Vortex Induced Vibration

Cross-bispectral analysis is used to investigate the coupling between a slender vibrating cylinder and the velocity field in its wake at low Reynolds numbers [Re = O(102)]. The velocity field was characterized by a complicated power spectrum, with low frequency peaks as well as upper and lower side band peaks near the primary Strouhal frequency peak and its harmonics. Power spectral and cross-bispectral analyses indicate that the rich structure of the wake velocity power spectra is a direct consequence of the vibrating cylinder. The data suggest the cylinder is coupled to its wake in the sense that the cylinder vibrations introduce motions into the wake at the vibration frequency, and these motions interact nonlinearly within the wake to produce fluctuations at other frequencies. Cross-bispectral analysis isolates the interactions between cylinder vibrations and wake fluctuations.

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Numerical simulations of the flow past a stationary circular cylinder at different Reynolds numbers (Re) have been performed using a computational fluid dynamics (CFD) solver that is based on the unsteady Reynolds-averaged Navier—Stokes equations (RANS). The results obtained are used to develop reduced-order models for the lift and drag coefficients. The models not only match the numerical simulation results in the time domain, but also in the spectral domain. They capture the steady-state region with excellent accuracy. Further, the models are verified by comparing their results in the transient region with their counterparts from the CFD simulations and very good agreement is found. The work performed here is a step towards building models for vortex-induced vibrations (VIV) encountered in risers, spars, and other offshore structures.

The study concerns the vortex-flow regimes around a cylinder exposed to an oscillatory flow and undergoing vibrations in the cross-flow direction. The flow of vortices was visualized, using the aluminum-powder technique. The study focuses on the lock-in regions. The tests have been carried out for the values of the Keulegan-Carpenter number KC = 10 and 20, and for a range of reduced velocities. The identified vortex-flow regimes were compared with the fixed cylinder case. The changes to the vortex-flow patterns were discussed with regard to their influence on the lift force. To complement the results of the experimental study, an attempt has been made to simulate the flow numerically, using the discrete vortex model. The numerical results agree with the experiments.

The complex behavior of an unsteady flow around two circular cylinders in tandem is of interest for many civil engineering applications across a wide range of aerospace, mechanical and marine applications. The present paper analyses Vortex-Induced Vibration (VIV) for flow around two circular cylinders. VIV of a circular cylinder has been shown to have the potential to generate clean energy. The efficiency of VIV power obtained from a downstream elastically mounted cylinder in the wake of an upstream cylinder is compared for different arrangements of cylinders. The upstream cylinder is stationary while the downstream one is mounted elastically with one degree of freedom normal to the mean flow direction. Scale-Adaptive Simulation (SAS) and Shear Stress Transport (SST) CFD models are utilized to analyze the validity of the SAS turbulence model. For this purpose, the spacing between the cylinders was varied whilst the Reynolds number is kept constant. The numerical simulation is validated against the mean pressure coefficients, lift and drag coefficients and Strouhal number experimentally measured on cylinders. The results indicate that both turbulence models predict the flow characteristics around the cylinders with reasonable precision; however, the predictions from SAS were more accurate compared to SST. Based on this comparison a SAS model was chosen as a tool to analyze the VIV power which can be captured by the downstream cylinder. To obtain the optimum efficiency of VIV power, the location of downstream cylinder has been altered in the wake of upstream one. The findings reveal that the arrangement of cylinders can significantly change the efficiency of VIV power. Cylinders offset from one another show a higher efficiency compared to aligned cylinders.

This chapter addresses the analysis of VIV e↵ects on the deck of the following bridges: Niterói (Brazil), Volgograd (Russia) and Trans-Tokyo Bay (Japan).These case studies, together with the Alconétar arch (cf. Chapter 2) o↵er real data that allow the validation of numerical simulations carried out. An equivalence can be established between complex cross sections and basic geometries analysed in Chapter 3, which leads to design criteria in order to diminish the sensibility to VIV. Strategies traditionally used to minimise vibrations are shown. The placement of damping devices on bridge decks tends to be the most common, although at times it is su cient to make small modifications in the cross section geometry to modify the vortex shedding regime as the excitation source. Other bridge elements can significantly influence its sensitivity to VIV, as the representative cross section geometry is changed, thus altering the wind structure interaction process. Lastly, VIV e↵ects can occur not only for the bridge in service, but also during construction, as happened in Alconétar. It is necessary, therefore, to take into consideration the dynamic structural properties and cross section geometries for the di↵erent construction phases of the bridge, to avoid any possible configuration that might lead to have VIV e↵ects, as well as some other aerodynamic problems. This section describes some of the most VIV representative cases that occurred on actual bridges in the previous years apart form Alconétar, which provide a basis for an overall analysis of the problem: Niterói, Volgograd and Trans-Tokyo Bay Bridge. 155 Chapter 5. Bridge case studies Vortex-induced vibrations on bridges Wind instability phenomena like flutter are very rare to happen on continuous steel girder bridges, due to the fact that these bridge types have a higher sti↵ness and also higher natural vibration frequencies. However, they are not safe from VIV e↵ects, associated with low regular wind velocities acting during a certain time period, typical from vast flat surfaces such as bays, estuaries or extensive rivers with no big obstacles that could introduce great disturbances in the wind regime. Continuous girder bridge typology is suggested as the best alternative in current projects for the crossing of large rivers and onshore areas to cover the overall bridge length, or as the structural solution for the approach spans of cablesstayed or suspension bridges. The three cases presented correspond to deck vertical oscillations for VIV e↵ects. However, as noted in Chapter 1, it should not be forgotten that other structural elements also are prone to this type of phenomenon, such as towers of cable-stayed or suspension bridges, as well as arch bridge hangers. Fortunately, vibration effects did not have serious consequences in the presented case studies, but they produced social alarm and media attention that has brought about a more exhaustive surveillance and minimisation of said oscillations through the addition of damping devices to avoid long-term fatigue problems. Numerical simulations are used to perform problem analysis, in the same way as done for the Alconétar Arch (cf. Chapter 2). The objective is to identify the interaction mechanism in complex sections and compare it with basic section geometries studied in Chapter 3, in order to give some design rules and avoid the resonance phenomena caused by dynamic wind actions. Numerical model validation is accomplished from the actual vibration episodes registered in every case study.

