17 results
Flow-induced vibrations with and without structural restoring force: convergence under the effect of path curvature
- Rémi Bourguet
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- Journal:
- Journal of Fluid Mechanics / Volume 984 / 10 April 2024
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- 04 April 2024, A51
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When a cylinder is free to move along a transverse rectilinear path within a current, the vibrations developing with and without structural restoring force (SRF) noticeably deviate: if the elastic support is removed, their onset is delayed from a Reynolds number ($Re$, based on the body diameter and inflow velocity) value of approximately 20 to 30, and their peak amplitudes and frequency bandwidths are substantially reduced. The present study examines the influence of a curved path on this deviation by considering that the cylinder, mounted on an elastic support or not, is free to translate along a circular path whose radius is varied. The investigation is carried out numerically at $Re=25$ and $100$, i.e. subcritical and postcritical values relative to the threshold of $47$ that marks the onset of flow unsteadiness for a fixed body. The principal result of this work is that the behaviours of the flow–structure systems with and without SRF tend to converge under the effect of path curvature. Beyond a certain curvature magnitude, both systems explore the same vibration ranges and the presence or absence of SRF becomes indiscernible. This convergence is accompanied by an enhancement of the responses appearing without SRF. It is analysed in light of the evolution of the effective added mass which determines the subset of responses reached with SRF that remain accessible without SRF. The apparent continuity of the physical mechanisms between the subcritical- and postcritical-$Re$ values suggests that the convergence phenomenon uncovered here could persist at higher $Re$.
Path curvature enhances the flow-induced vibrations of a cylinder without structural restoring force
- Rémi Bourguet
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- Journal:
- Journal of Fluid Mechanics / Volume 977 / 25 December 2023
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- 18 December 2023, A31
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A cylinder immersed in a current and free to translate along a circular arc is considered to investigate the impact of path curvature on the flow-induced vibrations (FIV) occurring without structural restoring force. Path curvature magnitude ($\kappa$, inverse of path radius non-dimensionalized by the body diameter $D$) is varied from $0$ (transverse rectilinear path) to $20$, over a wide range of values of the structure to displaced fluid mass ratio, $m^\star \in [0.05,10]$. The exploration is carried out numerically at subcritical and postcritical values of the Reynolds number ($Re$, based on $D$ and the inflow velocity), i.e. below and above the critical value $47$ for the onset of flow unsteadiness when the body is fixed, up to $100$. Path curvature triggers a desynchronized regime of the flow–body system in addition to the synchronized regime typical of vortex-induced vibrations, and alters the composition of fluid forcing. The most prominent effect uncovered here is, however, a global enhancement of FIV, with three principal results: (i) vibrations and flow unsteadiness are found to arise at lower subcritical $Re$ along a curved path, down to $19.5$ versus $31$ for $\kappa =0$; (ii) the $m^\star$ range where substantial responses develop is considerably extended and encompasses the entire interval under study, which contrasts with the narrow band of low $m^\star$ identified for $\kappa =0$; (iii) the vibrations are amplified, $+45\,\%$ relative to the peak amplitude measured along a rectilinear path at $Re =100$.
Forced rotation enhances cylinder flow-induced vibrations at subcritical Reynolds number
- Rémi Bourguet
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- Journal:
- Journal of Fluid Mechanics / Volume 955 / 25 January 2023
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- 24 January 2023, R3
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When a cylinder is mounted on an elastic support within a current, vortex-induced vibrations (VIV) may occur down to a Reynolds number (Re) close to $20$, based on the body diameter ($D$) and inflow velocity ($U$), i.e. below the critical value of $47$ reported for the onset of flow unsteadiness when the body is fixed. The impact of a forced rotation of the elastically mounted cylinder on the system behaviour is explored numerically for $Re \leqslant 30$, over wide ranges of values of the rotation rate (ratio between body surface velocity and $U$, $\alpha \in [0,5]$) and reduced velocity (inverse of the oscillator natural frequency non-dimensionalized by $D$ and $U$, $U^\star \in [2,30]$). The influence of the rotation is not monotonic, but the most prominent effect uncovered in this work is a substantial enhancement of the subcritical-Re, flow-induced vibrations beyond $\alpha =2$. This enhancement is twofold. First, the rotation results in a considerable expansion of the vibration/flow unsteadiness region in the $({Re},U^\star )$ domain, down to $Re=4$. Second, the elliptical orbits described by the rotating body are subjected to a major amplification, with a transition from VIV to responses whose magnitude tends to increase unboundedly with $U^\star$, even though still synchronized with flow unsteadiness. The emergence of such galloping-like oscillations close to the onset of vibrations disrupts the scenario of gradual vibration growth with Re, as amplitudes larger than $10$ body diameters may be observed at $Re=10$.
