7 results
Vortex-induced vibration prediction via an impedance criterion
- D. Sabino, D. Fabre, J. S. Leontini, D. Lo Jacono
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- Journal:
- Journal of Fluid Mechanics / Volume 890 / 10 May 2020
- Published online by Cambridge University Press:
- 04 March 2020, A4
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The vortex-induced vibration of a spring-mounted, damped, rigid circular cylinder, immersed in a Newtonian viscous flow and capable of moving in the direction orthogonal to the unperturbed flow is investigated for Reynolds numbers $Re$ in the vicinity of the onset of unsteadiness ($15\leqslant Re\leqslant 60$) using the incompressible linearised Navier–Stokes equations. In a first step, we solve the linear problem considering an imposed harmonic motion of the cylinder. Results are interpreted in terms of the mechanical impedance, i.e. the ratio between the vertical force coefficient and the cylinder velocity, which is represented as function of the Reynolds number and the driving frequency. Considering the energy transfer between the cylinder and the fluid, we show that impedance results provide a simple criterion allowing the prediction of the onset of instability of the coupled fluid-elastic structure case. A global stability analysis of the fully coupled fluid/cylinder system is then performed. The instability thresholds obtained by this second approach are found to be in perfect agreement with the predictions of the impedance-based criterion. A theoretical argument, based on asymptotic developments, is then provided to give a prediction of eigenvalues of the coupled problem, as well as to characterise the region of instability beyond the threshold as function of the reduced velocity $U^{\ast }$, the dimensionless mass $m^{\ast }$ and the Reynolds number. The influence of the damping parameter $\unicode[STIX]{x1D6FE}$ on the instability region is also explored.
Experimental investigation of flow-induced vibration of a sinusoidally rotating circular cylinder
- K. W. L. Wong, J. Zhao, D. Lo Jacono, M. C. Thompson, J. Sheridan
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- Journal:
- Journal of Fluid Mechanics / Volume 848 / 10 August 2018
- Published online by Cambridge University Press:
- 05 June 2018, pp. 430-466
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The present experimental investigation characterises the dynamic response and wake structure of a sinusoidally rotating circular cylinder with a low mass ratio (defined as the ratio of the total oscillating mass to the displaced fluid mass) undergoing cross-stream flow-induced vibration (FIV). The study covers a wide parameter space spanning the forcing rotary oscillation frequency ratio $0\leqslant f_{r}^{\ast }\leqslant 4.5$ and the forcing rotation speed ratio $0\leqslant \unicode[STIX]{x1D6FC}_{r}^{\ast }\leqslant 2.0$, at reduced velocities associated with the vortex-induced vibration (VIV) upper and lower amplitude response branches. Here, $f_{r}^{\ast }=f_{r}/f_{nw}$ and $\unicode[STIX]{x1D6FC}_{r}^{\ast }=\unicode[STIX]{x1D6FA}_{o}D/(2U)$, where $f_{r}$ is the forcing rotary oscillation frequency, $f_{nw}$ is the natural frequency of the system in quiescent fluid (water), $\unicode[STIX]{x1D6FA}_{o}$ is the peak angular rotation speed, $D$ is the cylinder diameter and $U$ is the free-stream velocity; the reduced velocity is defined by $U^{\ast }=U/(\,f_{nw}D)$. The fluid–structure system was modelled using a low-friction air-bearing system in conjunction with a free-surface recirculating water channel, with axial rotary motion provided by a microstepping motor. The cylinder was allowed to vibrate with only one degree of freedom transverse to the oncoming free-stream flow. It was found that in specific ranges of $f_{r}^{\ast }$, the body vibration frequency may deviate from that seen in the non-rotating case and lock onto the forcing rotary oscillation frequency or its one-third subharmonic. The former is referred to as the ‘rotary lock-on’ (RLO) region and the latter as the ‘tertiary lock-on’ (TLO) region. Significant increases in the vibration amplitude and suppression of VIV could both be observed in different parts of the RLO and TLO regions. The peak amplitude response in the case of $U^{\ast }=5.5$ (upper branch) was observed to be $1.2D$, an increase of approximately $50\,\%$ over the non-rotating case, while in the case of $U^{\ast }=8.0$ (lower branch), the peak amplitude response was $2.2D$, a remarkable increase of $270\,\%$ over the non-rotating case. Notably, the results showed that the amplitude responses at moderate Reynolds numbers ($Re=UD/\unicode[STIX]{x1D708}=2060$ and $2940$, where $\unicode[STIX]{x1D708}$ is the kinematic viscosity of the fluid) in the present study showed significant differences from those of a previous low-Reynolds-number ($Re=350$) numerical study at similar reduced velocities by Du & Sun (Phys. Fluids, vol. 14 (8), 2015, pp. 2767–2777). Remarkably, in an additional study examining the cylinder vibration as a function of $U^{\ast }$ while the fixed forcing rotary oscillation parameters were kept constant at $(f_{r}^{\ast },\unicode[STIX]{x1D6FC}_{r}^{\ast })=(1.0,1.0)$, the cylinder experienced substantially larger oscillations than in the non-rotating case, and a rotation-induced galloping response was observed for $U^{\ast }>12$, where the amplitude increased monotonically to reach approximately $3.0D$ at the highest reduced velocity ($U^{\ast }=20$) tested. Furthermore, new wake modes were identified in the RLO and TLO regions using particle image velocimetry measurements at selected points in the $f_{r}^{\ast }-\unicode[STIX]{x1D6FC}_{r}^{\ast }$ parameter space.