Two-dimensional space-time finite-element simulations are carried out to study the effect of oscillator mass ratio, m * on free inline and transverse vibrations of a rigid square cylinder. The effect of viscous damping or resistance is not considered and thus cylinders of mass ratio 1, 5, 10 and 20 execute free undamped vibrations. Results are presented for 50≤Re≤250 where Re is the Reynolds number. For m * = 1, synchronization between cylinder oscillation and vortex-shedding is 1:1 (periodic flow). Thus, the Cl-Y phase portraits must be single looped closed curves. For m * = 5-20, synchronization is 1:1 (periodic flow) prior to galloping. For m * = 5, the synchronization during galloping is quite different from 1:3. Along 'increasing Re', the entire galloping branch of m * = 5 cylinder is quasi-periodic. However, along 'decreasing Re', it is mostly quasi-periodic and it is periodic for Re≤190. An 1:3 sub-harmonic synchronization is seen for the m * = 10 and 20 cylinders only in the periodic regime of galloping occurring at higher Re. For m * = 1, only one kink is seen in the response curve signifying transition from initial to lower branch. Two kinks are captured for m * = 5 cylinder. For m * = 10 and 20, an additional third kink is resolved in the galloping branch that signifies transition from quasi-periodic flow/body motion to periodic flow/body motion.

The control of vortex-induced vibration of two side-by-side circular cylinders in a cross flow is carried out experimentally. One cylinder is elastically supported and the other is fix-supported at both ends. The two cylinders vibrate under the action of the unsteady flow-induced forces. A micro actuator is embedded on the surface of each cylinder to perturb the boundary layer. The spacing ratio is set at 1.2. The measurement shows that the structural vibration can be suppressed significantly when the reduced excitation frequency is around 2.655.

The control of vortex-induced vibration of two side-by-side circular cylinders in a cross flow is carried out experimentally. One cylinder is elastically supported and the other is fix-supported at both ends. The two cylinders vibrate under the action of the unsteady flow-induced forces. A micro actuator is embedded on the surface of each cylinder to perturb the boundary layer. The spacing ratio is set at 1.2. The measurement shows that the structural vibration can be suppressed significantly when the reduced excitation frequency is around 2.655.

This paper reviews the dynamics and vibration control techniques for marine riser systems. The riser pipes are modeled as Euler-Bernoulli beams that vibrate under the effects of ocean loads and the movements of the surface vessel, resulting in hybrid ODE-PDE equations. Chronological development of such hybrid models is first discussed, and their approximated ODE models for simulation are examined. Theoretical and experimental techniques for instability and fatigue analyses on the riser systems are also summarized. To increase the fatigue life against ocean currents, passive vibration suppression devices (e.g., strakes and spoilers) were mounted on the surface of the riser. Whereas to tackle the instability problem caused by sea waves, active control techniques utilizing the movements of the vessel were employed. In Conclusions, as future riser technologies, seven research issues are identified.

The DF1-1 submarine pipeline was investigated using a dual-frequency side-scan sonar and a swath sounder system. More than a hundred scour pits under the pipeline were found, most of which have caused the span of the pipeline to increase and threatened its safety. The maximum allowable free span length (MAFSL) of the pipeline was determined through the limitations regarding maximum allowable stress under static or quasi-static loads and the onset of Vortex Induced Vibrations (VIV) under different hydrodynamic actions. The results show that the MAFSL under static conditions is 56 m. However, the MAFSLs are 30 m and 20 m under ordinary weather conditions and hurricane-induced currents for the 100-year return period, respectively, to avoid VIV as calculated by using the highest safety class factor. It is suggested that spanning pipelines longer than 20 m should be supported. Additionally, eight successive spans which may also threaten the pipeline were proposed. The most hazardous scour pits are along the pipeline section from KP42 to KP51.