Flow-induced vibrations of a cylinder along a circular arc
- Rémi Bourguet
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- Journal:
- Journal of Fluid Mechanics / Volume 954 / 10 January 2023
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- 23 December 2022, A7
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An elastically mounted circular cylinder, immersed in a cross-current and free to move along a rectilinear path, is subjected to vortex-induced vibrations (VIV). These vibrations develop through a mechanism referred to as lock-in, where body motion and vortex shedding synchronize at a frequency that may deviate both from the oscillator natural frequency and from the vortex shedding frequency past a fixed cylinder. The present numerical study aims at extending the analysis to curved trajectories, by considering that the cylinder is free to translate along a circular path. The Reynolds number based on the body diameter ($D$) and current velocity ($U$) is set to $100$. A wide range of path radii, from $0.05D$ to $10D$, and values of the reduced velocity (inverse of the oscillator natural frequency non-dimensionalized by $D$ and $U$) up to $30$ are examined, for the concave and convex configurations, i.e. the circular path centre located upstream or downstream of the cylinder. Path curvature results in a major alteration of the flow–body system behaviour compared with rectilinear VIV, with substantially different evolutions in the concave and convex configurations. In addition to the typical lock-in mechanism, two subharmonic forms of synchronization, at half and one third of vortex formation frequency, are uncovered in the convex configuration. They coexist with a desynchronized regime where the body and the flow oscillate at incommensurable frequencies. The four interaction regimes exhibit contrasted trends in terms of structural response, spatiotemporal organization of the wake and associated forces. They particularly differ by their symmetry properties, which are closely linked to the possible reconfiguration of the oscillator due to mean fluid forcing.
An experimental study of flow–structure interaction regimes of a freely falling flexible cylinder
- Manuel Lorite-Díez, Patricia Ern, Sébastien Cazin, Jérôme Mougel, Rémi Bourguet
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- Journal:
- Journal of Fluid Mechanics / Volume 946 / 10 September 2022
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- 05 August 2022, A16
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The fluid–structure interaction problem composed of an elongated, finite-length, flexible cylinder falling in a fluid at rest is investigated experimentally. Tomographic reconstruction of the cylinder and three-dimensional particle tracking velocimetry of the surrounding fluid, based on the Shake-The-Box algorithm, are used jointly to capture both solid and fluid motions. Starting from the rectilinear vertical fall characterized by a steady wake, focus is placed on subsequent regimes involving, mainly in the horizontal direction, periodic rigid-body motions (RBM) of weak amplitude or periodic large-amplitude bending oscillations (BO). Two RBM regimes are explored: the TRA regime where the cylinder exhibits translational oscillations in a plane perpendicular to its axis, and the AZI regime in which the body displays an azimuthal oscillation around its centre. The associated unsteady wakes are composed of counter-rotating vortices bending near the body ends to connect with the adjacent vortex rows. Specific organizations of the vortical structures are uncovered, depending on the regime. In particular, in the AZI regime, they present an antisymmetrical distribution relative to the midspan point. For a sufficiently long cylinder, BO regimes emerge, resembling the structural modes of an unsupported beam. The associated wakes exhibit a cellular organization. Within each cell delimited by two deformation nodes, two counter-rotating vortex rows are shed per oscillation cycle. Flow velocity fluctuations are in phase opposition on each side of a deformation node. For both RBM and BO regimes, frequency and phase analyses of cylinder and wake behaviours, along the span, highlight the spatio-temporal synchronization of the unsteady flow and moving body.
