Vortex-induced Vibration Experiments Research Articles

Experimental investigation on hydrodynamic characteristics of water intake riser undergoing vortex-induced vibration in uniform flow

With the development of ocean engineering, freely hanging water intake riser (WIR) has aroused widespread interests. The WIR with one free end would be easier to deflect remarkably, along with more complex vortex-induced vibration (VIV) responses and hydrodynamic forces. To elucidate the VIV characteristics of WIR under uniform flow, a series of VIV experiments for a scaled WIR model are carried out. Then a hydrodynamic identification method for VIV along with large offset is developed. The response and hydrodynamic characteristics of the WIR in cross-flow (CF) and in-line (IL) directions are further analyzed. Results indicate that VIV displacements with higher exciting modes are unstable when uniform flow velocity gets large. The IL-to-CF dominant frequency ratio of VIV is found to be 1.0, not 2.0 in conventional VIV knowledge and the IL harmonic at nearly twice the dominant CF frequency is observed more complex for flow velocity greater than 0.3 m/s. The vortex-induced force coefficients are found to be time-varying. Moreover, it is found that within the spectrum of Reynolds number from 2 × 103 to 1.1 × 104, the drag coefficient distribution changes along the WIR model, and the average drag coefficient is in range of 1.0–1.5, which is slightly smaller than the results predicted by the other published empirical formulae.

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.

Flexible cylinder flow-induced vibration

In this paper, we conducted a selective review on the recent progress in physics insight and modeling of flexible cylinder flow-induced vibrations (FIVs). FIVs of circular cylinders include vortex-induced vibrations (VIVs) and wake-induced vibrations (WIVs), and they have been the center of the fluid-structure interaction (FSI) research in the past several decades due to the rich physics and the engineering significance. First, we summarized the new understanding of the structural response, hydrodynamics, and the impact of key structural properties for both the isolated and multiple circular cylinders. The complex FSI phenomena observed in experiments and numerical simulations are explained carefully via the analysis of the vortical wake topology. Following up with several critical future questions to address, we discussed the advancement of the artificial intelligent and machine learning (AI/ML) techniques in improving both the understanding and modeling of flexible cylinder FIVs. Though in the early stages, several AL/ML techniques have shown success, including auto-identification of key VIV features, physics-informed neural network in solving inverse problems, Gaussian process regression for automatic and adaptive VIV experiments, and multi-fidelity modeling in improving the prediction accuracy and quantifying the prediction uncertainties. These preliminary yet promising results have demonstrated both the opportunities and challenges for understanding and modeling of flexible cylinder FIVs in today's big data era.

Comparison of translational and pivoted oscillatory systems in vortex-induced vibration experiments with uncertainty analysis

Translational oscillatory (elastically mounted) apparatus is widely applied in the study of vortex-induced vibration (VIV). With good cost-efficiency, the pivoted oscillatory apparatus has also received increasing attention in the recent decade. However, quantitative comparison of these two oscillatory systems in terms of VIV response is not clear yet. The present article investigates their behaviors comprehensively by comparative experiments at low mass (m∗=2.1) and damping ratio (ζ=0.021) in the subcritical range (5×103<Re<2.5×104). Specifically, uncertainty analysis is performed concerning the damping fluctuation of the air-bearing system. According to the comparisons, the maximum relative deviation of response amplitudes is evaluated to less than 12% including the uncertainty caused by the damping fluctuation. Notably, the deviation of response amplitudes is found to be obvious from the resonant point (f*=1) on, while the response frequencies are shown in good overall agreement. According to the comparisons of oscillation mode and transient kinematics, those two oscillatory systems have certain differences in terms of transient characteristics, which are interpreted related to the nonlinearities in VIV.

