al. 2006a, 2006b; Raghavan et al. 2007). High damping is mandatory in energy harnessing and higher Reynolds numbers are required to reach the high lift TrSL3 regime. Roughness is added to the surface of the cylinder in the form of sandpaper strips to achieve three goals: (1) Trip separation in a controlled manner. (2) Fully correlate the flow regardless of the Reynolds number regime. (3) Energize the boundary layer, thus inducing higher vorticity and circulation and consequently lift. Our experiments show that it is possible to achieve all three goals and reach VIV amplitudes of 2.1 to 2.7 diameters under high damping. The range of synchronization increased dramatically but its end was not observed within the capabilities of our experimental facility.Copyright © 2008 by ASME

Vortex-induced vibration of marine risers : motion and force reconstruction from field and experimental data

Abstract: Vortex-induced vibration (VIV) of long flexible cylindrical structures enduring ocean currents is ubiquitous in the offshore industry. Though significant effort has gone into understanding this complicated fluid-structure interaction problem, major challenges remain in modeling and predicting the response of such structures. The work presented in this thesis provides a systematic approach to estimate and analyze the vortex-induced motions and forces on a marine riser, and develop suitable methods to improve riser VIV modeling and response prediction. In the first part of the thesis, a systematic framework is developed, which allows reconstruction of the riser motion from a limited number of sensors placed along its length. A perfect reconstruction criterion is developed, which allows us to classify when the measurements from the sensors contain all information pertinent to VIV response, and when they do not, in which case additional, analytical methods must be employed. Reconstruction methods for both scenarios are developed and applied to experimental data. The methods are applied to: develop tools for in-situ estimation of fatigue damage on marine risers; improve understanding of the vortex shedding mechanisms, including the presence of traveling waves and higher-harmonic forces; and estimate the vortex-induced forces on marine risers. In the second part of the thesis, a method is developed to improve the modeling of riser VIV by extracting empirical lift coefficient databases from field riser VIV measurements. The existing experiment-based lift coefficient databases are represented in a flexible parameterized form using a set of carefully chosen parameters. Extraction of the lift coefficient parameters is posed as an optimization problem, where the error between the prediction using a theoretical model and the experimental data is minimized. Predictions using the new databases are found to significantly reduce the error in estimating the riser cross-flow response. Finally, data from a comprehensive experiment is utilized to show that the riser response is resonant in the harmonic component, but non-resonant in the third-harmonic component. It is shown that this happens because the spatial dependence of the third-harmonic fluid force component is dominated by the first-harmonic wavelengths. This finding has significant implications for modeling the higher-harmonic forces and the resulting fatigue damage estimation methodologies. Thesis Supervisor: Michael S. Triantafyllou Title: Professor of Mechanical and Ocean Engineering Thesis Supervisor: Franz S. Hover Title: Assistant Professor of Mechanical and Ocean Engineering This thesis is dedicated with love and respect to the ever living memories of my elderly. Acknowledgments This PhD thesis is a culmination of five years of my stay here at MIT. Over these years, I have had the opportunity to learn, relearn and I have passed through some of the most difficult and frustrating moments of my life. I recognize that this rigorous learning experience will help me in several more difficult endeavors later in my life. Several people have helped me during this difficult endeavor with their encouraging words, helping hands and material assistance. My advisers during the thesis Prof. Michael Triantafyllou and Prof. Franz Hover have been exemplary, and I thank them for their motivation, mentorship and expert guidance through the labyrinth of research. Prof. Triantafyllou always had an open door and mind for several discussions and I sincerely thank him for his patience. Prof. Hover was always there to consolidate my learning, especially at times when Prof. Triantafyllou was not available. I share a special feeling for my previous advisor Prof. Nicholas Patrikalakis, from whom I have learned a lot and I remain indebted to him. I am grateful to Prof. Kim Vandiver and Prof. Eduardo Kausel who always had time for me in spite of their busy schedule. I also gratefully acknowledge the financial support from British Petroleum and technical support in the form of experimental data from Norwegian Deepwater Programme. I feel fortunate to have been taught by the masters in their corresponding fields at MIT, Professors A. Almazan, T. R. Akylas, K. J. Bathe, J. J. Connor, T. Copeland, T. Chen, D. M. Freeman, D. C. Gossard, V. K. Goyal, S. Hunter, E. Kausel, H. Kite-Powell, P. Koev, H. S. Marcus, D. Margetis, A. V. Oppenheim, A. T. Patera, N. M. Patrikalakis, J. Peraire, R. Stocker, N. P. Suh, J. Sussman, A. Techet, M. S. Triantafyllou, A. Toomre, K. J. Vandiver, D. Veneziano, J. K. White and D. K. P. Yue. I gratefully remember and acknowledge my Professors at IIT-Madras V. G. Idichandy, S. Surendran and R. Natarajan for their recommendation letters without which I would not have been at MIT in the first place. Xiaojing, my good friend has encouraged me in all my efforts and supported me during difficult times at MIT. Yahya and Phil have helped me enormously by proof reading my thesis and presenting several valuable comments. I thank my colleagues Arpit, Costas, Ding, Eric, Jason, Joe, Josh, Matt, Phil, Srini, Tianrun, Vikas, Vivek and Yahya for a pleasant working environment. Thanks are due to past and present administrative staff at Ocean Engineering S. Malley, K. de Zengotita, M. Munger, P. Pickard, J. Lewis, P. Tolan and E. Daniel, and also to the current Mechanical Engineering staff J. Kravit, L. Regan and D. Shea. I also thank the many unknown faces who have helped me in every aspect of my life in these years. I can never forget my Parents T. Mukundan and B. Lali for their unconditional support and encouragement for nearly 28 years without which I cannot even imagine (I don't want to imagine) where I would have been. Finally I thank the Almighty for guiding me through some of the most difficult and confusing times and for keeping up my morale.