Impact of body inclination on the flow past a rotating cylinder
- Rémi Bourguet
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- Journal:
- Journal of Fluid Mechanics / Volume 923 / 25 September 2021
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- 02 August 2021, A33
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The rotation applied to a circular cylinder, rigidly mounted in a current perpendicular to its axis, can result in the suppression of vortex shedding and of the associated force fluctuations. It also causes the emergence of a myriad of two- and three-dimensional flow regimes. The present paper explores numerically the impact of a deviation from the normal incidence configuration, by considering a rotating cylinder inclined in the current. The Reynolds number based on the body diameter and the magnitude of the current velocity component normal to its axis ($U_\perp$) is set to $100$. The range of values of the rotation rate (ratio between body surface velocity and $U_\perp$, $\alpha \in [0,5.5]$) encompasses the two unsteady flow regions and three-dimensional transition identified at normal incidence. The inclination angle ($\theta$) refers to the angle between the current direction and the plane perpendicular to the cylinder axis. A low inclination angle ($\theta \in \{15^\circ ,30^\circ \}$), i.e. slight deviation from normal incidence ($\theta =0^\circ$), has a limited influence on the global evolution of the flow with $\alpha$, which can be predicted via the independence principle (IP), based on $U_\perp$ only. This highlights the robustness of prior observations made for $\theta =0^\circ$. Some effects of the axial flow are, however, uncovered in the high-$\alpha$ range; in particular, the single-sided vortex shedding is replaced by an irregular streamwise-oriented structure. In contrast, a large inclination angle ($\theta =75^\circ$) leads to a major reorganization of flow evolution scenario over the entire $\alpha$ range, with the disappearance of all steady regimes, the occurrence of structures reflecting the pronounced asymmetry of the configuration (oblique shedding, strongly slanted vorticity tongues) and a dramatic departure of fluid forces from the IP prediction.
Bending oscillations of a cylinder freely falling in still fluid
- Patricia Ern, Jérôme Mougel, Sébastien Cazin, Manuel Lorite-Díez, Rémi Bourguet
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- Journal:
- Journal of Fluid Mechanics / Volume 905 / 25 December 2020
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- 04 November 2020, R5
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We investigate experimentally the behaviour of an elongated flexible cylinder settling at moderate Reynolds number under the effect of buoyancy in a fluid otherwise at rest. The experiments uncover the development of large-amplitude periodic deformations of the cylinder (of the order of its diameter) in specific parameter ranges. Bending oscillations are observed to occur for two base flow situations, involving either a steady or an unsteady wake. In both cases, the sequence of oscillatory deformations emerging when the cylinder length is increased involves the bending modes of an unsupported cylinder with free ends. Comparison of the deformation frequency measured for the falling cylinder with the vortex shedding frequency expected for a non-deformable cylinder at the same Reynolds number indicates that the deformation is coupled to the wake unsteadiness. It also suggests that the cylinder degrees of freedom in deformability allow wake instability to be triggered at Reynolds numbers that would be subcritical for fixed rigid cylinders.
Vortex-induced vibrations of a flexible cylinder at subcritical Reynolds number
- Rémi Bourguet
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- Journal:
- Journal of Fluid Mechanics / Volume 902 / 10 November 2020
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- 04 September 2020, R3
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The flow past a fixed rigid cylinder becomes unsteady beyond a critical Reynolds number close to $47$, based on the body diameter and inflow velocity. The present paper explores numerically the vortex-induced vibrations (VIV) that may develop for a flexible cylinder at subcritical Reynolds number ($Re$), i.e. for $Re<47$. Flexible-cylinder VIV are found to occur down to $Re\approx 20$, as previously reported for elastically mounted rigid cylinders. A detailed analysis is carried out for $Re=25$, in two steps: the system behaviour is examined from the emergence of VIV to the excitation of the first structural modes; and then focus is placed on higher-mode responses. In all cases, a single vibration frequency is excited in each direction. The cross-flow and in-line responses exhibit contrasting magnitudes (peak amplitudes of $0.35$ versus $0.01$ diameters), as well as distinct symmetry properties and evolutions (e.g. standing/travelling waves). The flow, unsteady once the cylinder vibrates, is found to be temporally and spatially locked with body motion. The synchronization with the cross-flow standing-wave responses is accompanied by the formation of cellular wake patterns, regardless of the modes involved in the vibrations. Body trajectory varies along the span, but dominant orbits can be identified. Despite the low amplitudes of the in-line responses, connections are uncovered between orbit orientation and flow–structure energy transfer, with different trends in each direction.