Mitigation of Vortex-Induced Vibration of Cylinders Using Cactus-Shaped Cross Sections in Subcritical Flow

Flexible cylinders, such as marine risers, often experience sustained vortex-induced vibrations (VIVs). Installing helical strakes on a riser is the most widely used technique to mitigate VIVs. This study was inspired by the giant Saguaro Cacti which can withstand strong wind with a shallow root system. In this study, numerical simulations of flow past a stationary cylinder of a cactus-shaped cross-section in a two-dimensional flow field at a subcritical Reynolds number of 3900 were performed. Results show that cylinders of a cactus-shaped cross-section have a lower lift coefficient without increasing drag compared to those of a circular cylinder. VIV experiments on a single flexible pipe as well as on a set of two tandem-arranged flexible pipes were conducted at different reduced velocities to investigate the effects of the streamwise spacing and wake of the cactus-like body shape on VIV mitigation. Experimental results show that the cactus-like body shape can mitigate VIV responses of the cylinder at upstream position with no cost of increased drag; however, similar to helical strakes, the efficiency of VIV mitigation for the cylinder at downstream position is reduced. Although the cactus-like body shapes tested in this study were not optimized for oscillation suppression, still this study suggests that modification of the cross-sectional shape to a well-designed cactus-like shape has potentials to be used as an alternative technology to mitigate VIV of marine risers.

Experimental investigation on the interference mechanical behavior of standing deepsea riser group coupling transient collision

Interference effect coupling transient collision between adjacent marine risers has become an important issue as the oil and gas industry moves to deepsea. The vortex-induced vibration experiment on a near-spaced, standing two-riser group was conducted to analyze the evolution from interference to collision and the mechanical behavior coupling transient collision. In this paper, the risers with the spacing of 2.0 D were arranged in two forms: tandem and side-by-side. Each riser had an effective length of 2.0 m and 40% of the riser length was submerged in the uniform flow. The result shows that the collision behavior of the risers was significantly constrained by flow velocity. The interference region, collision region, and destruction region in tandem and side-by-side arrangements were different. The cross-flow displacement amplitudes in side-by-side arrangement were larger than the isolated riser, and the wake and clearance flow caused that the displacement amplitude deviated to the interference side, so that the potential collision risk in the side-by-side arrangement was intensified. During the collision, the frequency components were abruptly changed, and the vibration dominated frequency was unstable. Simultaneously, the riser vibration trajectory deviated from the standard “8”, and the vibration state appeared unstable fluctuations. The response amplitude increased or decreased drastically in a short time, which increased the fatigue damage of the structure to a certain extent.

Image-based fluid data assimilation with deep neural network

Estimating unknown parameters or conditions based on observation in a numerical model is a problem considered in data assimilation. In this study, we investigate a data assimilation approach using an image-processing deep neural network (DNN) as a likelihood function. The DNN is trained by observed images and tests visualized images generated by numerical simulation. This approach does not provide exact comparison of observed and simulated images for constructing the likelihood function. However, it would be effective when exact comparison is difficult due to noise or complex measurement procedures. The effectiveness of this approach is investigated by a vortex flow around a circular cylinder moving freely in the crossflow direction, realizing vortex-induced vibration (VIV). The DNN trained by wake visualization images from the VIV experiment tests pseudo-wake images from the numerical simulation to estimate VIV amplitude and frequency parameters. By evaluating the confidence score of the DNN from the simulated images with different amplitude and frequency values, these input parameters can be estimated based on the experimental VIV images, resulting in data assimilation with a likelihood function based on the DNN. The results showed that the amplitude and frequency of the VIV can be approximated from the experimental images.

Effect of Top Tension on Vortex-Induced Vibration of Deep-Sea Risers

With the increase of water depth, the design and use of the top-tensioned risers (TTR) are facing more and more challenges. This research presents the effect of top tension on dynamic behavior of deep-sea risers by means of numerical simulations and experiments. First, the governing equation of vortex-induced vibration (VIV) of TTR based on Euler-Bernoulli theory and Van der Pol wake-oscillator model was established, and the effect of top tension on natural vibration of TTR was discussed. Then, the dynamic response of TTR in shear current was calculated numerically by finite difference method. The displacement, bending stress and vibration frequency of TTR with the variation of top tension were investigated. Finally, a VIV experiment of a 5 m long flexible top-tensioned model was carried out at the towing tank of Tianjin University. The results show that the vibration displacement of TTR increases and the bending stress decreases as the top tension increases. The dominant frequency of VIV of TTR is controlled by the current velocity and is barely influenced by the top tension. With the increase of top tension, the natural frequency of TTR increases, the lower order modes are excited in the same current.

