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A V Metrikine - One of the best experts on this subject based on the ideXlab platform.

  • modelling of coupled cross flow and in Line Vortex induced vibrations of flexible cylindrical structures part ii on the importance of in Line coupling
    Nonlinear Dynamics, 2021
    Co-Authors: A V Metrikine
    Abstract:

    To illustrate the influence of the in-Line coupling on the prediction of Vortex-induced vibration (VIV), the simulation results of the coupled cross-flow and in-Line VIVs of flexible cylinders- obtained with three different wake oscillator models with and without the in-Line coupling- are compared and studied in this paper. Both the cases of uniform and Linearly sheared flow are analysed and the simulation results of the three models are compared with each other from the viewpoints of response pattern, fluid force, energy transfer and fatigue damage. The differences between the simulation results from the three models highlight the importance of the in-Line coupling on the prediction of coupled cross-flow and in-Line VIVs of flexible cylindrical structures.

  • modelling of coupled cross flow and in Line Vortex induced vibrations of flexible cylindrical structures part i model description and validation
    Nonlinear Dynamics, 2021
    Co-Authors: A V Metrikine
    Abstract:

    This paper is first of the two papers dealing with the nonLinear modelling and investigation of coupled cross-flow and in-Line Vortex-induced vibrations (VIVs) of flexible cylindrical structures. As a continuation of the previous work (Qu and Metrikine in Ocean Eng 196:106732, 2020) where a new single wake oscillator model was proposed and studied for VIVs of rigid cylinders, the present paper focuses on applying it to flexible cylinders. In this paper, the structure is modelled as an extensible Euler–Bernoulli beam and its 3D nonLinear coupling motion is described in the absolute coordinate system. The single van der Pol wake oscillator model with nonLinear coupling to the in-Line motion of the structure, in addition to the classic Linear cross-flow motion coupling, is uniformly distributed along the structure to model the hydrodynamic force acting on it. The finite element method has been applied to solve the dynamics of the coupled system, and the experiments of the VIV of a top-tensioned straight riser subjected to a step flow have been taken for the validation of the model. The model has been shown to be able to capture most features of VIVs of flexible cylinders, and a good agreement between the simulation results and the experimental measurements has been observed with regard to the amplitude, frequency and excited mode of both cross-flow and in-Line vibrations, as well as the mean in-Line deflection due to the amplified in-Line force. While it is conventionally expected that the VIV of a flexible cylinder subjected to a uniform flow is dominated by a single frequency, a multi-frequency response is observed in the simulation results over the range of flow velocities through which the transition of the dominant mode of vibration occurs.

  • a single van der pol wake oscillator model for coupled cross flow and in Line Vortex induced vibrations
    Ocean Engineering, 2020
    Co-Authors: A V Metrikine
    Abstract:

    Abstract In this study a new wake oscillator model is proposed to describe the coupled cross-flow and in-Line Vortex-induced vibrations of an elastically supported rigid cylinder. Different from many other studies where two wake oscillators have been applied, the current model uses only one wake oscillator coupled to both cross-flow and in-Line motions. The new model is based on the van der Pol oscillator with the classic acceleration coupling between the wake and cross-flow motion, while the in-Line motion is coupled with the wake variable in a nonLinear manner. The predictions of this new model are compared with the existing experimental data and shown to be in good agreement. In addition to the conventional lock-in range that corresponds to reduced velocities between 5 and 8, another lock-in is predicted around reduced velocity of 2.5 due to the in-Line vibration. Most importantly, the new model is proved to be able to predict the appearance of the ‘super-upper’ branch at small mass ratios without changing the tuning parameters. The limitations of the model associated with unrealistic predictions of free vibrations with very small mass ratios and those of forced in-Line vibrations at high frequencies are also discussed along with a possible remedy.

Qu Y. - One of the best experts on this subject based on the ideXlab platform.

  • A single van der pol wake oscillator model for coupled cross-flow and in-Line Vortex-induced vibrations
    'Elsevier BV', 2020
    Co-Authors: Qu Y., Metrikine A.
    Abstract:

    In this study a new wake oscillator model is proposed to describe the coupled cross-flow and in-Line Vortex-induced vibrations of an elastically supported rigid cylinder. Different from many other studies where two wake oscillators have been applied, the current model uses only one wake oscillator coupled to both cross-flow and in-Line motions. The new model is based on the van der Pol oscillator with the classic acceleration coupling between the wake and cross-flow motion, while the in-Line motion is coupled with the wake variable in a nonLinear manner. The predictions of this new model are compared with the existing experimental data and shown to be in good agreement. In addition to the conventional lock-in range that corresponds to reduced velocities between 5 and 8, another lock-in is predicted around reduced velocity of 2.5 due to the in-Line vibration. Most importantly, the new model is proved to be able to predict the appearance of the ‘super-upper’ branch at small mass ratios without changing the tuning parameters. The limitations of the model associated with unrealistic predictions of free vibrations with very small mass ratios and those of forced in-Line vibrations at high frequencies are also discussed along with a possible remedy.Accepted Author ManuscriptOffshore EngineeringEngineering Structure