Experimental investigation of in-line flow-induced vibration of a rotating circular cylinder
- J. Zhao, D. Lo Jacono, J. Sheridan, K. Hourigan, M. C. Thompson
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- Journal:
- Journal of Fluid Mechanics / Volume 847 / 25 July 2018
- Published online by Cambridge University Press:
- 25 May 2018, pp. 664-699
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This study experimentally investigates the in-line flow-induced vibration (FIV) of an elastically mounted circular cylinder under forced axial rotation in a free stream. The present experiments characterise the structural vibration, fluid forces and wake structure of the fluid–structure system at a low mass ratio (the ratio of the total mass to the displaced fluid mass) over a wide parameter space spanning the reduced velocity range $5\leqslant U^{\ast }\leqslant 32$ and the rotation rate range $0\leqslant \unicode[STIX]{x1D6FC}\leqslant 3.5$ , where $U^{\ast }=U/(\,f_{nw}D)$ and $\unicode[STIX]{x1D6FC}=|\unicode[STIX]{x1D6FA}|D/(2U)$ , with $U$ the free-stream velocity, $D$ the cylinder outer diameter, $f_{nw}$ the natural frequency of the system in quiescent water and $|\unicode[STIX]{x1D6FA}|$ the angular velocity of the cylinder rotation. The corresponding Reynolds number (defined by $Re=UD/\unicode[STIX]{x1D708}$ , with $\unicode[STIX]{x1D708}$ the kinematic viscosity of the fluid) was varied over the interval $1349\leqslant Re\leqslant 8624$ , where it is expected that the FIV response is likely to be relatively insensitive to the Reynolds number. The fluid–structure system was modelled using a low-friction air-bearing system in conjunction with a free-surface water-channel facility. Three vibration regions that exhibited vortex-induced vibration (VIV) synchronisation, rotation-induced galloping and desynchronised responses were observed. In both the VIV synchronisation and rotation-induced galloping regions, significant cylinder vibration was found to be correlated with wake–body synchronisation within the rotation rate range $2.20\lesssim \unicode[STIX]{x1D6FC}\lesssim 3.15$ . Of significant interest, the frequency of the streamwise fluid force could be modulated by the imposed rotation to match that of the transverse lift force, resulting in harmonic synchronisation. Measurements using the particle image velocimetry (PIV) technique were performed to identify the wake structure. Interestingly, the imposed rotation can cause regular vortex shedding in in-line FIV at rotation rates that see suppression of the Bénard–von-Kármán vortex shedding in the case of a rigidly mounted cylinder ( $\unicode[STIX]{x1D6FC}\gtrsim 1.75$ ). There is a monotonic increase in the drag coefficient with rotation rate beyond $\unicode[STIX]{x1D6FC}=2$ for a non-oscillating rotating cylinder. This suggests that the mechanism for sustaining the large rotation-induced galloping oscillations at higher $\unicode[STIX]{x1D6FC}$ is due to a combination of aerodynamic forcing from the locked induced vortex shedding associated with the oscillations, assisted by aerodynamic forcing, evaluated using quasi-steady theory.