Hydrodynamic force investigation of a rigid cylinder under the coupling CF and IL motion

Extensive researches have been conducted to simulate the two-degree-of-freedom (2DOF) vortex induced vibration (VIV) of a cylinder during the past few decades. However, there are still very few publications in terms of the hydrodynamic force of a cylinder with 2DOF motion in cross-flow (CF) and in-line (IL) directions. This study employs the Reynolds-Average-Navier–Stokes (RANS) equations and shear stress transport (SST) k-ω turbulence model to investigate the hydrodynamic force characteristics. The numerical model is firstly validated based on the 2DOF VIV experiment of an elastically supported cylinder in the literature. The results indicate that the predicted displacement and hydrodynamic force are approximately harmonic, and are in reasonable agreement with the experimental data. By imposing harmonic motion on the cylinder in CF and IL directions with period ratio of 2, parametric analyses are carried out to simply broaden the understanding of the sensitivity of the hydrodynamic coefficients including force coefficient, excitation coefficient and added mass coefficient, to the motion-related parameters, such as the motion phase angle, non-dimensional amplitude and frequency. There would be a sudden change for the excitation coefficient and added mass coefficient at a certain motion phase angle approximately corresponding to the peak of the force coefficient. The varying trend of the hydrodynamic coefficient with non-dimensional amplitude is different from that obtained from one-degree-of-freedom (1DOF) forced vibration test in literatures, and is significantly related with the motion phase angle and non-dimensional frequency. The non-dimensional frequency seems to mainly affect the excitation coefficient, added mass coefficient and mean drag coefficient in the lock-in range.

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

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.

Vortex-induced vibration of a flexible cylinder: Interaction of the in-line and cross-flow responses

It has been known that in-line (IL) response will influence cross-flow (CF) vortex shedding forces and vice versa. However, empirical codes for prediction of vortex induced vibrations (VIV) of slender marine structures have so far been limited to handle CF or IL response separately without taking into account the interaction between the two response modes. The present study uses the data from two flexible cylinder VIV experiments. The IL and CF interaction is studied by investigating the response trajectories and the corresponding hydrodynamic force coefficients under different flow conditions. It is found that the hydrodynamic force coefficients estimated from flexible cylinder tests are clearly associated with the trajectories or the motion phase angles between IL and CF displacement. The energy transfer from the fluid to the pipe is related to counter-clockwise trajectories inside the excitation region; while clockwise trajectories are associated with hydrodynamic damping forces. It is also found that the spatial variation of the motion phase angle of a flexible cylinder is influenced by the presence of traveling waves in the response. The motion phase angle is one of the key parameters to be included in the empirical VIV prediction codes in order to model the IL and CF interaction of a flexible cylinder.

Vortex-induced vibration experiments with a long semi-immersed flexible cylinder under tension modulation: Fourier transform and Hilbert–Huang spectral analyses

An experimental investigation of the vortex-induced vibration phenomenon on a long semi-immersed flexible cylinder in constant current profile were carried out at a recirculating water channel facility. The experiments are part of a comprehensive research project which aims a better understanding of non-linear riser dynamics. Vertical and prescribed monochromatic harmonic motions were imposed to the top with amplitude \(A_t/L\approx 1\,\%\) causing eigenfrequencies modulations. The mass ratio parameter is \(m^*\approx 10\), and the ratio between the total length \(L\) and the immersed length \(L_\text {im}\) is \(L/L_\text {im}\approx 3.5\). The Reynolds number \(Re\) range lies within the interval \(10^3<Re<10^4\). Three values of motion frequency \(f_t\) were imposed, being them \(f_t:f_{N,1}=\)1:3, 1:2 and 1:1 where \(f_{N,1}\) is the first eigenfrequency. The application of both the Fourier Transform and the Hilbert–Huang Transform allowed identifying subharmonic components and also frequency modulation aspects in the data.