numerical investigations were unable to fully capture all flow regimes using the k-ω model (Guilminineau and Queutey 2003). It is observed that the SST model is able to cover all three branches of the flow-induced oscillation of the cylinder. Based on the numerical results of the SST model, the maximum obtained theoretical power coefficient of the VIV power is evaluated to be approximately 10% for a single cylinder.

numerical investigations were unable to fully capture all flow regimes using the k-ω model (Guilminineau and Queutey 2003). It is observed that the SST model is able to cover all three branches of the flow-induced oscillation of the cylinder. Based on the numerical results of the SST model, the maximum obtained theoretical power coefficient of the VIV power is evaluated to be approximately 10% for a single cylinder.

The phenomenon of flow-induced vibration (FIV) over a square cylinder at Reynolds numbers, Re ¼ (3.6-12.5  10 3 ) is numerically studied. This current study provides a detailed explanation of the behaviour of transverse motion of square cylinder with the mass damping ratio, m Ã ζ ¼ 2.48. The computation of FIV is conducted by numerical simulation based on the Unsteady Reynolds Navier-Stokes (URANS) flow field using OpenFOAM software. The first part of the numerical simulation consists of an isolated square cylinder to validate the solution with previous studies. The computation of FIV with a total number of cells, N ¼ 101,662 have shown a comparable pattern of amplitude curve. The coexistence of vortex-induced vibration (VIV) and galloping is observed for a single isolated cylinder. A downstream flat plate is introduced in the second part of the work. Different gaps separation between the cylinder and flat plate (0.1 ⩽G/D ⩽ 3) are simulated. Based on the amplitude curve against reduced velocities 4 ⩽ U R ⩽ 20, four regimes are identified. According to the power estimation, the optimum gap separation is G/D ¼ 0.1. The harnessed power is higher than a single isolated square cylinder while preserving the robustness for the remote harvesting purpose.

of hexagonal cylinders at Reynolds number of 1000 and mass ratio of 2 are studied numerically. In the numerical model, the Navier-Stokes equations are solved using finite volume method, and the fluid-structure interaction (FSI) is modelled using Arbitrary Lagrangian Eulerian (ALE) Scheme. The numerical model accounts for the cross-flow vibration of the cylinders, and is validated against published experimental and numerical results. In order to account for different angles of attack, the hexagonal cylinders are studied in the corner and face orientations. The results are compared with the published results of circular and square cylinders. Current results show that within the studied range of reduced velocities (up to 20), unlike circular and square cylinders, no lock-in response is observed in the hexagonal cylinders. The maximum normalized VIV amplitudes of the hexagonal cylinders are 0.45, and are significantly lower than those of circular and square cylinders. Vortex shedding regimes of the corner-oriented hexagons are mostly irregular. However, in the face-oriented hexagons, the shedding modes are more similar to the typical P + S and 2P modes.

Vortex-induced vibrations (VIV) of hexagonal cylinders at Reynolds number of 1000 and mass ratio of 2 are studied numerically. In the numerical model, the Navier–Stokes equations are solved using finite volume method, and the fluid-structure interaction (FSI) is modelled using Arbitrary Lagrangian Eulerian (ALE) Scheme. The numerical model accounts for the cross-flow vibration of the cylinders, and is validated against published experimental and numerical results. In order to account for different angles of attack, the hexagonal cylinders are studied in the corner and face orientations. The results are compared with the published results of circular and square cylinders. Current results show that within the studied range of reduced velocities (up to 20), unlike circular and square cylinders, no lock-in response is observed in the hexagonal cylinders. The maximum normalized VIV amplitudes of the hexagonal cylinders are 0.45, and are significantly lower than those of circular and squa...

This work presents a numerical study on the vortex-induced vibration (VIV) phenomenon of synchronization of a vertical, flexible, circular cylinder with a length-to-diameter ratio of 475, being free to move along the in-line (IL) and cross-flow (CF) directions for Reynolds numbers of 42K, 84K and 126K. It is found that the dominant mode numbers, the maximum root mean square amplitudes, the dominant frequencies and the lift coefficient increase with the Reynolds number, but the drag coefficient decreases. The in-line response shows a main frequency component at twice the cross-flow frequency. At some Reynolds number value and riser span location, a third harmonic frequency component is observed in the CF response. The aims of this paper are to study the lock-in phenomenon and the effect of the flow-induced tension in the riser frequency spectrum. The lock-in analysis shows that when both the cross-flow riser movement and the velocity transversal component frequency values are the same, lock-in takes place. The lock-in is established at the vibration mode predominant frequency for the three Reynolds numbers. The results show also that, taking into account the tension produced by the flow, the vibration frequency spectrum will be calculated accurately. The drag force produces a flow-induced tension that makes the riser behave as a tension-dominated riser, even if the riser was not pre-tensioned.

