Vortex-induced Vibration Prediction Research Articles

Vortex-induced vibration fatigue damage prediction method for flexible cylinders based on RBF neural network

Vortex-induced vibration (VIV) is an important factor resulting in fatigue failure of cylindrical structures in offshore engineering applications (such as risers and cables). The traditional empirical model for VIV prediction often yields unsatisfactory results due to its dependence on the selection of empirical coefficients. Thus, establishing a general approach to VIV damage prediction based on artificial neural networks and engineering experience is of great significance to engineering guidance. In this paper, a VIV damage prediction model of a flexible cylinder was established using a radial basis function neural network (RBFNN) and model test data. Compared with back propagation neural network (BPNN), genetic algorithm-back propagation neural network (GA-BPNN), and generalized regression neural network (GRNN), the RBFNN exhibited excellent performance with high accuracy and the shortest training time in predicting the VIV damage of flexible cylinders. In addition, the VIV fatigue of flexible cylinders fitted with passive control devices (control rods and helical strakes) was predicted. Using three and four control rods, the VIV reduction effect was observed to be better when the attack angle θ was in the range of 30°–45° and 15°–30°, respectively. Further, regarding helical strakes, the damage values of the cylinder were significantly reduced; however, the VIV suppression effect was significantly affected by the yaw angles. The VIV damage prediction model of this paper is expected to serve as important engineering guidance for the preliminary design, installation arrangement, operation, and maintenance of flexible cylindrical structures in offshore engineering applications.

Application of a modified wake oscillator model to vortex-induced vibration of a free-hanging riser subjected to vessel motion

A modified wake oscillator model for modelling vortex-induced vibrations (VIVs) of a free-hanging riser subjected to vessel motion is presented. Different from the conventional wake oscillators which have typically utilized in the prediction of steady flow VIVs , the new model can be applied to simulate VIVs introduced by the vessel motion where the flows around the riser are not known in advance. This has been achieved by introducing an equivalent flow based on the relative flow velocity and coupling the wake oscillator with the acceleration of the riser in the lift direction. The modified wake oscillator model is tuned against the experiments of riser VIVs subjected to steady flows and a set of empirical parameters is obtained. The model is then applied to simulate the VIVs of a large-scale free-hanging water intake riser (WIR) subjected to different vessel motions and the simulated results are compared to those of experiments. The comparison shows that the proposed model is able to predict response frequency in good agreement with the experiment, and generates satisfactory prediction with respect to the response amplitude for three test cases out of five that have been studied in the present paper.

Extraction of aerodynamic damping and prediction of vortex-induced vibration of bridge deck using CFD simulation of forced vibration

This study presents computational fluid dynamics (CFD) simulation of forced vibration of a bridge deck for extraction of amplitude-dependent aerodynamic damping in the vicinity of vortex lock-in region. The coordinate system fixed on the oscillating body is used to bypass the requirement of mesh updating. The three-dimensional flow around the vibrating deck is assumed to be homogeneous in spanwise direction and simulated using the efficient high-order spectral element/Fourier method. The aerodynamic damping is extracted from the simulated lift and then used to estimate the steady-state vortex-induced vibration (VIV) amplitude for given wind velocity and structural parameters. A good match of VIV amplitudes between simulations and wind tunnel tests validates the accuracy of simulation. The VIV mechanisms are discussed from the vortex shedding modes, distributions of dynamic pressure and energy transfer from flow to bridge deck. The leading-edge vortex shedding rather than Karman vortex shedding is identified to be responsible for the VIV. The hysteresis of VIV is observed which is related to the mode change of impinging leading-edge vortex and leading-edge vortex shedding. Finally, a novel forced vibration technique with a varying amplitude is proposed that permits extraction of aerodynamic damping by a single CFD run with improved efficiency.

Time domain simulation of marine riser vortex-induced vibrations in three-dimensional currents

One of the major uncertainties related to present Vortex-Induced Vibrations (VIV) prediction practice is related to the simplification of real multi-directional current profiles into equivalent uni-directional ones. This study addresses the correlation between a recently updated time domain model and data from VIV experiments including both two- and three-dimensional (2D and 3D) currents. The test riser and current profile were modeled in three different ways, including 2D & 3D current profiles and linear & nonlinear models, as a basis for discussing the results from various perspectives. As a first step, VIV simulations with a nonlinear riser model including actual current profiles were carried out, demonstrating a good correlation with test data for 2D current and low velocity 3D current. However, simulated VIV displacements were somewhat underestimated for the high velocity 3D current profiles. With reference to literature noting possible drag reduction for 3D current cases, drag coefficient sensitivity studies were carried out, demonstrating better correlation for 3D current cases at high velocities. Then, the VIV fatigue damages obtained by the three alternative structural and current modeling procedures were compared to each other in a case study. The locations of the maximum fatigue damages differed between methods, and it was demonstrated that prediction using uni-directional current does not always lead to the highest fatigue damage. In conclusion, it was demonstrated that the time domain VIV model can describe the 3D current VIV, and by doing that, it was found that present VIV prediction practice can be improved.

