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This study is inspired by the extraordinary suppression performance of the intrinsic vortex-induced vibration (VIV) of a harbor seal vibrissa, which is renowned for its excellent detection characteristics in sensing upstream flow information. A physical understanding of the suppression mechanism of vibrissal cylinder oscillation is mandatory for the design of a sensor detection system. In this study, the suppression performances are studied for a vibrissal cylinder at a reduced velocity U r = U/ (f nw D) of 5, with the corresponding Reynolds number of Re = 3,750, according to the swimming velocity and real geometrical scale for a harbor seal vibrissa. In addition, a verified fluid and structure interaction (FSI) solver implemented with an improved dynamic mesh strategy is adopted. Different transverse spring-mounted oscillation responses are obtained by alternating the angle of attack (AOA). It is found that the attachment and secondary separation of vortices in the near wake significantly influence the shedding pattern and can lead to the distinct suppression of oscillating responses. A pair of attached vortices are observed for the vibrissal cylinder at AOA = 0°, where the oscillating response is almost fully suppressed. Forced vibration is applied to address the role of attached vortices in the suppression mechanism. In contrast, when the suppression mechanism diminishes at AOA = 90°, the vortex-shedding pattern is characterized by unstable discrete secondary vortices. These secondary vortices re-separate from the primary vortices, indicating high stability of the primary vortices in the near wake, which represents the primary mechanism deteriorating the suppression of vortex-shedding. Furthermore, the interaction of attaching and secondary discrete vortices results in a partially suppressed oscillating response for the harbor seal vibrissa at AOA = 45°. Of note, these two mechanisms cause the shedding mode to switch between the "2S" and "P + S" modes while partially suppressing the oscillation response. [ABSTRACT FROM AUTHOR]

Studies on bluff-body aerodynamics have emphasized that galloping instability is strongly associated with the Kármán vortex. This study discusses the aerodynamic interactions between the galloping instability and Kármán and motion-induced vortices, analyzes the effects of these vortices on vortex-induced vibration and galloping, and investigates the stabilizing effects of various corner cuts on a rectangular cylinder. Wind tunnel tests were performed on a rectangular cylinder with a side ratio of 1.5 under a smooth flow for seven different corner shapes. The rectangular cylinder with cut corners significantly reduced both the aerodynamic force coefficients and the Kármán vortex shedding intensity. Furthermore, spring-supported free vibration tests indicated that the onset reduced wind velocities were high in the response amplitudes of all corner-cut sections that were analyzed despite the significantly low onset reduced wind velocities of the Kármán vortex-induced vibration, which were denoted as the reciprocal of the Strouhal number and closely associated with the onset of galloping. This was attributed to the motion-induced vortex dominating the vibration when the Kármán vortex shedding intensity was reduced. Therefore, this study clarified one of the factors that affected the onset-reduced wind velocity of galloping. [ABSTRACT FROM AUTHOR]

This study numerically investigates the aerodynamic and aeroelastic characteristics of a square prism (aeroelastic model) and wind field around it in sinusoidal oscillatory flows (SOFs). The reliability of the fluid-solid interaction (FSI) simulation is validated by a free vibration test and wind tunnel tests in smooth flow and SOF. The effects of the amplitude and frequency of SOFs are studied at the mean wind speed of vortex-induced resonance. The results show that increasing the amplitude and frequency of SOFs will amplify the root mean square (RMS) along-wind and across-wind base shear forces of the aeroelastic model but decrease the RMS across-wind displacement at the top of the aeroelastic model. The spectral analysis of the base shear forces indicates that the influence of vortex shedding on the across-wind base shear force is reduced by either increasing the amplitude or increasing the frequency of SOFs. The mean and instantaneous wind fields around the aeroelastic model in SOFs and smooth flow are compared, and the wake characteristics of the aeroelastic model in SOFs are analysed by dynamic mode decomposition. It is observed that when the frequency of SOFs is 1.5 times as large as the fundamental natural frequency of the aeroelastic model, the regular vortex shedding process is substantially affected. [ABSTRACT FROM AUTHOR]

This paper examines the ability of coupled system models, derived from the Facchinetti, de Langre and Biolley model, to fit the vortex-induced vibrations observed on a cantilevered rectangular flat plate, with zero angle of attack, in a hydrodynamic tunnel. Two VIV areas have been scrutinized. The first one for 0. 6 × 1 0 5 ≤ R e ≤ 1 0 5 and the second one for 4. 5 × 1 0 5 ≤ R e ≤ 7 × 1 0 5 where two distinct hydrodynamic excitation mechanisms, characterized by two Strouhal numbers, successively lock the first bending and first torsional mode of the plate respectively. For the bending VIV, exhibiting weak fluid–structure coupling, good results were obtained using a hybrid model for which a structure oscillator forced at ω v 1 = 2 π S t 1 U / D is coupled with a wake oscillator associated to the second Strouhal number. Regarding the torsional VIV, amplitude response and successive frequency lock-in have been satisfactorily reproduced using a structure oscillator (in rotation) coupled with two distinct wake oscillators. [ABSTRACT FROM AUTHOR]

The purpose of this study is to develop a reduced-order model (ROM) for the prediction of vortex-induced vibration of an elastically mounted circular cylinder constrained to oscillate purely in-line with a uniform free stream. The fluid force in the streamwise direction was modeled by an extended version of Morison's equation that comprises an inviscid inertial component opposing the cylinder acceleration, a viscous drag component proportional to the square of the relative velocity of the free stream and the oscillating cylinder, and a wake drag component representing the excitation from vortex shedding. A van der Pol equation was employed to model the oscillating wake drag component. The coupling between the oscillating wake and the oscillating cylinder was accomplished by forcing the wake oscillator with a term proportional to either the acceleration or the velocity of the cylinder. The ROM parameters were tuned against data from numerical solutions of the full system of non-linear equations governing the fluid and cylinder motions for a cylinder with a mass ratio of 10 and a Reynolds number of 180. The results showed that acceleration forcing can predict well the amplitude response when the ROM is properly tuned, whereas velocity forcing leads to underestimation of the amplitude response. In addition, acceleration forcing can predict satisfactorily the magnitude of the total streamwise force and of the wake drag. However, the trend in the frequency response was not well predicted by acceleration forcing, which was predicted more satisfactorily by velocity forcing. Both acceleration and velocity forcing predicted satisfactorily the phase lag between the wake drag and the cylinder displacement. Predictions with the calibrated ROM at different mass ratios captured well the amplitude response as a function of the reduced velocity, including the narrowing of the range where relatively high amplitudes occur, as in the full numerical simulations. Linear stability analysis based on the ROM equations indicated that the interaction between the fluid and the vibrating cylinder has the characteristics of nonlinear resonance, where the selection of the common frequency of the system is sensitive to nonlinearities, which cannot be faithfully captured by the van der Pol wake oscillator. [ABSTRACT FROM AUTHOR]

The present work presents an analytical model (AM) based on a nonlinear reduced-order model (ROM) for estimating the maximum efficiency (η m) of energy harvesting from vortex-induced vibration (VIV) of a circular cylinder. The proposed AM was validated through comparisons against both the origin ROM and experiments over a wide range of parameters. Of interest, it provided a low-cost analytical approximation relating the maximum efficiency η m to physical parameters. As inspired by the modified Griffin plot, the use of AM allowed the collapse of the maximum efficiency η m as a function of the combined mass-damping parameter α. Of significance, the presented AM should be not only effective for estimating the maximum efficiency η m but also useful to shed light on VIV dynamics. [ABSTRACT FROM AUTHOR]