Design of compact, lightweight, and robust heat recovery steam generators (HRSGs) is important for the implementation of offshore bottoming cycles. In this work, a novel methodology integrating flowinduced vibration analysis is proposed for the design optimization of HRSG. The vibration analysis considers four main flow-induced vibration mechanisms, namely turbulent buffeting, vortex shedding, fluidelastic instability, and acoustic resonance. The corresponding sub-models are selected from literature for estimating vibrations of finned tube bundles. The design criteria for vibration are included in the optimization problem as constraints, which affect the decision of the solver simultaneously. The methodology is demonstrated by three cases, in which not only geometric parameters but also operating parameters are set as design variables. With activating vibration constraints, the solver finds appropriate geometric and operating parameters to meet the design requirements. The comparison between the optimizations with and without the vibration constraints shows a trade-off between compactness, lightweight, efficient heat transfer, and the possibility of flow-induced vibration problems. This indicates the importance of consideration of vibration problems when designing such HRSGs.

Piezoelectric energy harvesters (PEHs) are an energy harvester that generate electricity based on pressure or vibration. Vortex-induced vibration (VIV) is another method to harvesting the wind energy used piezoelectric. The objective of this research is to know the effective triangular bluff body to generate VIV in PEHs. PVDF type LDT0-028K was the piezoelectric that used in this research. The bluff body with varied dimension (10, 20, 30 mm) were placed at the tip of fin with distance of 50 mm. bluff body and piezoelectric placed in wind tunnel with a cross-sectional area of 180 x 180 mm2. Simulation used finite element method and validated in wind tunnel with condition such as laminar flow and the velocity 3 m/s. Result showed that bluff body 10 mm and 20 mm create air flow with vortex in thin area and possible to hit fin and piezoelectric several time and lead to vibration. In simulation, the 30 mm triangular bluff body creates more vortex air flow but lower velocity hit the piezoelectric. In experimental result, it produces the smallest electrical voltage. It explains that vortex with minimum velocity hit the piezoelectric lead minimum vibration to generate voltage.

The flow and drag induced by active pitching of plates in the wake of a cylinder of diameter d were experimentally studied for various plate lengths L as well as pitching frequencies f p and amplitudes A 0 at Reynolds number Re = 1.6 × 10 4. Planar particle image velocimetry and a load cell were used to characterize the flow statistics and mean drag of a variety of cylinder-splitter assemblies. Results show the distinctive effect of active pitching on these quantities. In particular, flow recovery was significantly modulated by L, f p , or A 0. Specific pitching settings resulted in a wake with dominant meandering patterns and faster flow recovery. We defined a modified version of the amplitude-based Strouhal number of the system St A to account for the effect of the cylinder in active pitching. It characterizes the drag coefficient C d across all the cases studied, and reveals two regions intersecting at a critical value of St A ≈ 0.035. Below this value, the C d remained nearly constant; however, it exhibited a linear increase with increasing St A past this critical point. Inspection of the integral momentum equation showed the dominant role of velocity fluctuations in modulating C d past the critical St A .

The phenomenon of flow-induced vibration (FIV) over a square cylinder at Reynolds numbers, Re ¼ (3.6-12.5  10 3) is numerically studied. This current study provides a detailed explanation of the behaviour of transverse motion of square cylinder with the mass damping ratio, m Ã ζ ¼ 2.48. The computation of FIV is conducted by numerical simulation based on the Unsteady Reynolds Navier-Stokes (URANS) flow field using OpenFOAM software. The first part of the numerical simulation consists of an isolated square cylinder to validate the solution with previous studies. The computation of FIV with a total number of cells, N ¼ 101,662 have shown a comparable pattern of amplitude curve. The coexistence of vortex-induced vibration (VIV) and galloping is observed for a single isolated cylinder. A downstream flat plate is introduced in the second part of the work. Different gaps separation between the cylinder and flat plate (0.1 ⩽G/D ⩽ 3) are simulated. Based on the amplitude curve against reduced velocities 4 ⩽ U R ⩽ 20, four regimes are identified. According to the power estimation, the optimum gap separation is G/D ¼ 0.1. The harnessed power is higher than a single isolated square cylinder while preserving the robustness for the remote harvesting purpose.

Vortex-induced vibration (VIV) has been extensively studied for its significant role in the fatigue of marine structures subjected to excitations induced by vortex shedding. As one of the sub-topics, the Reynolds number (Re) effects on responses neglected by older studies have been verified as non-negligible by more recent studies. In the traditional plot of normalized response amplitude (A*-U*), with some exceptions, the reduced velocity (U*) is varied by changing the velocity of incoming flow, therefore the Reynolds number varies simultaneously. In the present article, a series of experiments are conducted with the variation of cylinder diameter, structural stiffness, and mass ratio, respectively, under the condition of subcritical Reynolds number regime and low mass-damping. Here, a non-dimensional parameter alfa (α�Re/U*) is introduced to relate the responses from experiments at different dimensions, making it feasible to obtain an overall view of responses. In addition to recognizing the Re effects, the influences of the mass ratio are also observed with consideration of the Re effects. Furthermore, a rough response surface of VIV is drawn out initially based on the present experimental results and those from several publications, especially at high Re regime. This new proposed response surface seems to lead to an improvement to the application in engineering.