A hybrid FEM-DNN-based vortex-induced Vibration Prediction Method for Flexible Pipes under oscillatory flow in the time domain

In this paper, a hybrid FEM-DNN-based vortex-induced vibration (VIV) prediction method for flexible pipes under an oscillatory flow in the time domain is proposed. In this method, a vortex-induced force coefficient model is regressed by a deep neural network (DNN) from experimental data. The model takes into account the effects of flow velocity variation, VIV responses and their coupling features on vortex-induced forces. Then, it is combined with finite element method (FEM) to predict the VIV responses of flexible pipes in time domain. In addition, a phase modulation model is developed to ensure that synchronization between forces and responses can be achieved. The proposed prediction method is used to predict the VIV responses of the flexible pipe used in DNN regression training under oscillatory flows. Comparisons between the predicted results and the experimental results are conducted to verify the feasibility and accuracy of the proposed method. Then, the generalizability of the proposed method is further verified via comparisons between the predicted VIV results and the experimental results of another flexible pipe whose structural parameters are different from the DNN training pipe.

Interpretable machine learning for insight extraction from rigid cylinder flow-induced vibration phenomena

As the amount of data from numerical simulations and physical experiments is ever-increasing, machine learning techniques have become a growing paradigm shift for scientific study. Developing advanced data analytics techniques is critical in assessing the similarities and differences among different datasets since they can lead to foundational insight into the physical phenomena. This paper presents a framework that combines machine learning predictions, feature selection, and machine learning interpretation to identify important vortex-induced vibration (VIV) features and quantify their contributions to the variability often observed in VIV response. Examples from rigid cylinder forced vibrations and free vibrations are employed to show the framework's effectiveness. Using this framework, we could quickly identify and summarize the key physical insights for VIV, such as the effect of damping on response amplitude, the role of reduced velocity, and the effect of phase angle in controlling the hydrodynamic coefficients. These extracted insights were general in that they correspond well with several independent flow-induced vibration experiments on flexible cylinders. The insights extracted using the proposed framework could help people better understand the structural dynamics in a fluid environment and check the consistency of VIV prediction models.

Numerical study on the characteristics of vortex-induced vibrations of a small-scale subsea jumper using a wake oscillator model

Vortex-induced vibrations (VIVs) of a small-scale subsea jumper has been studied numerically using a wake oscillator model. The dynamics of the jumper is described based on the linear Euler–Bernoulli beam theory and a wake oscillator model is uniformly distributed along the jumper to model the hydrodynamic force acting on it. The dynamics of the coupled system has been solved using the finite element method and the responses of the jumper subjected to uniform flows in different directions have been analysed. Some important characteristics of jumper VIVs, such as multi-frequency multi-mode response, single-frequency multi-mode response as well as uncoupled cross-flow and in-line VIVs without dual resonance, have been identified and the underlying mechanism have been explored. One unique character of the jumper mode is that some segmental elements of the jumper would undertake significant rigid body motions in their axial directions. A qualitative comparison shows that these axial rigid body motions of the segmental elements may influence the hydrodynamic force and could play an important role in the reliable prediction of the jumper VIV.

Illuminating the complex role of the added mass during vortex induced vibration

The role of the added mass coefficient in vortex induced vibration (VIV) of the bluff body is complex and elusive. It is certain that decoding the relationship between the added mass and the vibration pattern will benefit the prediction and prevention of VIV. We present a study on VIV of a long flexible cylinder and forced vibration of a rigid cylinder, in a combination of experimental optical measurements and high-fidelity numerical simulation. We focus on uniform flow over a uniform cylinder at a fixed Reynolds number, Red = 900, but systematically varied the motion amplitude in the in-line (Axd) and cross-flow direction (Ayd), as well as the phase angle (θ) between the motions. We show that θ∈[π2,3π2] is associated with negative added mass coefficients in the cross-flow direction (Cmy < 0), and there is a strong correlation between the vortex shedding mode of “2P” or “P+S” and Cmy < 0.