In published literature of the vortex-induced vibration of a sphere, limited attention has been devoted to spheres having three degrees of freedom (DOF) motion and the effect on the vibration response. In this study, the vortex-induced vibration response of a 3-DOF elastically-mounted sphere was examined numerically. The response of a sphere allowed to oscillate in all three spatial directions (3-DOF motion) was compared with the response of a sphere whose motion was restricted to only the transverse direction (1-DOF). Simulations were conducted over the reduced velocity range U ∗ ≡ U ∕ (D f n) ∈ [ 3. 5 , 16 ] , using a sphere of mass ratio m ∗ ≡ ρ s ∕ ρ = 3 , at a fixed Reynolds number of 2000, where U is the free-stream velocity of the flow, f n is the natural frequency of the system, ρ s and ρ are the solid and fluid densities, respectively. When the sphere was allowed 3-DOF movement, it was initially excited to vibrate along a linear path in the transverse direction, synchronized with the two-sided shedding of vortex loops behind the sphere. As the simulation time advanced, the sphere trajectory eventually converted into a circular orbit with a spiralling wake behind it. The transition time between these two modes was found to increase with decreasing Reynolds number. The vibration amplitude was significantly smaller when the sphere motion was in all three spatial directions than when it was restricted only to the transverse direction. The maximum vibration amplitudes were approximately 0.6 and 0.8 diameters, for 3-DOF and 1-DOF motion, respectively. In the synchronization regime, the maximum time-averaged drag coefficient was approximately 100% greater than that of a stationary sphere, when the sphere had 3-DOF, similar to previous observations for a tethered sphere. However, the maximum change to the time-averaged drag coefficient was only 76% when the motion was restricted to the transverse direction. [ABSTRACT FROM AUTHOR]

We present numerical prediction results of vortex-induced vibration (VIV) of a long flexible tensioned riser subject to uniform currents. The VIV model of long length-to-diameter ratio is considered and 'thick' strip technique based on high-order spectral/ hp element method is employed for computational simulation. The model parameter of the riser for the simulation is chosen according to the dimensional counterparts used in the experimental tests in Lehn (2003). The numerical results are displayed in terms of motion responses, hydrodynamic forces and wake patterns as well and compared and discussed with the available data in the literature. [ABSTRACT FROM AUTHOR]

Flow-induced vibration (FIV) of a circular cylinder with an attached rigid splitter plate is studied. A stabilized finite element formulation is utilized to solve the incompressible flow equations in two-dimensions. Three regimes of FIV are identified for the range of reduced speed (1 ≤ U ∗ ≤ 70) studied: vortex-induced vibration (VIV), steady flow and galloping. The effect of mass ratio, 2 ≤ m ∗ ≤ 1000 , is studied for the R e = 150 flow (Reynolds number is based on diameter of the cylinder, D) and splitter plate of length, L p = 3. 5 D. In contrast to an isolated cylinder, the peak response of the cylinder with splitter plate is associated with a significantly smaller amplitude and the lock-in occurs over a wider range of U ∗. The U ∗ for the onset of lock-in increases with increase in m ∗. The end of lock-in, however, is independent of m ∗. In addition, unlike the isolated cylinder, the transition from desynchronization to lock-in is gradual, except for very large m ∗. Although galloping occurs for all m ∗ , its onset is strongly dependent on m ∗. The end of lock-in regime of VIV is immediately followed by onset of galloping for small m ∗. For this range of m ∗ , the galloping is strongly influenced by vortex shedding. A steady regime, wherein the vibration is completely suppressed for a certain range of U ∗ , occurs beyond the lock-in regime for moderate m ∗. However, galloping eventually ensues and revives the unsteadiness at large U ∗. For very large m ∗ , the steady flow regime is replaced by a desynchronization region. In general, the onset of galloping is delayed to larger U ∗ with increase in m ∗. The effect of the length of the splitter plate (L p ∕ D = 1. 5 , 2.5 and 3.5) on FIV is studied for m ∗ = 10. The steady flow regime is observed only for L p = 3. 5 D. Increase in L p leads to a small increase in the peak amplitude and range of lock-in during VIV. The effect on galloping is more significant; the amplitude of vibration, at a given U ∗ , decreases with increase in L p. The frequency of vibration decreases, during both VIV and galloping, with increase in L p. The effect of R e , within the laminar regime, is studied. Its effect on the amplitude of vibration is found to be similar to that of L p. • 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. [ABSTRACT FROM AUTHOR]

Wake patterns behind two freely vibrating circular cylinders in side-by-side arrangements are numerically investigated through two-dimensional computational fluid dynamics simulations. The two cylinders, with an equal diameter and a low mass ratio (the cylinder mass/the displaced fluid mass) of 2, can only oscillate in the transverse direction, and they are subject to the uniform laminar flows with the Reynolds number of 100. The center-to-center spacing over a cylinder diameter ratio (s / D) is varied from 2 to 5. For each s / D , the reduced velocity U r (defined by U ∞ ∕ f n D , where U ∞ is the free-stream velocity and f n is the cylinder natural frequency) is varied from 0 to 30. A small parametric increment of Δ U r = 0. 1 and Δ s / D = 0. 1 is considered, providing a comprehensive analysis and insights into the wake patterns of the vortex shedding behind both cylinders. With more than 1500 new simulation cases, 8 distinctive wake patterns are identified through a map of U r and s / D , with several well-defined boundaries governing different flow patterns. Qualitatively distinctive wake characteristics are analyzed through the evaluations of vortex dynamics, hydrodynamic lift/drag forces and cylinder vibration responses. • Wake patterns of two transversely vibrating side-by-side circular cylinders are numerically investigated. • Cylinders with low mass ratio of 2 are subject to uniform laminar flows with Re = 100. • Cylinder spacing ratio is varied between 2 and 5 with small increment of 0.1. • Reduced flow velocity is varied between 0 and 30 with small increment of 0.1. • Over 1500 new simulation cases are characterized into 8 distinctive wake patterns. [ABSTRACT FROM AUTHOR]

In this study flow around a freely vibrating circular cylinder in two-degrees-of-freedom is investigated using two-dimensional numerical approach. The equations of motion, the continuity and the pressure Poisson equations are solved for incompressible constant property Newtonian fluid using the finite difference method. Systematic computations are carried out to investigate the effect of the natural frequency f N of the cylinder on the oscillation amplitudes and aerodynamic force coefficients. The mass ratio and the structural damping coefficient values are fixed at m* =10 and ζ = 0 , respectively. In addition to the typical distorted figure-eight motion, a raindrop-shaped path is also found whose range widens with increasing f N. Within this range the streamwise oscillation amplitude reaches higher values compared to figure-eight motions and the curves shift upwards with increasing f N. Different vortex structures belong to each cylinder path: 2S or C(2S) modes are observed for distorted figure-eight and P+S mode is found for raindrop-shaped trajectories. The time-mean values of lift jump abruptly between two solutions. Pre-and post-jump analysis reveals that these solutions are mirror images of each other. The root-mean-square (rms) values of lift plotted against U* St shift upwards in the raindrop-shaped motion range and downwards in the figure-eight motion range, while the rms of drag shifts to higher values in both regimes with increasing f N. Chaotic cylinder paths are observed when increasing the natural frequency over a critical value. • A freely vibrating cylinder is investigated at different natural frequencies. • Orbital and distorted-figure-eight cylinder paths are found. • The streamwise oscillation amplitude increases steeply for orbital paths. • For orbital motion asymmetric wake is found and time-mean of lift is non-zero. • A hysteretic boundary separates the two cylinder motion ranges. [ABSTRACT FROM AUTHOR]

The control of vortex-induced vibration (VIV) is very important for the safety of a bridge during its service life. In the present study, a self-issuing jet, which is a type of a passive control method, is installed on a bridge deck to suppress the VIV and to validate the control effect. This study focuses on the layout of the slits in a deck test model and compares the near-wake flow fields between the baseline and optimal-control cases. The experimental results show that the self-issuing jet plays an important role in controlling the vibration. The vertical VIV (V-VIV) is fully suppressed, and the maximum amplitude of the torsional VIV (T-VIV) is reduced by 45.5%. The Strouhal number (St = fB / U , where f and B are the frequency and reference length of the structure, respectively, and U is the incoming speed) of the best control case (Case II) in which the self-issuing jets are arranged at the upper half part of the lower inclined panel in the box section increases compared with that in the baseline case. The wake-flow structures in the V-VIV and T-VIV are also studied. The results show that the wake patterns are modified by the self-issuing jet. In the V-VIV case, the alternating wake-flow structures are changed into a relatively steady state and show a slight variation with time in the control case, i.e. Case II. In the T-VIV case, the strength of the spanwise vortex is reduced, whereas the streamwise vortex is increased by the self-issuing jet. Moreover, a Mode-B flow pattern is found in the wake flow field of the test model in Case II, which implies that the secondary wake instability is triggered by the self-issuing jet. [ABSTRACT FROM AUTHOR]