Experimental and Numerical Studies of Vortex Induced Vibration on Cylinder

Study of vibrations due to vortex shedding (VIV) in the wake of a cylinder that is exposed to current or oscillatory flow is very important, especially for marine risers which are used to extract oil and gas from the sea bed. The phenomenon of vortex induced vibration (VIV) has been one of the major concerns for hydrodynamic researchers due to its potential ability to cause severe fatigue damage. The hydrodynamics of VIV is very complex and still not fully understood. In this paper, some results (amplitude over diameter, lift and drag coefficients) of high Reynolds VIV experiments that are performed in UTM towing tank with an in-house test set-up are presented. A circular cylinder of 114 mm in diameter and 3 min length was towed at constant speed through the basin at Reynolds numbers up to 1.1x105. Model tests with a stationary cylinder and tests with a freely vibrating cylinder were carried out to investigate the influence of VIV on drag coefficient. Later these results are compared with results obtained through 3D numerical simulation, LES is used to solve turbulence flow. It was found that CFD results showed similar trends with experimental results. Results of this paper can be very important to design riser system and future endeavor to perform similar kind of experiments. Successful numerical study of the VIV can also be fruitful for designing efficient VIV suppression devices.

Concomitant vortex-induced vibration experiments: a cantilevered flexible cylinder and a rigid cylinder mounted on a leaf-spring apparatus

This paper presents experimental results of vortex-induced vibrations (VIV), concomitantly carried out in water with a flexible cylinder, rigidly fixed, and with a ‘rigid’ cylinder, mounted on an elastic apparatus. The experiments were run at IPT towing tank facility, in a side-by-side arrangement. The flexible cylinder is simply fixed at the upper end. For the flexible cylinder, two degrees of freedom (2DOF) are implied for each vibration mode: crosswise and aligned with respect to the incident flow. The elastic support to which the ‘rigid’ cylinder is mounted is made of two vertical leaf-springs, fixed to two thick horizontal plates, conferring to the cylinder a single degree of freedom (SDOF) to oscillate transversally with respect to the incident flow. The mass ratios of the cylinders are almost the same, around 1.2 and 1.4, respectively, very low values, typical of long ocean pipe structures, as risers and pipelines. The structural damping ratio is also typically low and such as to guarantee high-amplitude responses. Besides usual spectral and statistical analysis, the Hilbert–Huang spectral analysis technique is applied, as, strictly, VIV is a non-stationary oscillation emerged from a nonlinear dynamic system. A discussion is made on the distinct VIV behaviors of the SDOF and the 2DOF systems.

One and two degrees-of-freedom Vortex-Induced Vibration experiments with yawed cylinders

Vortex-Induced Vibration (VIV) experiments were carried out with yawed cylinders. The purpose was to investigate the validity of the Independence Principle (IP) for properly describing the flow characteristics and the dynamics of structures subjected to oblique flow. Five yaw angles in relation to the direction perpendicular to the free stream velocity were tested, namely θ=0°,10°,20°,30° and 45°. Both the upstream and downstream orientations were tested. The models were mounted on a leaf spring apparatus that allows experiments with one or two degrees of freedom. The Reynolds numbers based on the component normal to the cylinder axis fell in the interval 3×103<Ren<1.5×104. The mass ratio parameter was m⁎=2.6 and the cylinder aspect ratio was L/D≈13 for all the experiments. Time histories of displacement and hydrodynamic forces were acquired. Considering only the component of the free stream which is normal to the cylinder axis, the results of amplitude and force coefficients agreed reasonably well with the non-yawed ones for yaw angles up to 20° for both one and two degrees-of-freedom experiments. This indicates the validity of the IP for this yaw angle range. For yaw angles larger than 20°, a decrease in the maximum amplitude was observed. The decrease in the oscillation amplitudes was related to a larger modulation in the phase shift between force and displacement. Differences in the results for upstream and downstream were observed and were more evident for the larger yaw angle. These differences can be associated to the asymmetric cylinder end conditions.

An experimental investigation of one- and two-degree of freedom VIV of cylinders

In the paper, an experiment investigation was conducted for one- and two-degree of freedom vortex-induced vibration (VIV) of a horizontally-oriented cylinder with diameter of 11 cm and length of 120 cm. In the experiment, the spring constants in the cross-flow and in-line flow directions were regulated to change the natural vibration frequency of the model system. It was found that, in the one-degree of freedom VIV experiment, a “double peak” phenomenon was observed in its amplitude within the range of the reduced velocities tested, moreover, a “2T” wake appeared in the vicinity of the second peak. In the two-degree of freedom VIV experiment, the trajectory of cylinder exhibited a reverse “C” shape, i.e., a “new moon” shape. Through analysis of these data, it appears that, besides the non-dimensional in-line and cross-flow natural vibration frequency ratios, the absolute value of the natural vibration frequency of cylinder is also one of the important parameters affecting its VIV behavior.