Prediction of deepwater riser VIV with an improved time domain model including non-linear structural behavior

In this paper, vortex-induced vibrations (VIV) of deepwater risers including the VIV-induced tension fluctuations were simulated using a time domain VIV model. Data from a large-scale riser experiment was applied for correlation studies. As a first step, a new combination of in-line and cross-flow synchronization models with empirical parameters was proposed for better in-line response prediction. The performance of the improved load model was investigated based on a linear riser model. The results showed that the in-line response and its synchronization pattern were improved compared to the previous load model. Then, the improved load model was combined with a non-linear structural riser model to explore the effect of tension fluctuations. As observed in the tests, simulations showed that the tension fluctuations influenced the stability of VIV and possibly decreased the resulting vibration amplitudes. Lastly, the VIV prediction performance of the load model was evaluated with respect to frequency contents, stress levels, and fatigue damage. From the results, it was found that the improved load model in combination with the non-linear structural model reflected the response characteristics observed in the tests.

Simulating High-Mode Vortex-Induced Vibration of a Riser in Linearly Sheared Current Using an Empirical Time-Domain Model

Abstract As the development of offshore oil and gas continues to move into deeper waters, the risers connecting the seabed infrastructure to the offshore platform are getting longer. This means that vortex-induced vibration (VIV) may occur at different frequencies along the riser due to current variation, making the VIV prediction challenging. To increase the VIV prediction accuracy, an empirical time-domain VIV model has been developed. The advantage of the model is that it can account for structural non-linearities such as variable tension and time-varying flow. The robustness of the time-domain VIV model has been verified with respect to the several tests, but the length/diameter (L/D) ratio of the risers used in the experiments has so far been relatively small, and the response mode number accordingly low. Therefore, additional validation is needed to understand the uncertainties for the prediction of VIV for deep-sea riser systems at high-mode order. The purpose of the present study was to evaluate the use of time-domain model for high-mode VIV response prediction of deep-sea riser systems. This was done by a comparison study applying the Hanøytangen high-mode VIV test data. The main comparison was made regarding the dominating frequency and fatigue damage estimation. For selected empirical parameters, the model demonstrated a good correlation with the experiments. However, the simplified riser model that was utilized in the simulations caused some discrepancies between numerical simulations and the experimental results, which need to be further investigated.

Comparison of Two-Dimensional and Three- Dimensional Responses for Vortex-Induced Vibrations of a Rectangular Prism

The accurate prediction of the amplitudes of vortex-induced vibrations (VIV) is important in wind-resistant design. Wind tunnel tests of scaled section models have been commonly used. However, the amplitude prediction processes were usually inaccurate because of insufficient evaluations of three-dimensional (3D) effects. This study presents experimental measurements of VIV responses in a prototype rectangular prism and its 1:1 two-dimensional section model in smooth flow. The results show that the section model vibrates with the same Reynolds number, equivalent mass, frequency, and damping ratio as those of the prototype prism without scale effects. The VIV amplitudes can be qualitatively and quantitatively measured and analyzed. The measured VIV lock-ins of these two models agree with each other. However, the prototype prism produces a 20% higher maximum amplitude than the section model. Several classical VIV mathematical models are used to validate the wind tunnel test results. This confirms that the 3D coupling effects of the modal shape and the imperfect correlations of excitation forces positively contribute to the maximum amplitude. Based on the section model outcomes, the amplified factor of 1.2 is found to be appropriate for the amplitude prediction of VIV for the present prism, and it can also provide a reference for other structures.

Study on Vortex-Induced Vibration Predictions for Ship Rudders

Study on Vortex-Induced Vibration Predictions for Ship Rudders

Modelling of coupled cross-flow and in-line vortex-induced vibrations of flexible cylindrical structures. Part II: on the importance of in-line coupling

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.

A full three-dimensional vortex-induced vibration prediction model for top-tensioned risers based on vector form intrinsic finite element method

A novel three-dimensional model is proposed to study vortex-induced vibration (VIV) problems of top-tensioned risers. The model is based on the vector form intrinsic finite element (VFIFE) method and applicable to the fully-coupled cross-flow and in-line riser VIV scenarios. The axial time-varying dynamics and movement at the top end boundary of risers are also considered in the model. By using the VFIFE method, the structure is reduced to a set of particles whose motion satisfies Newton's second law and its response is described by the position change of particles in space. The fluctuating fluid force acting on the structure induced by vortex shedding is simulated using the classical van der Pol wake oscillator equation. In order to intuitively describe the time-varying top tension of the real structure, the real-time changes in top tension caused by the array of tensioners simulated by spring is also considered. The results of numerical simulation on top tension, dominant mode numbers, frequencies, and amplitudes are compared with the experiment to prove that the VFIFE method is feasible and effective in predicting the three-dimensional vortex-induced vibration response.