  • Modelling of coupled cross-flow and in-Line Vortex-induced vibrations of flexible cylindrical structures: Part II: on the importance of in-Line coupling
    'Springer Science and Business Media LLC', 2020
    Co-Authors: Qu Y., Metrikine A.
    Abstract:

    To illustrate the influence of the in-Line coupling on the prediction of Vortex-induced vibration (VIV), the simulation results of the coupled cross-flow and in-Line VIVs of flexible cylinders- obtained with three different wake oscillator models with and without the in-Line coupling- are compared and studied in this paper. Both the cases of uniform and Linearly sheared flow are analysed and the simulation results of the three models are compared with each other from the viewpoints of response pattern, fluid force, energy transfer and fatigue damage. The differences between the simulation results from the three models highlight the importance of the in-Line coupling on the prediction of coupled cross-flow and in-Line VIVs of flexible cylindrical structures.Accepted author manuscriptOffshore EngineeringEngineering Structure

  • A single wake oscillator model for coupled cross-flow and in-Line Vortex-induced vibrations of marine structures
    2019
    Co-Authors: Qu Y.
    Abstract:

    Vortex-induced vibration (VIV) is awell-known phenomenon for civil and offshore structures. Currently, the prediction of this type of vibration in practice currently mainly relies on the force-decomposition method. However, the limitations of this method have restricted the applicability of the method, and alternative models are therefore needed to meet increasing demands for the more accurate prediction of VIV under more complicated conditions. The wake oscillator model overcomes the main limitations of the force-decomposition method to some extent, and it is one of the promising models that has gained popularity in recent years. Although the concept of the wake oscillator was first proposed over half a century ago and has been developed much since then, the existing wake oscillator models still have some limitations, which have restricted their applications. The main objective of this study is to improve the wake oscillator model for better modelling of the VIV of cylindrical structures, and efforts are made in this thesis to (a) reproduce the free and forced vibration experiments by introducing nonLinear coupling, and (b) develop a single wake oscillator equation that is coupled to both cross-flow and in-Line motions for the prediction of coupled cross-flow and in-Line VIV...Offshore Engineerin

  • A single wake oscillator model for coupled cross-flow and in-Line Vortex-induced vibrations of marine structures
    2019
    Co-Authors: Qu Y.
    Abstract:

    Vortex-induced vibration (VIV) is awell-known phenomenon for civil and offshore structures. Currently, the prediction of this type of vibration in practice currently mainly relies on the force-decomposition method. However, the limitations of this method have restricted the applicability of the method, and alternative models are therefore needed to meet increasing demands for the more accurate prediction of VIV under more complicated conditions. The wake oscillator model overcomes the main limitations of the force-decomposition method to some extent, and it is one of the promising models that has gained popularity in recent years. Although the concept of the wake oscillator was first proposed over half a century ago and has been developed much since then, the existing wake oscillator models still have some limitations, which have restricted their applications. The main objective of this study is to improve the wake oscillator model for better modelling of the VIV of cylindrical structures, and efforts are made in this thesis to (a) reproduce the free and forced vibration experiments by introducing nonLinear coupling, and (b) develop a single wake oscillator equation that is coupled to both cross-flow and in-Line motions for the prediction of coupled cross-flow and in-Line VIV..

Narakorn Srinil - One of the best experts on this subject based on the ideXlab platform.

  • empirical sensitivity of two dimensional nonLinear wake cylinder oscillators in cross flow in Line Vortex induced vibrations
    Journal of Fluids and Structures, 2018
    Co-Authors: Narakorn Srinil, Pierreadrien Opinel, Francesca Tagliaferri
    Abstract:

    Abstract Phenomenological wake–cylinder oscillators have been extensively implemented for Vortex-induced vibration (VIV) predictions. Although such models capture fundamental VIV phenomena, the maximum response estimations and comparisons with different experimental data reveal some quantitative discrepancies due to the model empiricism embedding some uncertainties through system variables. This vital issue has not been well addressed in the literature of VIV modelling. This paper presents a new comprehensive investigation into the sensitivity to empirical input variables of nonLinear wake–cylinder oscillators simulating the two-dimensionally coupled cross-flow/in-Line VIV and amplified mean displacements of a flexibly mounted circular cylinder in uniform flows. The fluid–structure coupling terms are advanced by accounting for the higher-order nonLinear effects of fluctuating lift–drag forces and steady-drag dynamic magnifications, depending on the relative flow-cylinder velocities. A random sampling and variance-based sensitivity studies are carried out using Monte Carlo simulations which are computationally efficient based on the reduced-order model. This enables a large series of parametric examinations. Individual contribution, relative importance, coupling and interdependence of multiple input variables affecting output uncertainties are qualitatively and quantitatively evaluated. The Reynolds number dependence is also captured by correlating the wake and hydrodynamic coefficients with experimental data. Parametric studies highlight greater variations in the predicted amplitudes and mean displacements of the cylinder two-degree-of-freedom VIV with a lower mass ratio. Numerical findings allow for the identification of a few most influential variables to be treated as the empirically tuned coefficients. The improved understanding of model versatility and sensitivity enhances the calibration confidence and the response predictability with a reduced computational effort.

  • modelling of coupled cross flow in Line Vortex induced vibrations using double duffing and van der pol oscillators
    Ocean Engineering, 2012
    Co-Authors: Narakorn Srinil, Hossein Zanganeh
    Abstract:

    Many studies have typically applied a Linear structural spring–mass–damper oscillator and a van der Pol wake oscillator to model a one-dimensional cross-flow Vortex-induced vibration (VIV). In this study, an advanced model for predicting a two-dimensional coupled cross-flow/in-Line VIV of a flexibly mounted circular cylinder in a uniform flow is proposed and validated. The ensuing dynamical system is based on double Duffing–van der Pol (structural-wake) oscillators with the two structural equations containing both cubic and quadratic nonLinear terms. The cubic nonLinearities capture the geometrical coupling of cross-flow/in-Line displacements excited by hydrodynamic lift/drag forces whereas the quadratic nonLinearities allow the wake–cylinder interactions. Some empirical coefficients are calibrated against published experimental results to establish a new generic analytical function accounting for the dependence of VIV on a physical mass and/or damping parameter. By varying flow velocities in the numerical simulations, the derived low-order model captures several important VIV characteristics including a two-dimensional lock-in, hysteresis phenomenon and figure-of-eight trajectory tracing the periodically coupled in-Line/cross-flow oscillations with their tuned two-to-one resonant frequencies. By making use of a newly derived empirical formula, the predicted maximum cross-flow/in-Line VIV amplitudes and associated lock-in ranges compare well with several experimental results for cylinders with low/high mass or damping ratios. Moreover, the parametric studies highlight the important effect of geometrical nonLinearities through new displacement coupling terms and the ratio of in-Line to cross-flow natural frequencies of the freely vibrating cylinder.

Carl M Larsen - One of the best experts on this subject based on the ideXlab platform.

  • time domain simulation of riser viv in current and irregular waves
    Marine Structures, 2018
    Co-Authors: Jan Vidar Ulveseter, Svein Saevik, Mats Jorgen Thorsen, Carl M Larsen
    Abstract:

    Abstract A semi-empirical prediction tool for time domain analysis of cross-flow and in-Line Vortex-induced vibrations (VIV) is proposed. The model combines concepts from previous publications in a new way, which allows for time domain simulation of VIV together with response phenomena from Morison's equation. To test the prediction tool, a vertical riser exposed to two different flow profiles was modelled, and response predictions were compared to experimental data. The first flow case was steady uniform current, and the second case included irregular waves and current combined. The same empirical coefficients were used to simulate both experiments. In uniform flow, the results were good, although some discrepancies were seen. For the second flow case, the model provided a highly realistic representation of the riser motion. Both when the total response (containing wave and VIV frequencies) and the filtered signal (including only VIV frequencies) were analysed, the predictions followed the measurements closely. Different approaches were subsequently used to find maximum fatigue damage for the riser exposed to current and waves. The industry approach, finding the total damage from independent calculations of wave motion and VIV in uniform current, was compared to the damage produced from the combined analysis. Moderate differences in results between the two strategies were observed. Lastly, the experimental data was analysed to compare riser VIV for the two flow conditions.