Two-degree-of-freedom flow-induced vibrations of a rotating cylinder
- Rémi Bourguet
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- Journal:
- Journal of Fluid Mechanics / Volume 897 / 25 August 2020
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- 18 June 2020, A31
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The flow-induced vibrations of an elastically mounted circular cylinder, free to oscillate in the streamwise and cross-flow directions, and forced to rotate about its axis, are investigated via two- and three-dimensional simulations. The Reynolds number based on the body diameter and inflow velocity is equal to $100$. The impact of the imposed rotation on the flow–structure system behaviour is explored over wide ranges of values of the rotation rate (ratio between the cylinder surface and inflow velocities, $\unicode[STIX]{x1D6FC}\in [0,5.5]$) and of the reduced velocity (inverse of the oscillator natural frequency non-dimensionalized by the inflow velocity and body diameter, $U^{\star }\in [1,25]$). Flow-induced vibrations are found to develop over the entire range of $\unicode[STIX]{x1D6FC}$, including in the intervals where the imposed rotation cancels flow unsteadiness when the body is rigidly mounted (i.e. not allowed to translate). The responses of the two-degree-of-freedom oscillator substantially depart from their one-degree-of-freedom counterparts. Up to a rotation rate close to $2$, the body exhibits oscillations comparable to the vortex-induced vibrations usually reported for a non-rotating circular cylinder: they develop under flow–body synchronization and their amplitudes present bell-shaped evolutions as functions of $U^{\star }$. They are, however, enhanced by the rotation as they can reach $1$ body diameter in each direction, which represents twice the peak amplitude of cross-flow response for $\unicode[STIX]{x1D6FC}=0$. The symmetry breaking due to the rotation results in deviations from the typical figure-eight orbits. The flow remains close to that observed in the rigidly mounted body case, i.e. two-dimensional with two spanwise vortices shed per cycle. Beyond $\unicode[STIX]{x1D6FC}=2$, the structural responses resemble the galloping oscillations generally encountered for non-axisymmetric bodies, with amplitudes growing unboundedly with $U^{\star }$. The response growth rate increases with $\unicode[STIX]{x1D6FC}$ and amplitudes larger than $20$ diameters are observed. The cylinder describes, at low frequencies, elliptical orbits oriented in the opposite sense compared to the imposed rotation. The emergence of subharmonic components of body displacements, leading to period doubling or quadrupling, induces slight variations about this canonical shape. These responses are not predicted by a quasi-steady modelling of fluid forcing, i.e. based on the evolution of the mean flow at each step of body motion; this suggests that the interaction with flow unsteadiness cannot be neglected. It is shown that flow–body synchronization persists, which is not expected for galloping oscillations. Within this region of the parameter space, the flow undergoes a major reconfiguration. A myriad of novel spatio-temporal structures arise with up to $20$ vortices formed per cycle. The flow three-dimensional transition occurs down to $\unicode[STIX]{x1D6FC}\approx 2$, versus $3.7$ for the rigidly mounted body. It is, however, shown that it has only a limited influence on the system behaviour.