Vortex-induced vibration (VIV) has been extensively studied for its significant role in the fatigue of marine structures subjected to excitations induced by vortex shedding. As one of the sub-topics, the Reynolds number (Re) effects on responses neglected by older studies have been verified as non-negligible by more recent studies. In the traditional plot of normalized response amplitude (A*-U*), with some exceptions, the reduced velocity (U*) is varied by changing the velocity of incoming flow, therefore the Reynolds number varies simultaneously. In the present article, a series of experiments are conducted with the variation of cylinder diameter, structural stiffness, and mass ratio, respectively, under the condition of subcritical Reynolds number regime and low mass-damping. Here, a non-dimensional parameter alfa (α�Re/U*) is introduced to relate the responses from experiments at different dimensions, making it feasible to obtain an overall view of responses. In addition to recognizing the Re effects, the influences of the mass ratio are also observed with consideration of the Re effects. Furthermore, a rough response surface of VIV is drawn out initially based on the present experimental results and those from several publications, especially at high Re regime. This new proposed response surface seems to lead to an improvement to the application in engineering.

Vortex-Induced Vibrations (VIV) are well-known and related to the majority of cylindrical structures subjected to strong winds or currents. The VIV limit the lifetime of the structure because they increase the forces and so the fatigue. When several structures of this kind are put together in close interaction, the wake effects (Wake Induced Oscillations-WIO) sometimes involve strong instabilities. If these structures are flexible or mobile, oscillations of several diameters can be observed and collisions can occur ([4] & [6]). Such structures are widespread in the oil industry where the extraction of oil in deep water can be done by means of risers. In some cases, risers are connected to a floating support called FPSO (Floating Production Storage Offloading) and held in tension by buoys (figure 2). These buoys are located at depth where waves do not have any significant influence. However, in these areas, the magnitude of currents can be sometimes important. Consequently, engineering companies have to find solutions to prevent hydrodynamic interactions between risers and buoys. For a better understanding and characterization of wake effects, an experimental study is carried out at the Ifremer (Institut Fran¸cais de Recherche pour l'Exploitation de la Mer) flume tank in Boulogne-sur-Mer, France. This work is completed within the framework of the project Clarom cepm co 3007/04, in partnership with Doris engineering, Saipem s.a., Institut Fran¸cais du P´etrole, Oceanide, Ecole Centrale Marseille & Total. After presenting the experimental setup, we will focus on the interaction effects between two cylinders in close proximity. In this study, we quantify wake effects and risks of collision between structures. Several orientations and spacings between the cylinders are considered and tested. A short numerical study performed with the CFD code Fluent is also presented, for which hydrodynamic coefficients and motions of a single cylinder in a flow are numerically evaluated.

Vortex-induced vibrations (VIV) in systems with more than one degree of freedom often present complex synchronization among the motion components, also hidden by the randomness that characterizes the motion itself. A phase average method has been here developed and applied to the displacements of a tethered sphere, at low mass and damping, to analyze its xy trajectories over a wide range of reduced velocities, 5 B U* B 25 (Reynolds numbers, 5.1 9 10 3 B Re B 2.67 9 10 4). This method has allowed the identification of both the periodic and chaotic contribution of each motion component, accurately reconstructing the underlying trajectory periodic pattern. The two classical vibration modes, I and II, have been also observed. The method developed here was able to better rebuild the experimental data compared to other methods found in the relevant literature, providing useful insights into the study of the dynamic response of a freely-oscillating tethered sphere immersed in a steady flow.

We investigate the onset of vortex-induced vibrations in the wake of a spring-mounted circular cylinder by means of stability analyses and asymptotic methods. We carry out an expansion of the coupled flow-structure system, assuming that the Reynolds number departs from criticality at second order. The flow is forced by a third-order cylinder displacement under the form of an equivalent resonant blowing and suction velocity, applied at the wall of a virtually fixed cylinder. This imposes the classical compatibility conditions to be modified so as to encompass the effect of a resonant boundary condition. By analyzing the nonlinear dynamics of the associated limit cycles, we will show that the present model allows to recover the main phenomenology of vortex-induced vibrations, including subcritical vortex-shedding, lock-in and hysteretical behaviours. We will also use this model to assess the influence of the structural damping on the amount of energy that can be extracted from the flo...

Surge motion of top-end platform induced by periodic wave makes marine flexible riser encounter equivalent sheared-oscillatory flow, under which the Vortex-induced Vibration (VIV) response will be more complicated than pure sheared flow or oscillatory flow cases. Based on a time domain forcedecomposition model, the VIV response characteristics under sheared-oscillatory flows are investigated numerically in this paper. Firstly, the adopted numerical model is validated well against laboratory experiments under sheared flow and oscillatory flow. Then, 20 sheared-oscillatory flow cases with different oscillation periods and top maximum current velocities are designed and simulated. Under long and short oscillation period cases, the structural response presents several similar features owing to the instantaneous sheared flow profile at each moment, but it also has some different patterns because of the differently varying flow field. Finally, the effects and essential mechanism of oscillation period and top maximum current velocity on VIV response are discussed systematically.