Hydrodynamic performance of flexible risers subject to vortex-induced vibrations

The Vortex-Induced Vibration (VIV) displacements are determined from both the measured accelerations and strains in a series of VIV experiments. Based on the results, the forces in the longitudinal, transversal and tangential directions are estimated by using the finite element method with and without considering the interactions between adjacent elements. The numerical simulation indicates that the method considering the interactions performs better in the estimation of the forces. The component of the transversal force in phase with the acceleration is associated with the added mass coefficient. The estimated added mass coefficients take abnormally high values at the locations where the displacements are small. An improved formula based on the L'Hospital's rule is proposed to deal with this problem. The results show the advantage of this formula in estimating the added mass coefficients at the locations with small VIV displacements.

Identification of hydrodynamic coefficients from experiment of vortex-induced vibration of slender riser model

One of the challenges in predicting the dynamic response of deepwater risers under vortex-induced vibration (VIV) is that it runs short of believable fluid loading model. Moreover, the hydrodynamic loading is also difficult to be measured directly in the VIV experiments without disturbing the fluid field. In the present work, by means of a finite element analysis method based on the experimental data of the response displacements, the total instantaneous distributions of hydrodynamic forces together with the hydrodynamic coefficients on the riser model with large aspect ratio (length/diameter) of 1750 are achieved. The steady current speeds considered in the experiments of this work are ranging from 0.15 m/s to 0.60 m/s, giving the Reynolds Number between 2400 and 9600. The hydrodynamic coefficients are evaluated at the fundamental frequency and in the higher order frequency components for both in-line and cross-flow directions. It is found that the Root-Mean Squared hydrodynamic forces of the higher order response frequency are larger than those of the fundamental response frequency. Negative lift or drag coefficients are found in the numerical results which is equivalent to the effect of fluid damping.

Virtual damper–spring system for VIV experiments and hydrokinetic energy conversion

A device/system V CK is built to replace the physical damper/springs of the VIVACE Converter with virtual elements. VIVACE harnesses hydrokinetic energy of currents by converting mechanical energy of cylinders in Vortex Induced Vibrations (VIV) into electricity. V CK enables conducting high number of model tests rapidly as damping/springs are set by software rather than hardware. V CK consists of a cylinder, a belt–pulley transmission, a motor/generator, and a controller. The controller provides a damper–spring force feedback using displacement/velocity measurements, thus introducing no artificial force–displacement phase lag, which biases energy conversion. Damping is nonlinear, particularly away from the system natural frequency, and affects modeling near the VIV synchronization ends. System identification (SI) in air reveals nonlinear viscous damping, static and dynamic friction. Hysteresis, occurring in the zero velocity limit, is modeled by a nonlinear dynamic damping model Linear Autoregression with Nonlinear Static model (LARNOS). SI performed in air is verified using monochromatic excitation in air and VIV tests in water using physical damper and springs. A resistor bank added to the device provides an integrated V CK /Power Take-Off (PTO) system. VIV testing is performed in the Low Turbulence Free Surface Water Channel of the University of Michigan at 40,000< Re<120,000 and damping 0<ζ<0.16.

An application of system identification in the two-degree-freedom VIV experiments

Experiments on the two-degree-freedom vortex-induced vibration (VIV) of a flexibly-mounted, rigid, smooth cylinder were performed at MIT. The research reported here is an analysis of the cylinder’s trajectories. System identification methods were used to derive a best Fourier representation for these motions and to parse these motions into symmetric and asymmetric behaviors. It was postulated that the asymmetric behavior was due to distortions caused by the free surface and bottom used at the test facility, and that the symmetric behavior is representative of deepwater VIV. Further application of systems identification methods was used to associate the symmetric behavior and test conditions to a traditional vortex street model. These models were analyzed for their ability to predict details of VIV trajectories.