Abstract Two-degree-of-freedom (2-DOF) vortex-induced vibration (VIV) of an elliptic cylinder is numerically investigated at a Reynolds number R e = 150. The incompressible Navier–Stokes equations in arbitrary Lagrangian–Eulerian formulation are solved by a four-step fractional finite element method. The elliptic cylinder, with a variety of aspect ratios (AR = 1.0–2.5), is free to vibrate in both transverse and azimuthal directions. At each AR , the computations are conducted within a wide range of reduced velocities (2 ≤ U r ≤ 15). For comparison, the one-degree-of-freedom (1-DOF) responses of the cylinder vibrating only transversely are also provided. The results show that significant rotations occur under a condition of frequency synchronization similar to the lock-in condition of VIV in the transverse direction. Under the rotation effect, the transverse peak amplitude at AR = 1.0 reaches a higher value about 20% larger than its counterpart of 1-DOF VIV, whereas it fells by more than half at AR = 2.0 and 2.5, and the corresponding wake patterns present distinct characteristics. The typical phenomenon of phase switch can be captured successfully between the torque and rotation response, which occurs at the same reduced velocity to the phasing between the transverse force and motion. Our simulation results also elucidate that the free rotation may significantly affect the statistics of the drag, lift and moment coefficients. The most striking observation is that all of the force responses at AR = 2.0 and 2.5 collapse onto a very low level compared to those of the transverse-only VIV. This is consistent with the dramatic drop in the transverse vibration amplitude at the same AR. [ABSTRACT FROM AUTHOR]

Abstract The objective of this numerical study is to investigate the effect of spanwise grooves on the suppression of vortex-induced vibration (VIV) and the reduction of drag force. For this purpose, we consider a standard configuration of the elastically mounted circular cylinder, which is free to vibrate in both streamwise and transverse directions with identical natural frequency. We introduce a novel staggered groove configuration whose geometry is especially designed by offsetting the cross-sectional portion of the cylinder continuously along the spanwise direction. We assess the characteristic VIV responses of the proposed staggered groove configuration against the helical surface grooves for the identical dimensions and physical conditions. The staggered and helical groove configurations differ only in their arrangement of cross-section geometry along the spanwise direction. Three-dimensional coupled fluid–structure simulations are conducted at low mass and damping values with moderate Reynolds number of R e = 4800. The effective width and the depth of surface grooves are determined to characterize the size effects for the assessment of staggered and helical groove configurations. Results show that the staggered groove configuration is effective in suppressing VIV, wherein the net reductions of 37% in the peak transverse amplitude and about 25% in the mean drag coefficient are observed in comparison to the plain cylinder counterpart. Staggered groove configuration produces three dominant effects by introducing a continuous jump in the cross-sectional geometry along the spanwise direction: (i) reduction of the spanwise correlation, (ii) enhancement of the three-dimensional effects in the near-wake flow, and (iii) broadening of the frequency spectra of fluid forces. As a result of these physical effects, the transfer of energy from the surrounding fluid flow to the vibrating grooved-cylinder system is reduced as compared to its plain cylinder counterpart. Owing to the simplicity of mechanical design and the ease of installation, the proposed passive control concept has a potential application to deepwater marine risers and tall structures in a wind environment. Highlights • A new staggered spanwise groove device for VIV control and drag reduction. • Comparison of staggered groove with helical groove and plain cylinder. • Mechanism of VIV suppression and drag reduction via spanwise grooves. • Quantification of spanwise cross-correlation and three-dimensionality. • Broadening of frequency spectra of hydrodynamic forces. [ABSTRACT FROM AUTHOR]

Abstract Vortex-induced vibration (VIV) of an elliptic cylinder free to rotate about its center is numerical studied at Reynolds number 2580 ≤ Re ≤ 15490. The vibration and rotation responses occurring simultaneously are considered with normalized torsional friction varied from 3.34 × 10−4 to 1.17. A two-way fluid–structure interaction (FSI) approach is used to solve the incompressible flow equations and the structure motion including the vibration and rotation in two dimensions. The numerical results indicate that the torsional friction is a key factor, determining the rotation response and then affecting the vibration amplitude. When the torsional friction is too large or the reduced velocity is too small, the elliptic cylinder is unable to rotate, while too small friction leads to transverse galloping instead of the desynchronized region. For the elliptic cylinder with moderate frictions, the response occurs similar as the circular cylinder, but the amplitude in the desynchronized region is larger than the circular cylinder due to the hydrodynamic instability. The results show four kinds of rotation response, which are associated with the time-averaged torque and the flow field around the elliptic cylinder. Compared to the non-rotatable elliptic cylinder with 0° attack angle, the rotatable cylinder vibrates more vigorously. Highlights • The vibration and rotation of an elliptic cylinder are considered simultaneously. • Large torsional friction leads to no rotation while small friction results in galloping. • The elliptic cylinder undergoes unstable rotation at small reduced velocities. • The amplitude of rotatable cylinder is larger than that of non-rotatable one. • Rotation response is determined by friction, torque and flow field around the cylinder. [ABSTRACT FROM AUTHOR]

Abstract A large-scale model test of a truncated steel catenary riser (SCR) was performed in an ocean basin to investigate the riser responses under pure steady uniform current and pure vessel motions separately. Out-of-plane vortex-induced vibration (VIV) was confirmed to have occurred under both test conditions. A comparative analysis and discussion were carried out on selected cases in terms of out-of-plane VIV responses, VIV developing mechanisms and the fatigue damage contribution. Results indicate that both steady current-induced and vessel motion-induced VIV responses are dominated by strong travelling waves, but vessel motion-induced VIV responses are more 'intermittent' with respect to the response amplitude and frequency owing to its space- and time-varying shedding frequency. 'Power-in' regions are further estimated to understand the VIV developing mechanisms for both test conditions. Finally, fatigue damages are evaluated showing the damage by vessel motion-induced VIV is comparable and at the same level as uniform current-induced VIV, which highlights the great importance of vessel motion-induced VIV, which cannot be neglected in the design and analysis for SCR systems. [ABSTRACT FROM AUTHOR]

Numerous works have been conducted to study the wake control of a bluff body and more efficient strategies are still what we seek for. This paper studies the complex fluid–structure interaction (FSI) problem of a square cylinder mounted with dual flexible splitter plates from a new perspective: impinging the vortex core with flapping plate tip. The Reynolds number (Re) based on the side length of the square cylinder is fixed at 332.6. The splitter plates are attached at both sides of the square cylinder. Structural parameters, such as plate length and bending stiffness, are considered to achieve an optimal arrangement of this configuration. Compared to the traditional single splitter plate arrangement, only a short splitter length is needed for this dual splitter plates configuration. Flow visualization indicates that the drag reduction is associated with the increase of base pressure and the lift suppression is related to the inhabitation of the interaction of the shear layer and split effect of the tip vortices. The dimensionless bending stiffness plays a vital role in determining the behavior of the splitter plate. Two vibration modes, i.e., off-center vibration and asymmetry vibration modes are captured and the up and down splitter plates tend to vibration in-phase. The Lissajous trajectories of the free tip mainly exhibits arc-shape and sickle-shape. Given the performance of the square cylinder-dual splitter plates configuration, splitter length of l / L = 1. 5 is an optimal configuration, which is less affected by flexural rigidity. The maximum drag reduction could reach 23.1% and the lift force exerting on this configuration does not increase significantly, which is beneficial for the structural stability. • Wake control from a new perspective: impinging the vortex core with plate tip. • Pronounced drag reduction of a square cylinder with dual splitter plates. • Gap flow plays a vital role in determining the vibration modes. • Mechanism elaboration of the dynamical system. [ABSTRACT FROM AUTHOR]