Flow-induced vibrations of a rotating cylinder in an arbitrary direction
- Rémi Bourguet
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- Journal:
- Journal of Fluid Mechanics / Volume 860 / 10 February 2019
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- 11 December 2018, pp. 739-766
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The flow-induced vibrations of an elastically mounted circular cylinder, free to oscillate in an arbitrary direction and forced to rotate about its axis, are examined via two- and three-dimensional simulations, at a Reynolds number equal to 100, based on the body diameter and inflow velocity. The behaviour of the flow–structure system is investigated over the entire range of vibration directions, defined by the angle $\unicode[STIX]{x1D703}$ between the direction of the current and the direction of motion, a wide range of values of the reduced velocity $U^{\star }$ (inverse of the oscillator natural frequency) and three values of the rotation rate (ratio between the cylinder surface and inflow velocities), $\unicode[STIX]{x1D6FC}\in \{0,1,3\}$, in order to cover the reference non-rotating cylinder case, as well as typical slow and fast rotation cases. The oscillations of the non-rotating cylinder ($\unicode[STIX]{x1D6FC}=0$) develop under wake-body synchronization or lock-in, and their amplitude exhibits a bell-shaped evolution, typical of vortex-induced vibrations (VIV), as a function of $U^{\star }$. When $\unicode[STIX]{x1D703}$ is increased from $0^{\circ }$ to $90^{\circ }$ (or decreased from $180^{\circ }$ to $90^{\circ }$), the bell-shaped curve tends to monotonically increase in width and magnitude. For all angles, the flow past the non-rotating body is two-dimensional with formation of two counter-rotating spanwise vortices per cycle. The behaviour of the system remains globally the same for $\unicode[STIX]{x1D6FC}=1$. The principal effects of the slow rotation are a slight amplification of the VIV-like responses and widening of the vibration windows, as well as a limited asymmetry of the responses and forces about the symmetrical configuration $\unicode[STIX]{x1D703}=90^{\circ }$. The impact of the fast rotation ($\unicode[STIX]{x1D6FC}=3$) is more pronounced: VIV-like responses persist over a range of $\unicode[STIX]{x1D703}$ but, outside this range, the system is found to undergo a transition towards galloping-like oscillations characterised by amplitudes growing unboundedly with $U^{\star }$. A quasi-steady modelling of fluid forcing predicts the emergence of galloping-like responses as $\unicode[STIX]{x1D703}$ is varied, which suggests that they could be mainly driven by the mean flow. It, however, appears that flow unsteadiness and body motion remain synchronised in this vibration regime where a variety of multi-vortex wake patterns are uncovered. The interaction with flow dynamics results in deviations from the quasi-steady prediction. The successive steps in the evolution of the vibration amplitude versus $U^{\star }$, linked to wake pattern switch, are not captured by the quasi-steady approach. The flow past the rapidly-rotating, vibrating cylinder becomes three-dimensional over an interval of $\unicode[STIX]{x1D703}$ including the in-line oscillation configuration, with only a minor effect on the system behaviour.
Three-dimensional mode selection of the flow past a rotating and inline oscillating cylinder
- David Lo Jacono, Rémi Bourguet, Mark C. Thompson, Justin S. Leontini
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- Journal of Fluid Mechanics / Volume 855 / 25 November 2018
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- 19 September 2018, R3
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This paper studies the transition to three-dimensional flow in the wake of a cylinder immersed in a free stream, where the cylinder is externally forced to continuously rotate about its axis and to linearly oscillate in the streamwise direction. Floquet stability analysis is used to assess the stability of the nominal two-dimensional flows at a Reynolds number $Re=100$ and rotation rate $\unicode[STIX]{x1D6FC}=\unicode[STIX]{x1D714}D/U=3$ to three-dimensional perturbations, as a function of the amplitude and frequency of the linear oscillations. Two modes of instability are found, distinguished by their spatial structure, temporal behaviour and apparent mechanism. The first mode has a shorter wavelength in the spanwise direction and appears to be linked to a centrifugal instability in the layer of fluid near the rotating body. The second mode has a longer wavelength and is linked to an instability of the vortex cores in the wake that is subharmonic, leading to a period doubling. Either mode can be stable while the other is unstable, depending primarily on the frequency of the oscillation of the cylinder. This indicates that either mode can control the transition to a three-dimensional flow. The results are compared to the fully three-dimensional simulation results of a rotating cylinder elastically mounted and free to oscillate in the streamwise direction from Bourguet & Lo Jacono (J. Fluid Mech., vol. 781, 2015, pp. 127–165), and appear to be able to explain the surprising switching of the observed spanwise wavelength in that flow as a change in the dominant mode, and therefore mechanism, of instability.