Drive comfort and safety evaluation for vortex-induced vibration of a suspension bridge based on monitoring data

Vortex-induced vibration (VIV) caused by wind excitation can seriously affect traffic comfort and safety. Therefore, reasonable control measures in the initial stage of VIV shall be formulated by the bridge management department to ensure the safety of passing vehicles. In this paper monitoring data of VIV of a suspension bridge from 2013 to 2017 were analyzed and the characteristics of structural vibration response factors such as wind parameters and Energy Concentration Coefficient (ECC) of VIV were studied. A dynamic monitoring model which used for VIV prediction was established based on the massive data collected by the existing structural health monitoring system and dynamic monitoring system of VIV installed on the bridge which is located in the East China Sea. The data measured in 2017 by the anemometers and acceleration sensors from dynamic monitoring system was taken as input to simulate the prediction process of VIV. The warning mechanism of VIV with 5 levels and corresponding measures were proposed. Results show that the root mean square (RMS) value of acceleration and ECC can be used as further criteria for judging the occurrence of VIV. The comprehensive decision method and trend analysis method can be used for VIV judgment. The recognition rate of the occurrence VIV is 88.00% and 93.24% by the comprehensive decision method and trend analysis method respectively, based on the dynamic monitoring model. The dynamic monitoring model for VIV can be adopted to timely predict VIV, and can be effectively applied in practical engineering.

Application of wake oscillators to two-dimensional vortex-induced vibrations of circular cylinders in oscillatory flows

A nonlinear time-domain simulation model for predicting two-dimensional vortex-induced vibration (VIV) of a flexibly mounted circular cylinder in planar and oscillatory flow is presented. This model is based on the utilization of van der Pol wake oscillators, being unconventional since wake oscillators have typically been applied to steady flow VIV predictions. The time-varying relative flow–cylinder velocities and accelerations are accounted for in deriving the coupled hydrodynamic lift, drag and inertia forces leading to the cylinder cross-flow and in-line oscillations. The system fluid–structure interaction equations explicitly contain the time-dependent and hybrid trigonometric terms. Depending on the Keulegan–Carpenter number (KC) incorporating the flow maximum velocity and excitation frequency, the model calibration is performed, entailing a set of empirical coefficients and expressions as a function of KC and mass ratio. Parametric investigations in cases of varying KC, reduced flow velocity, cylinder-to-flow frequency ratio and mass ratio are carried out, capturing some qualitative features of oscillatory flow VIV and exploring the effects of system parameters on response prediction characteristics. The model dependence of hydrodynamic coefficients on the Reynolds number is studied. Discrepancies and limitations versus advantages of the present model with different feasible solution scenarios are illuminated to inform the implementation of wake oscillators as a computationally efficient prediction model for VIV in oscillatory flows.

Vortex-induced vibration prediction of a flexible cylinder by three-dimensional strip model

Numerical simulations on vortex-induced vibration (VIV) of a top tensioned flexible cylinder experiencing the uniform flow and the stepped flow conditions have been carried out by our in-house solver viv3D-FOAM-SJTU in this paper, which is developed by using the open source toolbox OpenFOAM and the three-dimensional strip model. Hydrodynamic forces in each three-dimensional fluid strip are calculated through the PIMPLE algorithm in the OpenFOAM adopting the Reynolds-averaged Navier-Stokes (RANS) method. The flexible cylinder is simplified as an Euler-Bernoulli bending beam, while structural vibrations are computed using the finite element method (FEM) in both the crossflow and the inline directions. The connection between three-dimensional fluid strips and the long flexible cylinder is realized through the interpolation of hydrodynamic forces and vibration displacements in each time step. Detailed comparisons on vibration Root Mean Square (RMS) amplitude, vibration frequency and vibration modes in both flow conditions among present simulation results, experimental results and published data are presented the validity of the viv3D-FOAM-SJTU solver. The fluid strip thickness dependence study shows that three-dimensional features of the wake field begins to appear at specific strip with dramatic vibration responses when the strip thickness reaches around 1/100 L (where L is the axial length of the flexible cylinder). The flow field of all fluid strips will be fully developed when the strip thickness reaches 3/100 L. Considering the cost of computational resources and the computational efficiency, the strip thickness around 1/50 L is recommend for future researches.