The effect of damping on vortex-induced vibration (VIV) of a circular cylinder with a fixed mass ratio ( m ∗ = 3 . 0 ) was studied through water-channel experiments. An eddy-current-based damping mechanism was constructed to provide controlled and adjustable damping values. It consisted of a permanent magnet connected to the cylinder that moves parallel to a copper plate at some predetermined gap, which determines the damping in the system. Increased damping was found to reduce the reduced-velocity range of the upper and lower branches, thus reducing the synchronization region. As the damping is increased, the lower branch remains easy to identify from the amplitude response curves, but the boundary between the initial and upper branch becomes less clear. However, the frequency response under higher damping shows similarities to that at the lowest damping and these similarities, for the first time, were used to delimit the different response branches. The existence of the upper branch was found to continue down to A ∗ ≈ 0 . 2 D . The experimental data was assembled to plot the peak amplitude response as a function of the mass–damping parameter in a “Griffin plot”. Due to a restricted variation in Reynolds number in the experiments, the measured data shows negligible scatter compared to the assembled literature data. Three sets of experiments using different sets of springs were conducted to quantify the Reynolds number effect previously established by Govardhan and Williamson (2006). An exponential fitting function was then used to successfully fit the data on the Griffin plot. Under higher damping, it was found that the total and vortex phases are no longer at either 0°or 180°, and take intermediate values throughout the response branches. The power extracted by the damping mechanism was also calculated. Maximum power extraction occurs for a combination of optimal damping and reduced velocity. The power was also found to increase with Reynolds number, correlated with the increase in vibration amplitude. At the highest Reynolds number examined, the dimensionless energy conversion ratio is 0.2, indicating that approximately 20% of the flow energy approaching the cylinder frontal cross-section can be converted to useful electrical energy. This factor increased substantially with Reynolds number from approximately 15 to 20% over the Reynolds number range considered ( R e ∼ 1700 –5900). The fit devised for the peak vibration amplitude was extended for expressing the average extracted power as a function of mass–damping and Reynolds number. [ABSTRACT FROM AUTHOR]

A number of offshore compliant structures such as risers, mono-columns, tension leg platforms (TLPs), spars, etc., which are deployed in the offshore field are subjected to vortex-induced vibration (VIV). The harsh environmental conditions often lead to failure of these structures well before their design life period resulting in fatal accidents. To minimize VIV, several investigations were conducted by other researchers. The present study focuses on the experimental investigations of an elastically mounted circular cylinder shrouded with net substructure called as Ventilated Net (VN) to suppress VIV. The VN is an omnidirectional, economical, customizablenet substructure comprising of flexible hollow tubes in a systematic arrangement. This device could be retrofitted to the offshore structures/risers, to attenuate VIV, thereby preventing the structure from fatigue failure. Experimental investigations are conducted to address the reduction of VIV and drag forces acting on the oscillating cylinder shrouded with VN of various configurations. The experiments are performed in towing tank facility in Department of Ocean Engineering, IIT Madras, India. The primary objective of the present investigation is to quantify the VIV amplitudes by employing VN shrouding the cylinder, at high Re regime, ranging from (0.22–2.50) × 10 5 . The effect of spaces between the flexible hollow tubes and shrouding radii of VN around the cylinder are also addressed in this article. From the measurements, it is observed that cylinder with VN of dense mesh at a radial spacing of twice the diameter of the bare cylinder, suppresses VIV by 98% and drag force by 40% at Re of 1.2 × 10 5 . [ABSTRACT FROM AUTHOR]

The effect of varying the structural mode excitation on bending-dominated flexible cylinders undergoing vortex-induced vibrations was investigated. The response of the bending-dominated cylinders was compared with the response of a tension-dominated cylinder using multivariate analysis techniques. Experiments were conducted in a recirculating flow channel with a uniform free stream with Reynolds numbers between 650 and 5500. Three bending-dominated cylinders were tested with varying stiffness in the cross-flow and in-line directions of the cylinder in order to produce varying structural mode shapes associated with a fixed 2:1 (in-line:cross-flow) natural frequency ratio. A fourth cylinder with natural frequency characteristics determined through applied axial tension was also tested for comparison. The spanwise in-line and cross-flow responses of the flexible cylinders were measured through motion tracking with high-speed cameras. Global smooth-orthogonal decomposition was applied to the spatio-temporal response for empirical mode identification. The experimental observations show that for excitation of low mode numbers, the cylinder is unlikely to oscillate with an even mode shape in the in-line direction due to symmetric drag loading, even when the system is tuned to have an even mode at the expected frequency of vortex shedding. In addition, no mode shape changes were observed in the in-line direction unless a mode change occurs in the cross-flow direction, implying that the in-line response is a forced response dependent on the cross-flow response. The results confirm observations from previous field and laboratory experiments, while demonstrating how structural mode shape can affect vortex-induced vibrations. [ABSTRACT FROM AUTHOR]

The vortex-induced vibration (VIV) of a rotating blade is studied in this paper. Uniform Euler–Bernoulli cantilever beam equation is used to model the rotating blade, while the nonlinear oscillator satisfying Van der Pol equation is used to represent the vortex shedding. The fluctuating lift force on the blade due to the wake dynamics is expressed such that it is proportional to the square of the magnitude of the relative velocity at which the blade interacts with the fluid. An unsteady lift due to vortex shedding acts on the blade, where as the blade is coupled with the fluid through a linear inertial coupling, resulting in a fluid–structure interaction problem. The coupled equations are discretized by using modal functions which satisfy the Eigenvalue problem. The work attempts to understand the interaction between the blade modes and the wake modes. The influence of the mass ratio and the reduced flow velocity on the coupling of the modes is explored. It is concluded form the study that the first mode is dominant and the effect of higher modes is less significant. Therefore, the single mode approximation is considered to study the instabilities associated with the frequency lock-in phenomenon of a rotating blade. The influence of fluid and blade damping, mass ratio and the coupling parameter on the lock-in phenomenon is investigated in this study. [ABSTRACT FROM AUTHOR]

This paper proposes the Forgetting Factor Least Squares (FF-LS) method for identification of time-varying hydrodynamics of a flexible riser under multi-frequency vortex-induced vibrations (VIV). Differing from the traditional least squares method used to identify hydrodynamics, with equal weights on all sampled data, this method introduces a forgetting factor to give higher weight to data closer to the present moment. FF-LS allows the possibility of handling the time-varying parameters. By following this procedure for all sampled data of a flexible riser undergoing multi-frequency VIV, the corresponding time-varying hydrodynamics in the Cross-Flow (CF) direction will be obtained considering multi-frequency coupling. The results show that, under multi-frequency coupling, vortex-induced force coefficients of flexible risers change periodically and differ from coefficients at the basic frequency usually used in VIV prediction. This difference is a result of the coupling effect between the basic frequency and high frequency. Time-varying coefficients considering multi-frequency coupling contain steady components and vibration components whose frequencies are the sum of coupling frequencies and the difference of coupling frequencies. To show the advantages of time-varying hydrodynamics, we compare forces reconstructed from different vortex-induced force coefficients, assuming vortex-induced force obtained from the inverse analysis is the real force. The results show that time-varying vortex-induced force coefficients considering a coupling effect between multiple frequencies can accurately reconstruct vortex-induced force, while the sum of vortex-induced force coefficients under multiple frequencies will produce the cross-term that overestimates the vortex-induced force. [ABSTRACT FROM AUTHOR]

We experimentally investigated the self-excited vibration of a cylindrical pendulum in a uniform flow as a fundamental step in evaluating its feasibility for energy harvesting. In the pendulum configuration, a cylinder is fixed to rigid plates via a thin elastic sheet so that it can swing perpendicularly to the free stream and cause periodic deformation of the elastic sheet. The dynamics of the pendulum were examined by varying free-stream speed and several geometric parameters of the cylinder. In contrast to past studies of vortex-induced vibration, which mainly used reduced velocity to characterize the pendulum dynamics, our study introduced non-dimensional free-stream velocity, which is the relative magnitude of the restoring bending moment of the sheet to the hydrodynamic moment acting on the cylinder. We confirmed that all pendulums converted to an oscillatory mode at similar non-dimensional velocity and had almost identical amplitude and frequency responses at given non-dimensional velocity. The pendulum was able to oscillate with the amplitude much larger than its diameter in the limited range of the non-dimensional velocity. Mutual interactions between the two cylindrical pendulums closely arranged in tandem were also investigated, and the dependence of the amplitude response on center-to-center distance and the transition from vortex-induced vibration to wake galloping were observed. [ABSTRACT FROM AUTHOR]