  • time domain model for calculation of pure in Line Vortex induced vibrations
    Journal of Fluids and Structures, 2017
    Co-Authors: Jan Vidar Ulveseter, Svein Saevik, Carl M Larsen
    Abstract:

    Abstract A time domain model for prediction of cross-flow Vortex-induced vibrations (VIV) of slender structures with circular cross section has been under development since 2012. As an extension of this work, a time domain model for pure in-Line VIV is here proposed, with the same underlying theory. The in-Line force model, consisting of added mass, damping and excitation, is based on empirical data from forced oscillation tests of rigid cylinders. Damping and excitation is tuned to give the best fit of the excitation force coefficient calculated from experiments, whereas a strip theory approach is utilized to determine the force is phase with cylinder acceleration, i.e. added mass. The excitation force model represents the time varying drag force induced by Vortex shedding, and consists of two frequency-regions with positive excitation. Within these regions the excitation force is able to synchronize with the response vibrations, so that energy is transferred to the cylinder. Numerical simulations are performed to compare the present model with experimental results of free oscillations of rigid and flexible pipes with circular cross section, in uniform current. For the flexible cylinder case, a simple Linear finite element structural model is combined with the in-Line force model. The numerical simulations and the experiments are seen to match fairly well, both concerning frequency content, amplitude ratio and dominating vibration mode. Some discrepancies are observed, mostly concerning amplitude ratio. However, due to the complexity of VIV as a phenomenon, and the simplicity of the present model, it is concluded that the results are satisfactory. Consequently, this paper shows that the original idea of synchronization between excitation force and cylinder response is seen to work, not only for cross-flow VIV, but for pure in-Line VIV as well.

  • blind predictions of laboratory measurements of Vortex induced vibrations of a tension riser
    Journal of Fluids and Structures, 2005
    Co-Authors: J R Chaplin, P W Bearman, Y Cheng, E Fontaine, J M R Graham, K Herfjord, F Huera J Huarte, M Isherwood, Kostas F Lambrakos, Carl M Larsen
    Abstract:

    This paper compares laboratory measurements of the Vortex-induced vibrations of a riser in a stepped current with blind predictions obtained with 11 different numerical models. Results are included on in-Line and transverse displacements and curvatures, and dominant frequencies. In general, empirical models were more successful at predicting cross-flow displacements and curvatures than current codes based on CFD. Overall ratios between predictions and measurements of cross-flow displacements were around 95% and 75%, respectively. Predictions of cross-flow curvatures were more scattered, and almost all were unconservative. In-Line Vortex-induced curvatures, which may cause as much damage as cross-flow curvatures, could not be computed by any of the empirically based codes, and in general those based on CFD were in very poor agreement with the measurements.

Metrikine A. - One of the best experts on this subject based on the ideXlab platform.

  • A single van der pol wake oscillator model for coupled cross-flow and in-Line Vortex-induced vibrations
    'Elsevier BV', 2020
    Co-Authors: Qu Y., Metrikine A.
    Abstract:

    In this study a new wake oscillator model is proposed to describe the coupled cross-flow and in-Line Vortex-induced vibrations of an elastically supported rigid cylinder. Different from many other studies where two wake oscillators have been applied, the current model uses only one wake oscillator coupled to both cross-flow and in-Line motions. The new model is based on the van der Pol oscillator with the classic acceleration coupling between the wake and cross-flow motion, while the in-Line motion is coupled with the wake variable in a nonLinear manner. The predictions of this new model are compared with the existing experimental data and shown to be in good agreement. In addition to the conventional lock-in range that corresponds to reduced velocities between 5 and 8, another lock-in is predicted around reduced velocity of 2.5 due to the in-Line vibration. Most importantly, the new model is proved to be able to predict the appearance of the ‘super-upper’ branch at small mass ratios without changing the tuning parameters. The limitations of the model associated with unrealistic predictions of free vibrations with very small mass ratios and those of forced in-Line vibrations at high frequencies are also discussed along with a possible remedy.Accepted Author ManuscriptOffshore EngineeringEngineering Structure

  • Modelling of coupled cross-flow and in-Line Vortex-induced vibrations of flexible cylindrical structures: Part II: on the importance of in-Line coupling
    'Springer Science and Business Media LLC', 2020
    Co-Authors: Qu Y., Metrikine A.
    Abstract:

    To illustrate the influence of the in-Line coupling on the prediction of Vortex-induced vibration (VIV), the simulation results of the coupled cross-flow and in-Line VIVs of flexible cylinders- obtained with three different wake oscillator models with and without the in-Line coupling- are compared and studied in this paper. Both the cases of uniform and Linearly sheared flow are analysed and the simulation results of the three models are compared with each other from the viewpoints of response pattern, fluid force, energy transfer and fatigue damage. The differences between the simulation results from the three models highlight the importance of the in-Line coupling on the prediction of coupled cross-flow and in-Line VIVs of flexible cylindrical structures.Accepted author manuscriptOffshore EngineeringEngineering Structure