Induced Separation and Vorticity Using Roughness in VIV of Circular Cylinders at 8×103 < Re < 2.0×105

Abstract: Results of an experimental investigation on fluid flow past an elastically mounted circular cylinder with rectangular surface roughness strips are presented. Flow characteristics change depending on the strip width, roughness grit size, and location. Roughness size and distribution can be designed to enhance or reduce/suppress VIV amplitude and increase or reduce the range of synchronization, respectively. To the authors’ knowledge this is the first study in passive control of VIV using properly distributed roughness.Copyright © 2008 by ASME

Apparatus and method for inhibiting vortex-induced vibration

Abstract: Apparatus and methods for reducing vortex induced vibration are provided. An apparatus for reducing vortex induced vibration can include a first body at least partially disposed within a second body. The first body can be made of a material having more weight per linear foot than the second body. The apparatus can also include one or more helical fins.

Full-Scale Measurement and Numerical Simulation of Flow Around High-Speed Train in Tunnel

Abstract: Vibration of high-speed trains increases in tunnels caused by aerodynamic force whose mechanism is unknown. To investigate this aerodynamic force, the authors conduct an analysis of the running test data and a numerical simulation. The running test data indicates that the aerodynamic force acts to vibrate the train in the tunnel and it originates from large-scale coherent structures in the space between the tunnel wall and the train. These flow structures develop from the head toward the 6th to 8th cars, and become steady thereafter to the tail of the train set. The computation reveals that the flow becomes unstable under the train. The resulted vortices are spread on the train side by the tunnel wall, and then the unsteady aerodynamic force arises when the vortices pass.

Analysis of the Slider Off-Track Vibration Caused by the Aerodynamic Loads Associated With Different Components of a Head Stack Assembly in a Disk Drive

Abstract: The paper describes a numerical investigation of the problem of flow-induced, off-track vibration of the slider in a disk drive. The flow field was simulated in a 3.5-in, 10 000 rpm disk drive using FLUENT flow-modeling software, which yielded the aerodynamic forces and moments for various components of the head stack assembly (HSA). The finite-element model of the HSA was developed using ANSYS software. Then off-track vibration of the slider caused by the aerodynamic loads of different HSA components was computed. The computations showed that the arm experiences the largest fluctuations of aerodynamic load. However, the loadbeam base region appears to make the largest contribution to off-track vibration of the slider, due to the vortex shedding generated by the side tabs of the loadbeam base region.

Vortex induced vibration suppression systems and methods

Abstract: A system comprising a subsea structure defining an interior of the system, the structure subject to a water current; a sleeve exterior to the subsea structure, covering at least a portion of an outside surface of the subsea structure; and a vortex induced vibration suppression device exterior to the sleeve.

Comparison and Analysis of Vortex Induced Vibration for Twin-Box Bridge Sections Based on Experiments in Different Reynolds Numbers

Abstract: In order to compare the performance of the vortex induced vibrations(VIV) of twin-box bridge sections in different Reynolds numbers,two groups of experiments with 40 cases were carried out in TJ-1 and TJ-3 wind tunnel.The results show that VIV occurs in a more widely range of damping ratios with rather larger amplitudes in the low Reynolds number experiments.While in the high Reynolds number experiments,VIV only occurs in a much closer range of damping ratios with the amplitude being smaller.As for the guide vanes,they do decrease the amplitude of VIV in the attack angle of zero no matter the Reynolds number is high or low.However,when the attack angle exists,they only can decrease the amplitudes in the high Reynolds number experiments while the amplitudes of VIV will be increased a lot in the low Reynolds number experiments.Comparisons are also done with the experiments of the Great Belt East Bridge and the Stonecutters Bridge.The mechanisms of the experiments are detailed hereafter together with the reasons of comparisons' divergence and agreement.

VSIV (Vortex Self-Induced Vibration) Kinematics

Abstract: The Vortex Induced Vibration (VIV) of cylindrical lines that may occur when the lines are submitted to currents has been extensively discussed in the past few years and its behavior has become well known. However, it is not so well known that the vibrations may occur in a current-less situation, induced by the lateral motion of the structure itself. The present work refers to the last as the Vortex Self-Induced Vibration, the VSIV. This occurrence has been made clear in the LOC/COPPE/UFRJ (Laboratory of Waves and Currents of COPPE, the Graduate School of Federal University of Rio de Janeiro) by specifically designed tests. In these tests, a totally submerged horizontal cylinder was submitted to harmonic forced oscillations, being free to move in the transverse direction of the forced excitation. The VSIV then showed up, with the cylinder segment, describing vertical trajectories in two (vertical 8-shape), three, four, etc., almost circular trajectories (called the rings in the work). Subsequently, the work shows that the measurements in full scale with the VIV bottle on a Steel Catenary Riser in the PETROBRAS 18 platform also indicate the existence of the VSIV. The tests were carried out with Keulegan-Carpenter equal to 10, 20 and 30 and for several amplitudes. The response of the cylinder was represented in non-dimensional parameters corresponding to the amplitude, the excitation and the response frequencies.© 2008 ASME