An experimental investigation was carried out on the effects of attachments on the aerodynamic characteristics and vortex-induced vibration (VIV) of a twin-box girder. In the study, the flow pattern, pressure distribution, vortex shedding frequency, and VIV of five models, namely Model A (twin-box girder without attachments), Model B (twin-box girder with handrails and crash barriers), Model C (twin-box girder with handrails, crash barriers, and wind barriers), Model D (twin-box girder with maintenance rails), and Model E (twin-box girder with full ancillaries), are compared with a bare deck in detail. The results demonstrate that, owing to the large leading flow separation induced by the attachments, the flow characteristics and VIVs of the twin-box girder with attachments differ considerably from the bare twin-box girder. Because of the large porous ratio, the handrails and crash barriers exert relatively weak influences on the surface pressure distribution. However, the leading wind barrier and maintenance rail generate large flow separation at the leading edges, resulting in larger pressure fluctuations, a wider wake, and larger scale vortex with lower shedding frequency than that of the bare deck. Furthermore, owing to the large leading flow separation, the vertical VIV induced by the motion-induced vortex in the gap is suppressed. However, a torsional VIV is generated by the large-scale alternately shedding vortex in the wake of the downstream box girder. [ABSTRACT FROM AUTHOR]

The vortex-induced vibrations of a flexible cylinder oscillating harmonically in still water have been numerically simulated using a CFD method based on the strip theory. The algorithm PIMPLE in OpenFOAM is adopted to compute the flow field while the small-displacement Bernoulli–Euler bending beam theory is used to model the cylinder. Two ends of the flexible cylinder are forced to oscillate harmonically. The simulation results have been compared with experimental results and further analyzed. Features such as the hysteresis phenomenon and the build-up–lock-in–die-out cycle are observed in the cross-flow vibration. The in-line vibrations consist of three components, the low-frequency oscillation, the first-natural-frequency vibration during the cylinder reversal, and the second-natural-frequency vibration due to vortex shedding. The butterfly-shaped trajectory has been observed. Detailed wavelet analyses of the vibrations have been given at the end. [ABSTRACT FROM AUTHOR]

Vortex-Induced Vibration (VIV) for flexible cylinders is a typical fluid–structure interaction problem, and it becomes more complex when the time-varying axial tension effect is considered. An available force–decomposition model is proposed in this paper to investigate the cross-flow VIV response characteristics with time-varying tension. VIV hydrodynamic forces are all based on forced vibration experimental data, and structural stiffness will be updated at each time step to take the tension variation into account. Firstly, this VIV model is compared against the published experimental results of a small-scale cylinder with constant and time-varying tensions. Next, 60 cases of a long flexible cylinder are designed to investigate the time-varying tension effect comprehensively. Several new phenomena such as amplitude modulation, time-lag, frequency transition, mode jump and multi-frequencies response superposition are captured in the response comparison with the constant tension case. The effects of initial phase, amplitude and frequency of the varying tension are respectively discussed in detail. The Mathieu-type resonance between VIV and time-varying tension excitation is proved existent. The response displacement and strain will enlarge significantly at ω T = 2 ω C T , to which enough attention needs to be paid. [ABSTRACT FROM AUTHOR]

Rotating small control rods in the near wake of a circular cylinder is a combination of both passive and active control techniques. The effect of rotation direction of control rods on vortex-induced vibration (VIV) suppression is investigated numerically by computational fluid dynamics (CFD) models coupling with a fluid–structure interaction (FSI) computational method. Two small rods with diameter ratio of 0.06 are located symmetrically at 135°right behind the main cylinder, and the gap between the main cylinder and control rods is 0.09 D . The Reynolds number and the reduced velocity are 3484 and 6.0, respectively. Five different rotation modes are considered including the no-rotating case. The results indicate that the presence of control rods changes the momentum and kinetic energy distribution. Placing stationary rods in this specific position can achieve a good suppression effect, and the effect is enhanced when the control rods are inward counter rotating. Inward counter-rotating rods facilitate the momentum injection from the outer flow into the boundary layer, resulting in the delay of boundary layer separation and the shift of separation point. Therefore, the wake becomes narrower. However, control rods rotating outwardly play a counterproductive role in flow control and the vibration is enhanced. Rotating rods upwardly and downwardly have almost the same vibration responses. The cross-flow vibration is suppressed while the in-line vibration is enhanced at these two mirrored cases. [ABSTRACT FROM AUTHOR]

The present work investigates the use of a polar array of 8 wake-control cylinders as a means of suppressing the vortex-induced vibration (VIV) of a larger circular cylinder. The diameter of the control cylinders and their rotation speed were the main parameters investigated. Experiments have been performed in water at Reynolds numbers between 5000 and 50,000. The rotating cylinders suppressed the peak amplitude of displacement by around 70% when compared to that of a bare cylinder. A similar response was obtained even if the rotation speed of the control cylinders was kept constant in relation to the flow speed. A specific configuration with 8 non-rotating control cylinders achieved an even better 99% suppression. As a consequence of reduced vibrations, the fluctuation of lift and mean drag were not as amplified due to VIV. The results pave the way for further studies concerning system optimization and support the development of efficient VIV suppressors and dynamic positioning systems for large floating offshore platforms and other applications. [ABSTRACT FROM AUTHOR]

A vibration energy harvester was developed using a magnetostrictive material as a power generator and a prism as a wind receiver to harvest energy from low-speed wind by exploiting flow-induced vibration. The present cantilevered vibration power generator has originality for the structure which consists of a U-shaped unimorph beam of Galfenol (iron-gallium alloy). Wind tunnel experiments were conducted for prisms having circular, rectangular, filleted triangular, and V-shaped cross-sections. We focused on transverse vortex-induced vibration for a circular cylinder and low-speed galloping vibration for a rectangular prism with a depth-to-height ratio of 0.2, a filleted triangular prism, and a V-shaped prism. The effect of the width of prisms having a span length of L = 200 mm on the transverse vibration characteristics and the power extracted from the vibration generator was investigated. The maximum power generated by cylindrical, rectangular, filleted triangular, and V-shaped prisms with heights of 50, 50, 60, and 50 mm was 1.28, 3.5, 7.83, and 5.02 mW, respectively. These maximum power levels are enough to run a wireless sensor. Moreover, the angle of the V-shaped prism having a width of 50 mm was varied (i.e., β = 60 ° , 90°, 120°, and 150°) and tested in wind tunnel experiments. The V-shaped prism with β = 120 ° was best from several viewpoints, including low excitation wind speed, safe operation at high wind speed, efficient operation in environmental conditions, and sustainability. [ABSTRACT FROM AUTHOR]

Vortex-induced vibration (VIV) and galloping are two typical flow-induced vibration (FIV) problems of bluff bodies, which often cause fatigue damage or destruction of structures. Therefore, the accurate and effective FIV prediction has become a requirement of the industry in order to prevent structural damage and guide design. Although the semi-empirical model (such as the wake oscillator model) is an efficient FIV prediction tool, the precision ability of existing models is constrained due to empirical parameters and unideal mathematical equations. Generally, the numerical simulation and wind tunnel experiments are relatively accurate methods, but for complex fluid–structure interaction problems, their cost is too high. As a result, a data-driven FIV prediction model is proposed to accomplish effective and high-precision prediction in this study. The Ensemble Kalman filter (ENKF) data assimilation (DA) technique is used to combine the experimental data with a semi-empirical model. First, the sensitivity of empirical parameters is analyzed, and the most sensitive ones are selected as the target parameters for data assimilation. The mean deviation of ensemble (MDE) is proposed to represent the sensitivity of parameters. Then DA-based model is used to predict the galloping and VIV of a rectangular cylinder. The results indicate that the accuracy of the DA-based model is up to 10 times greater than that of the initial semi-empirical model. With the data-driven technology, the typical 'quasi-steady' theory of galloping has been improved for low Scruton number values. For the wake oscillator model, assimilating the parameters of the acceleration and velocity coupled terms can improve the prediction accuracy of VIV. Also, this modeling procedure can be regarded as a new parameter identification method from forced and free vibration. [Display omitted] • By fusing the empirical model, the DA-based FIV model relies on small amounts of high-precision data to make predictions. • Parameter sensitivity analysis and MDE become the selection method and quantitative indexes of assimilation target parameters. • The DA-based model has an accurate prediction for galloping and VIV even in extrapolation. [ABSTRACT FROM AUTHOR]