With influence of current, the platform and the connected riser form relative motion in the flow. In this paper, A computational fluid dynamics (CFD) model is built by strip theory and Bernoulli-Euler bending beam model. Based on this numerical model, the platform motions and corresponding current velocities are employed to investigate the responses of the riser. The numerical results indicate that the dominant crossflow and inline vibration modes are increased with the current velocity. The amplitudes of crossflow and inline vibration modes show periodic variations with the increase in the amplitude of the surge motion and the current velocity. The similar vortex shedding mode in the uniform flow is observed during vortex-induced vibrations of riser under the low frequency platform motions.

The interference phenomenon among multiple flexible cylinders is an important factor in designing offshore structures, which has been extensively studied by researchers in the past decades. In this paper, Vortex-induced Vibration (VIV) responses of two flexible cylinders experiencing the stepped flow with tandem arrangement are mainly studied using the viv-Thick-SJTU solver, which is developed based on the open source toolbox OpenFOAM and the Bernoulli-Euler bending beam model combining with the thick strip model. Hydrodynamic forces are calculated in each three-dimensional (3D) fluid strip, while vibrations of the cylinder are computed using the Finite Element Method (FEM) through the Newmarkβ algrithm. The dynamic mesh technique is adopted to update the computational mesh through the cubic spline interpolation algorithm at every time step. Effects of the gap ratio between two tandem cylinders on vibration characteristic are mainly studied, while gap ratios are set as 4 and 10. Firstly, the validation of the code is conducted. Then, the modal decomposition method and Fast Fourier Transform (FFT) method are used to get the dominant vibration mode and vibration features of both cylinders.

Vortex-induced vibration (VIV) of a tapered cylinder is investigated numerically at a constant Reynolds number of 500 using three-dimensional numerical simulations. The objectives of the study is to identify the difference between the response of a tapered cylinder and that of a uniform cylinder. Simulations are conducted for the lengths and the taper ratios the same as those in the published experimental studies. Two cylinders are considered: one with a length to diameter ratio of 4.3 and a mass ratio of 2.27 and another one with a length to diameter ratio of 12.3 and a mass ratio of 6.1. Detailed analysis of the vibration amplitude and frequency, the vortex shedding flow mode and the lift coefficient are performed for the longer cylinder. It was found that the frequencies of the vortex shedding and the lift coefficient synchronize with the vibration frequency in the lock-in regime, but vary along the cylinder span outside the lock-in regime. For some reduced velocities, it is found that vortex shedding is in 2P mode at the smalldiameter part and 2S mode at the larger-diameter part of the cylinder, forming a hybrid flow mode. The change of the flow mode on the cylinder span corresponds to the change of the phase difference between the lift coefficient and the displacement for about 180 • . The lock-in regime of a tapered cylinder is found to be wider than that of an equivalent uniform cylinder.

Vibration of two elastically mounted cylinders in an oscillatory flow at a Keulegan-Carpenter number of 10 is simulated numerically. The two cylinders are rigidly connected with each other and are allowed to vibrate in the cross-flow direction only. The aim of this paper is to identify the effects of the orientation of the cylinders and the gap between the cylinders on the vibration. The two-dimensional Reynolds-Averaged Navier-Stokes equations are solved to predict the flow and the cylinder vibration is predicted using the equation of motion. When the two cylinders are in a tandem arrangement, a combined single pair flow regime and attached pair flow regime are observed as reduced velocity exceeds 10 and this combined regime and the single pair regime occurs intermittently. Periodic vibration is found when the two cylinders are in a staggered arrangement with a 45 • flow attack angle. When the two cylinders are in a side-by-side arrangement, a new single vortex regime is observed. This single vortex remains attached to the cylinder surface and rotates around the cylinder. The intermittent switch between this single vortex regime and the single pair regime are observed.

g r a p h i c a l a b s t r a c t h i g h l i g h t s • Flow-induced vibration of circular cylinder with a rigid splitter plate. • Three regimes of FIV: VIV, steady flow and galloping. • Effect of mass ratio, plate length and Re on response during different regimes. • VIV galloping interaction for low mass ratio. • Steady flow and classical galloping for high mass ratio.

Two-dimensional space-time finite-element simulations are carried out to study the effect of oscillator mass ratio, m * on free inline and transverse vibrations of a rigid square cylinder. The effect of viscous damping or resistance is not considered and thus cylinders of mass ratio 1, 5, 10 and 20 execute free undamped vibrations. Results are presented for 50≤Re≤250 where Re is the Reynolds number. For m * = 1, synchronization between cylinder oscillation and vortex-shedding is 1:1 (periodic flow). Thus, the Cl-Y phase portraits must be single looped closed curves. For m * = 5-20, synchronization is 1:1 (periodic flow) prior to galloping. For m * = 5, the synchronization during galloping is quite different from 1:3. Along 'increasing Re', the entire galloping branch of m * = 5 cylinder is quasi-periodic. However, along 'decreasing Re', it is mostly quasi-periodic and it is periodic for Re≤190. An 1:3 sub-harmonic synchronization is seen for the m * = 10 and 20 cylinders only in the periodic regime of galloping occurring at higher Re. For m * = 1, only one kink is seen in the response curve signifying transition from initial to lower branch. Two kinks are captured for m * = 5 cylinder. For m * = 10 and 20, an additional third kink is resolved in the galloping branch that signifies transition from quasi-periodic flow/body motion to periodic flow/body motion.