Vortex-induced vibration of a rotating sphere
- A. Sareen, J. Zhao, D. Lo Jacono, J. Sheridan, K. Hourigan, M. C. Thompson
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- Journal:
- Journal of Fluid Mechanics / Volume 837 / 25 February 2018
- Published online by Cambridge University Press:
- 20 December 2017, pp. 258-292
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Vortex-induced vibration (VIV) of a sphere represents one of the most generic fundamental fluid–structure interaction problems. Since vortex-induced vibration can lead to structural failure, numerous studies have focused on understanding the underlying principles of VIV and its suppression. This paper reports on an experimental investigation of the effect of imposed axial rotation on the dynamics of vortex-induced vibration of a sphere that is free to oscillate in the cross-flow direction, by employing simultaneous displacement and force measurements. The VIV response was investigated over a wide range of reduced velocities (i.e. velocity normalised by the natural frequency of the system): $3\leqslant U^{\ast }\leqslant 18$, corresponding to a Reynolds number range of $5000<Re<30\,000$, while the rotation ratio, defined as the ratio between the sphere surface and inflow speeds, $\unicode[STIX]{x1D6FC}=|\unicode[STIX]{x1D714}|D/(2U)$, was varied in increments over the range of $0\leqslant \unicode[STIX]{x1D6FC}\leqslant 7.5$. It is found that the vibration amplitude exhibits a typical inverted bell-shaped variation with reduced velocity, similar to the classic VIV response for a non-rotating sphere but without the higher reduced velocity response tail. The vibration amplitude decreases monotonically and gradually as the imposed transverse rotation rate is increased up to $\unicode[STIX]{x1D6FC}=6$, beyond which the body vibration is significantly reduced. The synchronisation regime, defined as the reduced velocity range where large vibrations close to the natural frequency are observed, also becomes narrower as $\unicode[STIX]{x1D6FC}$ is increased, with the peak saturation amplitude observed at progressively lower reduced velocities. In addition, for small rotation rates, the peak amplitude decreases almost linearly with $\unicode[STIX]{x1D6FC}$. The imposed rotation not only reduces vibration amplitudes, but also makes the body vibrations less periodic. The frequency spectra revealed the occurrence of a broadband spectrum with an increase in the imposed rotation rate. Recurrence analysis of the structural vibration response demonstrated a transition from periodic to chaotic in a modified recurrence map complementing the appearance of broadband spectra at the onset of bifurcation. Despite considerable changes in flow structure, the vortex phase ($\unicode[STIX]{x1D719}_{vortex}$), defined as the phase between the vortex force and the body displacement, follows the same pattern as for the non-rotating case, with the $\unicode[STIX]{x1D719}_{vortex}$ increasing gradually from low values in Mode I of the sphere vibration to almost $180^{\circ }$ as the system undergoes a continuous transition to Mode II of the sphere vibration at higher reduced velocity. The total phase ($\unicode[STIX]{x1D719}_{total}$), defined as the phase between the transverse lift force and the body displacement, only increases from low values after the peak amplitude response in Mode II has been reached. It reaches its maximum value (${\sim}165^{\circ }$) close to the transition from the Mode II upper plateau to the lower plateau, reminiscent of the behaviour seen for the upper to lower branch transition for cylinder VIV. Hydrogen-bubble visualisations and particle image velocimetry (PIV) performed in the equatorial plane provided further insights into the flow dynamics near the sphere surface. The mean wake is found to be deflected towards the advancing side of the sphere, associated with an increase in the Magnus force. For higher rotation ratios, the near-wake rear recirculation zone is absent and the flow is highly vectored from the retreating side to the advancing side, giving rise to large-scale shedding. For a very high rotation ratio of $\unicode[STIX]{x1D6FC}=6$, for which vibrations are found to be suppressed, a one-sided large-scale shedding pattern is observed, similar to the shear-layer instability one-sided shedding observed previously for a rigidly mounted rotating sphere.
Flow-induced vibration of two cylinders in tandem and staggered arrangements
- Martin D. Griffith, David Lo Jacono, John Sheridan, Justin S. Leontini
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- Journal:
- Journal of Fluid Mechanics / Volume 833 / 25 December 2017
- Published online by Cambridge University Press:
- 02 November 2017, pp. 98-130
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A numerical study of the flow-induced vibration of two elastically mounted cylinders in tandem and staggered arrangements at Reynolds number $Re=200$ is presented. The cylinder centres are set at a streamwise distance of 1.5 cylinder diameters, placing the rear cylinder in the near-wake region of the front cylinder for the tandem arrangement. The cross-stream or lateral offset is varied between 0 and 5 cylinder diameters. The two cylinders are identical, with the same elastic mounting, and constrained to oscillate only in the cross-flow direction. The variation of flow behaviours is examined for static cylinders and for elastic mountings of a range of spring stiffnesses, or reduced velocity. At least seven major modes of flow response are identified, delineated by whether the oscillation is effectively symmetric, and the strength of the influence of the flow through the gap between the two cylinders. Submodes of these are also identified based on whether or not the flow remains periodic. More subtle temporal behaviours, such as period doubling, quasi-periodicity and chaos, are also identified and mapped. Across all of these regimes, the amplitudes of vibration and the magnitude of the fluid forces are quantified. The modes identified span the parameter space between two important limiting cases: two static bodies at varying lateral offset; and two elastically mounted bodies in a tandem configuration at varying spring stiffnesses. Some similarity in the response of extremely stiff or static bodies and extremely slack bodies is shown. This is explained by the fact that the slack bodies are free to move to an equilibrium position and stop, effectively becoming a static system. However, the most complex behaviour appears between these limits, when the bodies are in reasonably close proximity, and the natural structural frequency is close to the vortex shedding frequency of a single cylinder. This appears to be driven by the interplay between a series of time scales, including the vortex formation time, the advection time across the gap between the cylinders and the oscillation period of both bodies. This points out an important difference between this multi-body system and the classic single-cylinder vortex-induced vibration: two bodies in close proximity will not oscillate in a synchronised, periodic manner when their natural structural frequencies are close to the nominal vortex shedding frequency of a single cylinder.