The onset of fluid-elastic instability in tube arrays is thought to depend primarily on the mean flow velocity, the Scruton number and the natural frequencies of the tubes. However, evidence from experiments suggests that the Reynolds number is also an important parameter, although the available data are insufficient to understand or quantify this effect. We use high resolution direct numerical simulations to solve the penalized two-dimensional Navier–Stokes equations in order to accurately model turbulent flow through tube arrays with a pitch to diameter ratio P ∕ D = 1 . 5 . To investigate the Reynolds number effect we perform simulations that independently vary the mean flow velocity and the Reynolds number at fixed Scruton number. Parameters are chosen so that the simulations are well outside the lock-in regime of resonant vortex excitation. Increasing Reynolds number and mean flow velocity both have strong de-stabilizing effects for rotated arrays. For in-line arrays the effect is weaker and not monotonic with increasing Reynolds number and mean flow velocity. This study clarifies how the onset of fluid-elastic instability depends on Reynolds number (and hence turbulence intensity) and reduces uncertainties arising from the experimental data, which usually do not account for the effect of Reynolds number. It also demonstrates the usefulness of two-dimensional direct numerical simulations to investigate fluid-elastic instability at turbulent Reynolds numbers. [ABSTRACT FROM AUTHOR]

The sharp interface immersed boundary method is assessed for suitability and accuracy in simulating flow-induced vibration, specifically the phenomenon of vortex-induced vibration (VIV) in the two-dimensional flow past an elastically-mounted cylinder. Inherent to immersed boundary methods are the spurious force oscillations observed when the immersed boundary moves across the underlying grid. This deficiency in immersed boundary methods is acute in flows featuring fully-coupled fluid–structure interaction, where these oscillations are fed directly back into the coupled system and have the potential to significantly affect the solution. Here, the immersed boundary method is tested and compared directly and in detail to an accurate and validated spectral-element method. The immersed boundary method performs well for the given problem, excepting a few cases, such as those featuring disordered vortex-shedding. A heuristic model is developed to analyze the frequency content of the observed spurious force oscillations and their potential to affect the global solution. A guide to the resolution required for spatial accuracy is proposed, that around 40 points should span the peak-to-peak distance of any significant oscillation. [ABSTRACT FROM AUTHOR]

Long-span cable-supported bridges with twin-box decks have been constructed in recent years to enhance the stability of the bridges against flutter, but some of these bridges suffer large-amplitude vortex-induced vibration (VIV). Large-amplitude VIV can cause fatigue damage accumulation in structural components and reduce safety and comfort levels of road vehicles, trains, and pedestrians. Therefore, in the design of long-span bridges with twin-box decks, it is essential to know vortex-induced forces (VIF) and predict vortex-induced responses (VIR). Currently, there are several semi-empirical models for the estimation of VIF but most of them are established based on measured VIR for single-box decks and without considering turbulent wind effects. This paper presents a semi-empirical model for vertical VIF on a twin-box deck with and without turbulent wind flow. First, the two existing semi-empirical single-degree-of-freedom (SDOF) VIF models are discussed, and a refined SDOF model for VIF under smooth wind flow is proposed. The proposed SDOF VIF model includes all the non-conservative motion-induced force terms. An approximate analytical solution for the maximum amplitude of VIR is then deduced based on the proposed VIF model. Second, based on quasi-static assumption, the proposed VIF model is further extended to take account of turbulence wind effects by introducing a turbulence-induced positive damping term into the model. Finally, the validity of the proposed VIF model with and without turbulent wind effects is examined using a newly-developed wind tunnel test technique for an elastically-mounted twin-box section model. The comparative results show that the proposed VIF model can effectively predict the maximum VIR of a twin-box deck under different turbulent fields with different structural damping ratios. It is also found that the maximum VIR decreases nonlinearly with the increase of turbulence intensity. [ABSTRACT FROM AUTHOR]

Inspired by the drag reduction mechanism in a tandem cylinder arrangement, the objective of this study is to numerically investigate the characteristic response of shear layer reattachment for the suppression of vortex-induced vibration (VIV). A new device termed as connected-C is introduced, whereby its geometry can be constructed by attaching a C-shaped foil at the end of a splitter plate behind a circular cylinder. We investigate the significance of each part of the connected-C device and compare the proposed device with conventional fairings, the splitter plate, and one of its variant disconnected-C device without the connector plate between the main cylinder and the C-shaped foil. To simplify the numerical study and to concentrate on the wake stabilization mechanism, the cylinder-device systems are only allowed to move in the transverse direction. There is no relative rotation between the cylinder and the attached devices. Two- and three-dimensional studies are carried out at low Reynolds number Re =100 and subcritical Reynolds numbers Re ∈ [ 6150 , 7400 ] . We observe the counter-rotating pair of recirculations and the shear layer reattachment for the vibrating connected-C device. These flow characteristics of the connected-C device result into a similar VIV performance as the fairing with respect to the suppression of VIV. Similar to fairings, there is a net reduction in the total drag force exerted over the combined cylinder-device system. Based on the results from the low Reynolds number study, while the presence of C-shaped device is essential to prevent the occurrence of galloping at high reduced velocity, the presence of connector plate is not necessary to suppress VIV. [ABSTRACT FROM AUTHOR]

Experiments are performed on three spring-supported separated-box girder section models in a closed wind tunnel to investigate the vortex-induced vibration (VIV) characteristics. A particle image velocimetry (PIV) instrument is employed to obtain the flow field around the models. Four VIV-sensitive velocity intervals are observed for the test model, in which vertical and torsional oscillations are repeatedly induced. Comparisons with the control groups show that vortex shedding in the gap is the crucial factor that induces VIVs. Wake-induced vibration (WIV) is observed to occur with VIV in the gap, accompanied by a dramatic magnification in vibrational amplitude. The WIV becomes weaker and disappears as the wind velocity increases, and the wake vortex-shedding mode changes from the classic 2 S mode to a 2(2 S ) mode. The vortex-shedding frequency shows that it is a special lock-in mode of 1:2 phase-locking, which is found by taking the vibrating model and vortex-shedding as two coupled oscillators. [ABSTRACT FROM AUTHOR]

This paper presents a series of two-dimensional numerical simulations of flow-induced vibrations (FIV) and coupled wake flow behind two identical square cylinders in a side-by-side configuration at Re =200. The computational results of stationary configuration are firstly investigated and compared with the existing experimental observations as a function of gap ratio g * , which is the ratio of spacing between the inner cylinder surfaces to the diameter of the cylinder. Consistent with the experimental measurements, four distinct regimes have been observed namely: (i) the single bluff-body, (ii) the gap flow with narrow and wide streets, (iii) the coupled-vortex street regime, and (iv) the quasi-independent vortex shedding. We estimate the merging downstream distance of two vortex streets and compare it against the measurement trend for the gap ratio range 0.4 ≲ g * ≲ 2.0 . We next investigate the configuration of two elastically mounted square cylinders, which are free to oscillate in both streamwise and transverse directions. Instead of independent vibrations due to the individual fluid forces, the two square cylinders are tied together as a single rigid body with a fixed relative position between them. The role of flow passing through the gap between two cylinders is examined by exploring interactions of shear layers with the gap flow in the near-wake region. Through controlled numerical experiments, we show that the gap flow mechanism has a profound role on both vortex-induced vibration and galloping regimes corresponding to low and high reduced velocities, respectively. The fluid–structure simulations are performed via a nonlinear partitioned iterative scheme for the variational coupled system based on the Navier–Stokes equations and rigid body dynamics. For the freely vibrating condition, all the 2D simulations are computed at Reynolds number Re = 200, mass ratio m * = 10 , damping ratio ζ = 0 and reduced velocity U r ∈ [ 1 , 50 ] and the four regimes are considered based on the stationary analysis. The effects of reduced velocity on the force variation, the vibration amplitudes and the vorticity contours are analyzed systematically to understand the underlying FIV physics of side-by-side cylinders in the four regimes. For a fixed mass-damping parameter, we introduce simple correlations for the prediction of amplitudes for the vortex-induced vibration and galloping modes of freely vibrating side-by-side cylinders in the four regimes. [ABSTRACT FROM AUTHOR]

A new concept of power generator using self-induced oscillating foil to extract energy from fluid is proposed and tested in the present study. The pitching motion of the elliptical foil is induced by the downward shed vortexes of the foil. Two-dimensional Navier-Stokes simulations at Re =10 4 are conducted to study the fluid-foil interaction as well as the performance of energy extraction. Three kinds of motion trajectories are concerned, i.e. pitching motion, pitching motion combined with streamwise plunging and pitching motion combined with transverse plunging. The investigations are undertaken over a wide range of structural parameters (mass ratio, stiffness factor and damping coefficient). Numerical results reveal that the pitching motion combined with transverse plunging can achieve best energy extraction performance. With well-tuned structural parameters, the power generator can reach maximum power coefficient of 32% and energy extraction efficiency of 20%. Hence, it demonstrates that the promising self-induced oscillating foil can achieve satisfactory energy extraction performance without pitching motion actuator. [ABSTRACT FROM AUTHOR]