The effects of surface roughness on the vortex-induced vibration (VIV) performances of a circular cylinder were studied numerically. The VIV response amplitude, response frequency, vortex force, vortex phase and vortex shedding flow pattern with different degrees of surface roughness were compared. The numerical results show that the VIV response amplitude decreases with increased surface roughness. For a smooth cylinder and a cylinder with small surface roughness, the VIV response could be divided into three branches: initial branch, upper branch and lower branch. In the initial and upper branches, the vortex shedding flow pattern displays a 2S mode. However, the VIV response produces a 2P mode vortex shedding pattern in the lower branch. For a cylinder with large surface roughness, the VIV response can be divided into only two branches: an initial branch and a lower branch. In the initial branch, the vortex shedding flow pattern displays a 2S mode, but in the lower branch, the pattern changes to the 2P mode. The transition of 2S mode to 2P mode is caused by the abrupt change of the vortex phase between the vortex force and the VIV response.

Drilling risers used in oil and gas operations are subjected to external loads such as wave and current. One of the phenomena that arise from the external loads is the Vortex-Induced Vibration (VIV), which affects the performance of the riser due to excessive vibration from the vortex shedding. A significant factor influencing the VIV is the design of the drilling riser and its auxiliary lines. Until now, the optimum geometrical size and gap between the auxiliary and the main riser are still very scarcely studied. In this paper, the main objective is to study the effects of the gap ratio (G/D) on the vortex shedding phenomenon on a fixed and freely vibrating riser. The riser system was modelled with a main drilling riser and six auxiliary lines with a constant diameter ratio (d/D) of 0.45 and gap ratio (G/D) = 0 to 2.0 in the laminar flow regime with Reynold Number, Re = 200. The simulations were conducted for Single Degree of Freedom (SDOF) using Computational Fluid Dynamics (CFD) ...

Article History: Received: 24 May. 2016 Accepted: 15 Sep. 2016 Free-span in subsea pipelines occur at manmade supports, uneven seabed or pipeline crossing. Free spanning may induce pipeline vibration due to vortex shedding which makes pipeline susceptible to some failures such as fatigue, fracture, etc. Free spanning analysis is an important subject because fatigue is the most effective factor in reducing the pipeline design life. Free spanning analysis includes static analysis and dynamic analysis. DNV-RP-F105 suggests a methodology of dynamic analysis for long pipeline with multi-mode responses, but the fatigue analysis method for multi-modes is not detailed. In addition, the fatigue analysis of multi-spanning pipeline is not clear. Based on the methodology of DNV-RP-F105 fatigue life relates to natural frequencies of pipeline, the method of determination of effective natural frequencies still is not clear. In this paper, a fatigue analysis for multi-spanning pipeline in Persian s...

PurposeThe purpose of this paper is to mainly review the state-of-the-art developments in the field of hydrodynamics of offshore pipelines, identifying the key tools for analysis of pipeline free spans, their applications, their qualifying characteristics and capabilities and limitations.Design/methodology/approachThese different analytical, numerical and semi-empirical tools available for predicting such hydrodynamic loads and their effects include VIVANA, PIPESIN, VIVSIM, SIMULATOR, FATFREE, amongst others. Inherent in these models are current effects, wave effects and/ or pipe–soil interactions.FindingsAmongst these models, the most attention was given to the new VIVANA model because this model take into account the vortex-induced effects with respect to free-spanning pipelines (which have dominant effect in the span analysis in deep water) better than other semi-empirical models (such as Shear 7). Recent improvements in VIVANA include its ability to have arbitrary variation in s...

Vortex-induced vibration (VIV) of a sphere represents one of the most generic fundamental fluid–structure interaction problems. Since vortex-induced vibration can lead to structural failure, numerous studies have focused on understanding the underlying principles of VIV and its suppression. This paper reports on an experimental investigation of the effect of imposed axial rotation on the dynamics of vortex-induced vibration of a sphere that is free to oscillate in the cross-flow direction, by employing simultaneous displacement and force measurements. The VIV response was investigated over a wide range of reduced velocities (i.e. velocity normalised by the natural frequency of the system): $3\leqslant U^{\ast }\leqslant 18$, corresponding to a Reynolds number range of $5000

As a means of understanding the phenomenon of vortex-induced vibration, the forced oscillation of a cylinder in cross-flow has been examined. A lack of information regarding the connections between energy transfer and the wake mode of oscillating bodies in low Reynolds number flow prompted a two-dimensional numerical investigation at Re = 200. The region in which vortex shedding synchronized with the cylinder forcing frequency was defined and the energy transfer calculated. The mode of Kármán vortex shedding incorporated a gradual change from positive to negative energy transfer as the amplitude of the motion was increased. A mode of shedding was detected in the negative energy region in which a pair and a single vortex (P+S) were shed each motion cycle. Energy contours were established in the region of primary lock-in and the boundary of zero energy transfer was defined.