Experimental investigation of flow-induced vibration of a rotating circular cylinder
- K. W. L. Wong, J. Zhao, D. Lo Jacono, M. C. Thompson, J. Sheridan
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- Journal:
- Journal of Fluid Mechanics / Volume 829 / 25 October 2017
- Published online by Cambridge University Press:
- 21 September 2017, pp. 486-511
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While flow-induced vibration of bluff bodies has been extensively studied over the last half-century, only limited attention has been given to flow-induced vibration of elastically mounted rotating cylinders. Since recent low-Reynolds-number numerical work suggests that rotation can enhance or suppress the natural oscillatory response, the former could find applications in energy harvesting and the latter in vibration control. The present experimental investigation characterises the dynamic response and wake structure of a rotating circular cylinder undergoing vortex-induced vibration at a low mass ratio ($m^{\ast }=5.78$) over the reduced velocity range leading to strong oscillations. The experiments were conducted in a free-surface water channel with the cylinder vertically mounted and attached to a motor that provided constant rotation. Springs and an air-bearing system allow the cylinder to undertake low-damped transverse oscillations. Under cylinder rotation, the normalised frequency response was found to be comparable to that of a freely vibrating non-rotating cylinder. At reduced velocities consistent with the upper branch of a non-rotating transversely oscillating cylinder, the maximum oscillation amplitude increased with non-dimensional rotation rate up to $\unicode[STIX]{x1D6FC}\approx 2$. Beyond this, there was a sharp decrease in amplitude. Notably, this critical value corresponds approximately to the rotation rate at which vortex shedding ceases for a non-oscillating rotating cylinder. Remarkably, at $\unicode[STIX]{x1D6FC}=2$ there was approximately an 80 % increase in the peak amplitude response compared to that of a non-rotating cylinder. The observed amplitude response measured over the Reynolds-number range of ($1100\lesssim Re\lesssim 6300$) is significantly different from numerical predictions and other experimental results recorded at significantly lower Reynolds numbers.
Control of confined vortex breakdown with partial rotating lids
- L. Mununga, D. Lo Jacono, J. N. Sørensen, T. Leweke, M. C. Thompson, K. Hourigan
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- Journal:
- Journal of Fluid Mechanics / Volume 738 / 10 January 2014
- Published online by Cambridge University Press:
- 29 November 2013, pp. 5-33
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Experiments were conducted to determine the effectiveness of controlling vortex breakdown in a confined cylindrical vessel using a small rotating disk, which was flush-mounted into the opposite endwall to the rotating endwall driving the primary recirculating flow. The results show that the control disk, with relatively little power input, can modify the azimuthal and axial flow significantly, changing the entire flow structure in the cylinder. Co-rotation was found to precipitate vortex breakdown onset whereas counter-rotation delays it. Furthermore, for the Reynolds-number range over which breakdown normally exists, co-rotation increases the bubble radial and axial dimensions, while shifting the bubble in the upstream direction. By contrast, counter-rotation tends to reduce the size of the bubble, or completely suppress it, while shifting the bubble in the downstream direction. These effects are amplified substantially by the use of larger control disks and higher rotation ratios. A series of numerical simulations close to the onset Reynolds number reveals that the control disk acts to generate a rotation-rate-invariant local positive or negative azimuthal vorticity source away from the immediate vicinity of the control disk but upstream of breakdown. Advection of this source along streamlines modifies the strength of the azimuthal vorticity ring, which effectively controls whether the flow reverses on the axis, and thus, in turn, whether vortex breakdown occurs. The vorticity source generated by the control disk scales approximately linearly with rotation ratio and cubically with disk diameter; this allows the observed variation of the critical Reynolds number to be approximately predicted.