A series of experiments was performed to investigate the modal excitation of a tensioned flexible cylinder in a uniform flow due to flow-induced vibrations. Experiments were conducted in a recirculating flow channel using a flexible cylinder with relatively low aspect ratio such that low structural mode numbers were excited. The spanwise motion of the cylinder was measured using standard motion tracking techniques with high-speed cameras. Hysteresis is observed in the response of the cylinder dependent on whether the flow speed is increased or decreased from the previous experiment. This observation differs from predictions in the literature regarding hysteresis and is attributed to hysteresis in the transition between excited structural modes coupled with the cylinder wake. It is also found that the flexible cylinder is unable to sustain excitation of asymmetric modes in the in-line direction unless the fluid-structure interaction excites a 1:1 frequency response between the in-line (IL) and cross-flow (CF) directions, resulting in a pedaling mode response. The inability to excite asymmetric modes is consistent with the response of linear systems undergoing a symmetric drag load and is consistent with the cylinder undergoing a preferred figure eight shape motion when excited. Distributed fluid forces are derived from the structural characteristics and body motions illustrating the transition of the distribution of added mass and excitation forces on the body. [ABSTRACT FROM AUTHOR]

The hydrodynamic coefficients for straked cylinders were obtained from a set of forced motion experiments with a constant towing speed. Based on results from previous studies, two helical strake configurations were tested. The amplitude and frequency ratios of the in-line (IL) and cross-flow (CF) oscillations were fixed. This gave three free parameters of forced motion: CF oscillation amplitude, CF oscillation frequency, and phase angle between IL and CF oscillations. The hydrodynamic coefficients obtained for the straked cylinders were compared with results from forced oscillation tests using a bare cylinder. Clear differences between the designs were demonstrated. The hydrodynamic coefficients obtained from this study can be used in empirical models to predict vortex-induced vibration for structures with strakes. [ABSTRACT FROM AUTHOR]

The existence of multiple response in vortex-induced vibration of a circular cylinder is explored in the laminar flow regime by carrying out computations with different initial conditions for several mass ratios. In addition to primary and secondary hystereses near the ends of the lock-in regime, multiple states of the fluid-structure system exist in the middle of the lock-in regime as well. One of the two states is associated with large amplitude of cylinder oscillation. The response of the cylinder, for this state, lies on the classically known lower branch . In the other state, the cylinder exhibits low oscillation amplitude. Depending on the mass ratio and reduced speed this state may either be synchronized or desynchronized with the wake. A new branch of cylinder response has been identified for the lock-in state in this regime. The time histories of cylinder displacement and force coefficients on this branch are similar to that observed on the initial branch. In view of this, the new branch is referred to as initial branch (II) . The variation of frequency of cylinder oscillation and phase between cylinder displacement and lift coefficient along IB ( II ) is studied. [ABSTRACT FROM AUTHOR]

Vortex-induced vibrations sustained by immersed bodies may induce early structural fatigue and acoustic noise radiation. The present study consists of an experimental characterization of the vortex shedding and induced vibrations of a cantilevered blunt rectangular aluminium plate of chord to thickness ratio 16.7, immersed in a uniform water flow in the hydrodynamic tunnel of the French Naval Academy Research Institute. Experiences have been conducted at zero degrees incidence for Reynolds numbers R e c (based on chord length) ranging from 2. 5 × 1 0 5 to 9. 5 × 1 0 5 , leading to turbulent wake conditions. The hydrodynamic properties of the wake have been evaluated by statistical analysis, Proper Orthogonal Decomposition (POD) and vortex core identification of Time Resolved Particle Image Velocimetry fields. The structural response of the plate has been examined by laser vibrometry through modal analysis. The fluid–structure interactions have been analysed for three different vibration regimes: out of resonance, lock-off resonance and lock-in resonance with the 1st twisting mode. The features of the first nine POD modes were analysed according to the different vibration regimes to understand the physics proper to the near wake. In the out of resonance regime, the strength of the primary Karman vortices is preferentially controlled by the Strouhal and Reynolds numbers. The thickness based Strouhal number is 0.195. A secondary hydrodynamic excitation source of characteristic Strouhal number 0.227 coexists with the primary Karman vortex shedding mode. At the lock-off resonance, this secondary excitation source is identified as a second Karman vortex shedding mode which is coupled with the natural frequency. A local maximum of vibration magnitude is attained for this regime. The coexistence of these two Karman modes is responsible for the early occurrence of resonance. The lock-in resonance occurs when the primary vortex shedding mode locks with the natural frequency. It is characterized by an enlargement of the wake and by a reinforcement of the primary Karman vortices strength. [ABSTRACT FROM AUTHOR]

The control effect and mechanism of a passive–suction–jet control scheme on the vortex-induced vibration (VIV) of a stay-cable model were experimentally investigated in this study. The vibration responses and flow characteristics of an elastically mounted rigid cable model under passive–suction–jet control were measured at damping ratios (ξ) of 0.11%–0.29% and Reynolds numbers (Re) of 12700–29200. The control scheme was realized using specially designed pipes that could spontaneously produce jet flow from the holes on the leeward surface of the pipes. The results of the vibration response and lock-in region show that the passive–suction–jet control was effective in suppressing cable VIV, especially at ξ ≥ 0.17%. The evolution of the wake vortices at a low damping ratio (ξ = 0. 11 %) and high damping ratio (ξ = 0. 17 %) was discussed to study the vortex dynamics of the cable model subjected to VIV. The control mechanism of the pipe was also explored in terms of swirling strength distributions, streamlines, and velocity profiles in the wake field. The jet flow from the pipes was unsteady and formed periodic small-scale vortices at low damping ratios. At a high damping ratio, the jet flow was a steady flow and could suppress VIV more effectively. [ABSTRACT FROM AUTHOR]

The phenomenon of interference between vortex-induced vibration (VIV) and galloping in the transverse degree of freedom was studied in the wind tunnel in the case of a spring-mounted slender rectangular cylinder with a side ratio of 1.5 having the short side perpendicular to the flow. The tests were carried out in a wide Scruton number range, starting from low values and increasing it in small steps by using eddy-current viscous dampers. This study helped understanding the dynamics of the interaction between the two excitation mechanisms and clearly highlighted the transition through four regimes of VIV-galloping interference. It was found that a high value of the mass-damping parameter is required to decouple the ranges of excitation of vortex-induced vibration and galloping completely, and for the quasi-steady theory to predict the galloping critical wind speed correctly. This conclusion is also relevant from the engineering point of view, as it means that structures and structural elements with ordinary mass-damping properties can exhibit sustained vibrations in flow speed ranges where no excitation is predicted by classical theories of vortex-induced vibration and galloping. Although most of the experimental tests were conducted in smooth flow at zero angle of attack, the paper also discusses the sensitivity of the results to a small variation of the mean flow incidence and to the presence of a low-intensity free-stream turbulence. [ABSTRACT FROM AUTHOR]

The slot width ratio of a twin-box girder plays an important role in the aerodynamic performance of the corresponding bridge. Therefore, it is necessary to investigate the VIV performance of a twin-box girder with various slot width ratios and evaluate the effectiveness of present VIV countermeasures considering the influence of different slot width ratio. Through a series of wind tunnel tests, the present study firstly investigated the VIV performance of a twin-box girder suspension bridge with six representative slot width ratios ranging from 0% to 100%. Then, four countermeasures, including increasing structural damping ratio, installing grid plates, wind barriers and guide vanes, were checked and comprehensively evaluated with the aim of identifying the most effective VIV control measure. The experimental results showed that the VIV performance of a twin-box girder with 0% slot width ratio (a single box girder) is superior to those with central slot. The maximum normalized heaving VIV amplitude occurs at the slot width ratio of 60% for the 1:60 geometrical scale model (around 0.093) and 80% for the 1:30 scale model (around 0.1), respectively. The significant difference of the torsional VIV responses between the two models with different geometrical scale is found at the slot width ratio of 20%. The comparison of VIV countermeasures for the 20% and 60% slot width ratios suggested that increasing damping ratio is helpful for reducing VIV responses, while installing guide vanes may have adverse effects. The application of grid plates and wind barriers can have positive effects in suppressing the heaving VIV responses, while the effectiveness of grid plates is more obvious with the decrease of grid porosity. Furthermore, the application of the grid plates with 0% grid porosity can perfectly enable the VIV performance of the twin-box girder bridge to meet the requirements of both Sperling indicator and Wind-resistance specification of China. [ABSTRACT FROM AUTHOR]