Direct Numerical Simulations of Horizontally Oblique Flows Past Three-Dimensional Circular Cylinder Near a Plane Boundary

Abstract Understanding hydrodynamics of a free-spanning pipeline subjected to omni-directional flows is important to engineering design. In this study, horizontally oblique flows past a three-dimensional circular cylinder in the vicinity of a plane boundary are numerically investigated using direct numerical simulations. Parametric studies are carried out at the normal Reynolds number of 500, a fixed gap-to-diameter ratio of 0.8 and five flow inclination angles (α) ranging from 0 deg to 60 deg with an increment of 15 deg. Two distinct vortex-shedding modes are observed: parallel (α ≤ 15 deg) and oblique (α ≥ 30 deg) vortex shedding. The wake evolution is further divided into two or three stages depending on α. The occurrence of the oblique vortex shedding is accompanied by the base pressure gradient along the cylinder span and the resultant axial flows near the cylinder base. The total hydrodynamic drag and lift force coefficients decrease from being the parallel mode to the oblique mode, owing to the intensified three-dimensionality of wake flows and the phase differences in the spanwise vortex shedding. The independence principle (IP) is found to be valid in predicting hydrodynamic forces and wake patterns when α ≤ 15 deg. This IP might produce unacceptable errors when α > 15 deg. In comparison with the mean drag force, the fluctuating lift force is more sensitive to the inclination angle. The IP validity range is substantially smaller than that in the case of flow past a wall-free cylinder. Such finding would be practically useful for vortex-induced vibration prediction.

VIV of flexible structures in 2D uniform flow

The paper investigates Vortex-Induced Vibration (VIV) of a horizontal flexible structure with pinned-pinned ends in uniform flow. The fluid – structure interaction is modelled using the wake oscillator approach to describe fluctuations of the fluid forces based on the results, previously reported by Pavlovskaia et al. (2016), Postnikov (2016), and Kurushina et al. (2018). New two-dimensional wake oscillator models are developed in this study employing a number of alternative damping types in the fluid equation. The proposed models are calibrated with the published experimental data by Sanaati and Kato (2012), for VIV of a horizontal flexible structure for the middle cross-section. The calibration is performed with the focus on the maximum of the observed displacement amplitude in the cross-flow direction. One of the models having the Van der Pol – Krenk–Nielsen damping allowed achieving the lowest objective function during the calibration and, hence, was selected for the detailed analysis in 3 and 5 mode approximations. The dynamics of this model is considered in terms of the time histories, changes in the standard deviations of the modal coefficients along the reduced velocity range and frequency response. Also, a comparison with the alternative model versions in terms of the displacements generated at different locations is provided.In contrast to the previous findings by Kurushina et al. (2018) for VIV of rigid structures, for the flexible structures it was found in this study that both Van der Pol and Rayleigh damping types appear to be applicable for the VIV prediction. A detailed consideration of the 3 mode approximation of the Krenk–Nielsen – Van der Pol model reveals a presence of co-existing solutions in a number of regions of the reduced velocity. They are present in short ranges of reduced velocity in between the lock-in peaks of the in-line displacement coefficient of the first mode. Modulations and co-existing solutions in the in-line modal coefficients make the in-line displacement prediction challenging while the cross-flow displacement amplitudes are described reasonably well by the proposed models.

Comparative Analysis of Vortex-Induced Vibration Models on Risers Caused by Vessel Motion

Over the past recent years, several models for prediction of vortex-induced vibrations on risers and cables in slender marine structures have been proposed. This study provided a consistent discussion and critical evaluation of the most commonly applied models, highlighting their strengths, mathematical equations, principles, assumptions and their implications, and the apparent limitations associated with each model. The study critically evaluated and compared vortex-induced vibration models for dynamic response of water risers induced by vessel motion using a multi-criteria analysis tool (AHP). Seven alternatives which include: the DNV model, the LIC engineering model, the MARINTEK model, the MIT-Trianfyllou model, the MIT-Vandiver model, the NTH model and the UCL model were compared against a set of five broad criteria which include: Robustness, Reliability, and Accuracy, Time, Ease of application and cost. The robustness as a broad criterion contains sub-criteria like Reynold’s number range for which the simulation is valid, ability to be deployed for multimode problems, ability to describe spatial attenuation, ability to define excitation zones and how the load process is correlated in the zone. From the AHP analysis, the UCL model came out on top as the best and optimum VIV model compared to the other alternatives with an overall priority score of 1.3694. MIT-Vandiver came second with an overall priority score of 0.9656. The worst model from the outcome of the AHP analysis is the DNV model with an overall priority score of 0.6972. Keywords: vortex-induced vibration, optimum VIV model, multi-criteria analysis tool, vortex-induced model comparison DOI: 10.7176/IEL/10-2-05 Publication date: March 31 st 2020