One of the most basic examples of fluid-structure interaction is provided by a tethered cylinder or sphere in a fluid flow. The tendency of a tethered sphere to oscillate when excited by waves is a well-known phenomenon and it has only recently been found that the same system will act in a similar fashion when exposed to a uniform flow at moderate Reynolds numbers, with a transverse peak-to-peak amplitude of approximately two diameters over a wide range of velocities. The present paper presents results of DNS of the flow past a tethered cylinder. The coupled Navier-Stokes equations and the equations of motion of the cylinder are solved using a spectral element method. The fluid forces acting on the cylinder as well as the tension in the tether are computed and used to determine the resulting motion of the object. It is found that the mean amplitude response is greatest at high reduced velocities, i.e. when the cylinder is oscillating predominantly transverse to the fluid flow. Furthermore, the oscillation frequency is found to correspond to the vortex shedding frequency of a stationary cylinder, except at high reduced velocities. This is in contrast to a tethered sphere in which the oscillation frequency does not correspond to either the vortex shedding frequency or the natural frequency. Visualizations of the vortex structures in the wake reveal the mechanisms behind the motion of the cylinder, and suggest that the induced oscillations are highly significant in the prediction of cylinder response in a steady flow.

Flow-induced vibration of a tethered body immersed in a uniform flow represents a fundamental example of fluid-structure interaction. However, to date little research in this problem has been undertaken. This paper presents results from a twodimensional numerical simulation of the flow past a tethered circular cylinder with mass ratio, m * = 0.833. The Navier-Stokes and dynamic equations of motion of the cylinder are solved using a Galerkin spectral-element/Fourier method. The fluid forces acting on the cylinder, as well as the tension in the tether, are computed and used to determine the resulting cylinder motion. A large peak in the cylinder oscillation is noted for a reduced velocity u * 19 corresponding to a negative maximum in the mean lift and a positive maximum in the RMS drag force. Analysis of the vortex structures in the wake of the cylinder, for the cases of u * = 15.4, 19 and 21, reveal a subtle asymmetry in the shedding process which may provide a mechanism to explain this peak in oscillation amplitude.

The current experimental work focuses on studying the effects of surface roughness on vortex induced vibration (VIV) of an elastically mounted circular cylinder which is free to vibrate in a direction transverse to the flow. Our objective is to identify configurations which lead to high amplitudes of vibrations and a greater range of synchronization that can be successfully used for energy harvesting. Different configurations such as smooth cylinders, cylinder with zero roughness strips, and prescribed roughness (using sand paper) were used. Experiments were also conducted with the zero roughness strips at different angles around the cylinder to verify the effect of the position of the strip. All results were also found to be dependent on the spring stiffness. Variations were observed in the amplitude and frequency response profiles for the different cases investigated.

The current experimental work using circular cylinders aims to identify mechanisms that amplify vortex-induced vibrations (VIV) and galloping oscillations, within the Reynolds number range, 1.5 × 10 3 < Re < 3 × 10 4. Our experiments fall in the TrSL2 (Transition in Shear Layer) regime and complement studies in the TrSL3 regime (3 × 10 4 < Re < 1.2 × 10 5) performed by Bernitsas and his group. Smooth rectangular strip pairs of varying thickness (1.6%-31% of cylinder diameter) were attached to a circular cylinder at 60°from the frontal stagnation point. Amplified VIV and high amplitude galloping oscillations were observed for all configurations with strips, except at the least thickness, where we observed reduced VIV amplitudes compared to the smooth cylinder at the lower flow velocities and high amplitude galloping at the higher flow velocities. Thicker strips led to higher VIV and galloping amplitudes, accompanied by an increase in steadiness within the transition regime. Thicker strips also led to the earlier initiation of galloping, indicating capability for increased energy transfer even at the lower flow speeds. Higher mass-damping led to lower vibration amplitudes and frequencies in both modes, however, the potential of smooth strips to incite galloping was evident even at the higher values tested.

The traditional models for VIVPEH (piezoelectric energy harvesting from vortex-induced vibration) are focused on a circular cylinder. In the present paper, a VIVPEH model was built to solve an aero-electromechanical coupling problem during VIVPEH with different cross-sectional shapes. The model was verified by a wind tunnel test. One key benefit of the present model is that the region from the initial position to the threshold of the lock-in region along the voltage time history curve can be calculated quite accurately compared with traditional methods. The present method will significantly contribute to the design and optimization of the key factors of VIVPEH, e.g., the bluff body shape and the electrical circuit.