Vortex-induced vibrations of a cylinder in planar shear flow
- Simon Gsell, Rémi Bourguet, Marianna Braza
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- Journal of Fluid Mechanics / Volume 825 / 25 August 2017
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- 20 July 2017, pp. 353-384
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The system composed of a circular cylinder, either fixed or elastically mounted, and immersed in a current linearly sheared in the cross-flow direction, is investigated via numerical simulations. The impact of the shear and associated symmetry breaking are explored over wide ranges of values of the shear parameter (non-dimensional inflow velocity gradient, $\unicode[STIX]{x1D6FD}\in [0,0.4]$) and reduced velocity (inverse of the non-dimensional natural frequency of the oscillator, $U^{\ast }\in [2,14]$), at Reynolds number $Re=100$; $\unicode[STIX]{x1D6FD}$, $U^{\ast }$ and $Re$ are based on the inflow velocity at the centre of the body and on its diameter. In the absence of large-amplitude vibrations and in the fixed body case, three successive regimes are identified. Two unsteady flow regimes develop for $\unicode[STIX]{x1D6FD}\in [0,0.2]$ (regime L) and $\unicode[STIX]{x1D6FD}\in [0.2,0.3]$ (regime H). They differ by the relative influence of the shear, which is found to be limited in regime L. In contrast, the shear leads to a major reconfiguration of the wake (e.g. asymmetric pattern, lower vortex shedding frequency, synchronized oscillation of the saddle point) and a substantial alteration of the fluid forcing in regime H. A steady flow regime (S), characterized by a triangular wake pattern, is uncovered for $\unicode[STIX]{x1D6FD}>0.3$. Free vibrations of large amplitudes arise in a region of the parameter space that encompasses the entire range of $\unicode[STIX]{x1D6FD}$ and a range of $U^{\ast }$ that widens as $\unicode[STIX]{x1D6FD}$ increases; therefore vibrations appear beyond the limit of steady flow in the fixed body case ($\unicode[STIX]{x1D6FD}=0.3$). Three distinct regimes of the flow–structure system are encountered in this region. In all regimes, body motion and flow unsteadiness are synchronized (lock-in condition). For $\unicode[STIX]{x1D6FD}\in [0,0.2]$, in regime VL, the system behaviour remains close to that observed in uniform current. The main impact of the shear concerns the amplification of the in-line response and the transition from figure-eight to ellipsoidal orbits. For $\unicode[STIX]{x1D6FD}\in [0.2,0.4]$, the system exhibits two well-defined regimes: VH1 and VH2 in the lower and higher ranges of $U^{\ast }$, respectively. Even if the wake patterns, close to the asymmetric pattern observed in regime H, are comparable in both regimes, the properties of the vibrations and fluid forces clearly depart. The responses differ by their spectral contents, i.e. sinusoidal versus multi-harmonic, and their amplitudes are much larger in regime VH1, where the in-line responses reach $2$ diameters ($0.03$ diameters in uniform flow) and the cross-flow responses $1.3$ diameters. Aperiodic, intermittent oscillations are found to occur in the transition region between regimes VH1 and VH2; it appears that wake–body synchronization persists in this case.
The onset of vortex-induced vibrations of a flexible cylinder at large inclination angle
- Rémi Bourguet, Michael S. Triantafyllou
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- Journal:
- Journal of Fluid Mechanics / Volume 809 / 25 December 2016
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- 09 November 2016, pp. 111-134
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The onset of the vortex-induced vibration (VIV) regime of a flexible cylinder inclined at $80^{\circ }$ within a uniform current is studied by means of direct numerical simulations, at Reynolds number $500$ based on the body diameter and inflow velocity magnitude. A range of values of the reduced velocity, defined as the inverse of the fundamental natural frequency, is examined in order to capture the emergence of the body responses and explore the concomitant reorganization of the flow and fluid forcing. Additional simulations at normal incidence confirm that the independence principle, which states that the system behaviour is determined by the normal inflow component, does not apply at such large inclination angle. Contrary to the normal incidence case, the free vibrations of the inclined cylinder arise far from the Strouhal frequency, i.e. the vortex shedding frequency downstream of a fixed rigid cylinder. The trace of the stationary body wake is found to persist beyond the vibration onset: the flow may still exhibit an oblique component that relates to the slanted vortex shedding pattern observed in the absence of vibration. This flow component which occurs close to the Strouhal frequency, at a high and incommensurable frequency compared to the vibration frequency, is referred to as Strouhal component; it induces a high-frequency component in fluid forcing. The vibration onset is accompanied by the appearance of novel, low-frequency components of the flow and fluid forcing which are synchronized with body motion. This second dominant flow component, referred to as lock-in component, is characterized by a parallel spatial pattern. The Strouhal and lock-in components of the flow coexist over a range of reduced velocities, with variable contributions, which results in a variety of mixed wake patterns. The transition from oblique to parallel vortex shedding that occurs during the amplification of the structural responses, is driven by the opposite trends of these two component contributions: the decrease of the Strouhal component magnitude associated with the progressive disappearance of the high-frequency force component, and simultaneously, the increase of the lock-in component magnitude, which dominates once the fully developed VIV regime is reached and the flow dynamics is entirely governed by wake–body synchronization.