Experiments with a flexible cylinder of circular cross-section towed behind a stationary circular cylinder at various distances are described in this paper. The dynamics of the model immersed in the wake of the leading one are fully characterised after data processing and computation of amplitude and frequency responses, as well as modal content as a function of reduced velocity. Reduced velocity was controlled by changing the towing speed of the system and by adjusting different pre-tensions on the model. The amplitude response of the model has been found to be dominated by resonance or lock-in regions in the reduced velocity axis at the different structural modes of the cylinder model. Separation distance does not produce significant qualitative changes in the response amplitudes and frequencies observed when compared to the response of the same cylinder without wake interference. However, there is a clear effect of the incoming wake, because amplitude responses are considerably higher than those expected in isolated cylinders at the same reduced velocities. Also, the response has been found to be more multi-modal with responses made of large amplitude contributions of several adjacent modes. [ABSTRACT FROM AUTHOR]

In this study, a coupled numerical model for simulation of 3D flow interacting with a slender body is proposed. A discrete-forcing immersed boundary method is implemented to couple the fluid motion and structure deformation. To model the dynamic response of a slender structure, an efficient nonlinear beam model is developed. A new surface reconstruction method is proposed to accurately represent the occupation of the moving structure with different cross-sections in the fixed Cartesian grids of the fluid domain. A quadratic interpolation based on the least-squares method is proposed to realize the local fluid velocity near the relocated solid surface. To validate the model, several fluid–structure interaction problems including a flexible plate attached to a circular cylinder in a flow, a flexible flag in a flow, etc. are simulated. The numerical results are in good agreement with reference results from either experiments or numerical modelling. The model is further employed to investigate the vortex-induced vibration of a flexible riser under unidirectional and oscillatory flow conditions as well as impulse force analysis for a swinging carangiform swimmer in different swing frequencies, the latter has a varying cross-sectional shape along the body's central axis. [ABSTRACT FROM AUTHOR]

The existence of a bidirectionally sheared flow field caused by internal solitary waves has recently been confirmed according to a field survey in the South China Sea. A model test of a tensioned flexible pipe in bidirectionally sheared flow was performed in an ocean basin. The model was 28.41 mm in diameter and 7.64 m in length. The purpose of the model test was to understand the response performance and obtain benchmark data of vortex-induced vibration (VIV) in bidirectionally sheared flow. The test was performed on a rotating test rig to simulate bidirectionally sheared flow conditions. Fiber Bragg Grating (FBG) strain sensors were arranged along the test pipe to measure bending strains, and the modal analysis approach was used to determine the VIV response. Reduced velocities based on the tested first natural frequency in water reached 30.79. The cross-flow and in-line VIV amplitudes, response frequencies in statistics and time-domain analysis, traveling wave characteristics, phase synchronization and trajectories are presented in this article. The maximum root mean square (RMS) VIV amplitudes in the test reached 0.51 diameters and 0.18 diameters in the cross-flow and in-line directions, respectively. A relative low Strouhal number of 0.10 was found in the CF direction. • An experiment of a flexible pipe that was 28.41 mm in diameter and 269 diameters in length under bidirectionally sheared flow was conducted. The bending strain of VIV was measured in the CF and IL directions at 9 and 14 locations, respectively. A total of 58 test conditions with maximum flow velocities varying from 0.30 to 1.39 m/s were used in this experiment. • The RMS VIV response in the CF and IL directions reached maximum levels of 0.51 D and 0.18 D , respectively. The '2n/n' and '2n/2n-1' modal groups, modal groups '3/1' and '4/1' existed under low flow velocity due to the special flow profile. • The Strouhal numbers under the bidirectional sheared flow were approximately 0.10 and 0.24 in the CF and IL directions, respectively, which were smaller than the St numbers under uniform flow and linearly sheared flow. • A strong traveling wave and 'multi-frequency' response phenomenon were found, particularly in the IL directions. [ABSTRACT FROM AUTHOR]

We study Vortex-Induced Vibration (VIV) of an elastically mounted cylinder placed in uniform flow using numerical simulations based on Immersed Boundary Method and theoretical analysis. We characterize the effect of bistability in elastic potential on energy harvesting from fluid flow, in which the energy generator is modeled as structural damping. A non-dimensional distance of 2 β separates the two minima of the bistable spring potential. An increase in the peak energy extraction efficiency and widening of lock-in range is observed for VIV with bistable springs compared to VIV with linear springs, especially for low values of β. We report peak efficiency of 10% for VIV with bistable springs, whereas the peak efficiency for VIV with linear spring is 4%. These results can be explained by the presence of an "Equilibrium Constraint (EC)", between oscillation amplitude and structure frequency, obtained by balancing the rate of energy production and the rate of energy dissipation due to either structural damping or electrical generators. The EC-based theory predicts that the maximum displacement amplitude depends on damping parameter b ∗ = 2 π 2 m ∗ ξ / U r as suggested by Klamo et al. (2005), where m ∗ is the solid-to-fluid mass ratio, ξ is the damping ratio and U r is the reduced velocity of the incident fluid flow. We briefly discuss the wake structures for bistable springs for different β and b ∗. We also derive a theoretical expression for oscillation amplitude and energy extraction efficiency for VIV with bistable springs, which agrees well with data from high fidelity numerical simulations, carried out in the present work. [ABSTRACT FROM AUTHOR]

Although vortex-induced vibration (VIV) has been extensively studied, much of existing literature deals with uniform flow in the absence of a boundary. The VIV flow field of a structure close to a boundary generally remains unexplored, but it can have important engineering implications, such as pipeline scour if the boundary is an erodible seabed. In this paper, laboratory experiments are performed to investigate the flow characteristics of an elastically mounted circular cylinder undergoing VIV, and a rigid plane boundary is considered to simplify the problem. The initial gap-to-diameter ratio is fixed at 0.8, and six different reduced velocities are considered. The velocity field is measured using a high resolution particle image velocimetry (PIV) system, which has several advantages over traditional PIV systems, including high sampling rate and the ability to mitigate scatter of laser light near the boundary, allowing accurate measurements at the viscous sublayer. This paper presents the vibration amplitude and oscillation frequency for different V r ; in addition, the mean velocity field, turbulence characteristics, vortex behavior, gap flow velocity, and normal/shear stresses on the boundary were measured/calculated, leading to new insights on the flow field behavior. [ABSTRACT FROM AUTHOR]

In this study, a method to obtain the hydrodynamic forces of a flexible riser undergoing vortex-induced vibration (VIV) based on measured strain is proposed. The tensioned riser is approximated as an Euler–Bernoulli beam, and an inverse method is adopted for the calculation of the hydrodynamic forces in the cross flow (CF) and inline (IL) directions. Based on these hydrodynamic forces, and combined with the VIV velocities and accelerations of the riser, the excitation and added-mass coefficients are obtained through a least-squares method. As an illustration example, the hydrodynamic characteristics of a flexible riser model undergoing VIV in a uniform flow are investigated. Results indicate that VIV leads to non-uniform distribution of the drag coefficient and amplifies the drag coefficient along the riser. Similar distribution of the energy transfer coefficient has been found between the entire riser and that of the CF direction. For synchronized VIV occurring in both CF and IL directions, the energy transferred from the fluid is all dissipated by the structural damping, and hence leads to the energy balance in both CF and IL directions. It further shows that the excitation coefficients on flexible riser undergoing VIV do not agree with those of the forced oscillation tests: excitation coefficients sometimes even become negative within the normally excitation regime, and are related with not only the non-dimensional frequency and amplitude but also the phase angles between the CF and IL vibrations. The added-mass coefficient of flexible risers does not keep a constant value of 1.0 anymore, but depends on the non-dimensional frequency and amplitude of vibration. [ABSTRACT FROM AUTHOR]