In-line flow-induced vibrations of a rotating cylinder
- Rémi Bourguet, David Lo Jacono
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- Journal:
- Journal of Fluid Mechanics / Volume 781 / 25 October 2015
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- 16 September 2015, pp. 127-165
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The flow-induced vibrations of an elastically mounted circular cylinder, free to oscillate in the direction parallel to the current and subjected to a forced rotation about its axis, are investigated by means of two- and three-dimensional numerical simulations, at a Reynolds number equal to 100 based on the cylinder diameter and inflow velocity. The cylinder is found to oscillate up to a rotation rate (ratio between the cylinder surface and inflow velocities) close to 2 (first vibration region), then the body and the flow are steady until a rotation rate close to 2.7 where a second vibration region begins. Each vibration region is characterized by a specific regime of response. In the first region, the vibration amplitude follows a bell-shaped evolution as a function of the reduced velocity (inverse of the oscillator natural frequency). The maximum vibration amplitudes, even though considerably augmented by the rotation relative to the non-rotating body case, remain lower than 0.1 cylinder diameters. Due to their trends as functions of the reduced velocity and to the fact that they develop under a condition of wake-body synchronization or lock-in, the responses of the rotating cylinder in this region are comparable to the vortex-induced vibrations previously described in the absence of rotation. The symmetry breaking due to the rotation is shown to directly impact the structure displacement and fluid force frequency contents. In the second region, the vibration amplitude tends to increase unboundedly with the reduced velocity. It may become very large, higher than 2.5 diameters in the parameter space under study. Such structural oscillations resemble the galloping responses reported for non-axisymmetric bodies. They are accompanied by a dramatic amplification of the fluid forces compared to the non-vibrating cylinder case. It is shown that body oscillation and flow unsteadiness remain synchronized and that a variety of wake topologies may be encountered in this vibration region. The low-frequency, large-amplitude responses are associated with novel asymmetric multi-vortex patterns, combining a pair and a triplet or a quartet of vortices per cycle. The flow is found to undergo three-dimensional transition in the second vibration region, with a limited influence on the system behaviour. It appears that the transition occurs for a substantially lower rotation rate than for a rigidly mounted cylinder.
Flow-induced vibrations of a rotating cylinder
- Rémi Bourguet, David Lo Jacono
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- Journal:
- Journal of Fluid Mechanics / Volume 740 / 10 February 2014
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- 06 February 2014, pp. 342-380
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The flow-induced vibrations of a circular cylinder, free to oscillate in the cross-flow direction and subjected to a forced rotation about its axis, are analysed by means of two- and three-dimensional numerical simulations. The impact of the symmetry breaking caused by the forced rotation on the vortex-induced vibration (VIV) mechanisms is investigated for a Reynolds number equal to $100$, based on the cylinder diameter and inflow velocity. The cylinder is found to oscillate freely up to a rotation rate (ratio between the cylinder surface and inflow velocities) close to $4$. Under forced rotation, the vibration amplitude exhibits a bell-shaped evolution as a function of the reduced velocity (inverse of the oscillator natural frequency) and reaches $1.9$ diameters, i.e. three times the maximum amplitude in the non-rotating case. The free vibrations of the rotating cylinder occur under a condition of wake–body synchronization similar to the lock-in condition driving non-rotating cylinder VIV. The largest vibration amplitudes are associated with a novel asymmetric wake pattern composed of a triplet of vortices and a single vortex shed per cycle, the ${\rm T} + {\rm S}$ pattern. In the low-frequency vibration regime, the flow exhibits another new topology, the U pattern, characterized by a transverse undulation of the spanwise vorticity layers without vortex detachment; consequently, free oscillations of the rotating cylinder may also develop in the absence of vortex shedding. The symmetry breaking due to the rotation is shown to directly impact the selection of the higher harmonics appearing in the fluid force spectra. The rotation also influences the mechanism of phasing between the force and the structural response.
Distributed lock-in drives broadband vortex-induced vibrations of a long flexible cylinder in shear flow
- Rémi Bourguet, George Em Karniadakis, Michael S. Triantafyllou
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- Journal:
- Journal of Fluid Mechanics / Volume 717 / 25 February 2013
- Published online by Cambridge University Press:
- 01 February 2013, pp. 361-375
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A slender flexible body immersed in sheared cross-flow may exhibit vortex-induced vibrations (VIVs) involving a wide range of excited frequencies and structural wavenumbers. The mechanisms of broadband VIVs of a cylindrical tensioned beam of length-to-diameter aspect ratio 200 placed in shear flow, with an exponentially varying profile along the span, are investigated by means of direct numerical simulation. The Reynolds number is equal to 330 based on the maximum velocity, for comparison with previous work on narrowband vibrations in linear shear flow. The flow is found to excite the structure at a number of different locations under a condition of wake–body synchronization, or lock-in. Broadband responses are associated with a distributed occurrence of the lock-in condition along the span, as opposed to the localized lock-in regions limited to the high inflow velocity zone, reported for narrowband vibrations in sheared current. Despite the instantaneously multi-frequency nature of broadband responses, the lock-in phenomenon remains a locally mono-frequency event, since the vortex formation is generally synchronized with a single vibration frequency at a given location. The spanwise distribution of the excitation zones induces travelling structural waves moving in both directions; this contrasts with the narrowband case where the direction of propagation toward decreasing inflow velocity is preferred. A generalization of the mechanism of phase-locking between the in-line and cross-flow responses is proposed for broadband VIVs under the lock-in condition. A spanwise drift of the in-line/cross-flow phase difference is identified for the high-wavenumber vibration components; this drift is related to the strong travelling wave character of the corresponding structural waves.
Vortex-induced vibrations of a long flexible cylinder in shear flow
- REMI BOURGUET, GEORGE E. KARNIADAKIS, MICHAEL S. TRIANTAFYLLOU
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- Journal:
- Journal of Fluid Mechanics / Volume 677 / 25 June 2011
- Published online by Cambridge University Press:
- 27 April 2011, pp. 342-382
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We investigate the in-line and cross-flow vortex-induced vibrations of a long cylindrical tensioned beam, with length to diameter ratio L/D = 200, placed within a linearly sheared oncoming flow, using three-dimensional direct numerical simulation. The study is conducted at three Reynolds numbers, from 110 to 1100 based on maximum velocity, so as to include the transition to turbulence in the wake. The selected tension and bending stiffness lead to high-wavenumber vibrations, similar to those encountered in long ocean structures. The resulting vortex-induced vibrations consist of a mixture of standing and travelling wave patterns in both the in-line and cross-flow directions; the travelling wave component is preferentially oriented from high to low velocity regions. The in-line and cross-flow vibrations have a frequency ratio approximately equal to 2. Lock-in, the phenomenon of self-excited vibrations accompanied by synchronization between the vortex shedding and cross-flow vibration frequencies, occurs in the high-velocity region, extending across 30% or more of the beam length. The occurrence of lock-in disrupts the spanwise regularity of the cellular patterns observed in the wake of stationary cylinders in shear flow. The wake exhibits an oblique vortex shedding pattern, inclined in the direction of the travelling wave component of the cylinder vibrations. Vortex splittings occur between spanwise cells of constant vortex shedding frequency. The flow excites the cylinder under the lock-in condition with a preferential in-line versus cross-flow motion phase difference corresponding to counter-clockwise, figure-eight orbits; but it damps cylinder vibrations in the non-lock-in region. Both mono-frequency and multi-frequency responses may be excited. In the case of multi-frequency response and within the lock-in region, the wake can lock in to different frequencies at various spanwise locations; however, lock-in is a locally mono-frequency event, and hence the flow supplies energy to the structure mainly at the local lock-in frequency.