The Experts below are selected from a list of 324 Experts worldwide ranked by ideXlab platform
Stavroula Balabani - One of the best experts on this subject based on the ideXlab platform.
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wake modes of a Cylinder undergoing free streamwise vortex induced vibrations
Journal of Fluids and Structures, 2013Co-Authors: Neil Cagney, Stavroula BalabaniAbstract:Abstract Simultaneous measurements of the response of a circular Cylinder experiencing vortex-induced vibrations (VIVs) in the streamwise direction and the resulting wake field were performed for a range of reduced velocities using time-resolved Particle-Image Velocimetry in the Reynolds number range 450–3700. The dominant vortex shedding mode was identified using phase-averaged vorticity fields. The Cylinder response amplitude was characterised by two response branches, separated by a low amplitude region at resonance, as has been previously reported in the literature. During the first response branch the wake exhibited not only the symmetric S-I mode, but also the alternate A-II mode at slightly higher reduced velocities. For both modes, the vortices were observed to be shed at the Cylinder response frequency, but rearranged downstream into a more stable structure in which the velocity fluctuations were no longer synchronised to the Cylinder Motion. A special case of the A-II mode, referred to as the SA mode, was found to dominate in the second response branch and the low amplitude region, while the far wake and the Cylinder Motion were synchronised (lock-in). A change in the timing of the vortex shedding with respect to the Cylinder Motion was observed between the low amplitude region and the second response branch. This is likely to correspond to a change in the fluid forcing and levels of excitation, and may explain the variation in the Cylinder amplitude observed in this region. Lock-in and the second response branch were found to coincide with a contraction of the wake and an increase in strength of the shed vortices. This work reveals the inherent differences between the extensively studied case of transverse-only VIV and the streamwise-only case, which is crucial if the wealth of information available on transverse VIV is to be extended to the more practical two degree-of-freedom case.
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Mode competition in streamwise-only vortex induced vibrations
Journal of Fluids and Structures, 2013Co-Authors: Neil Cagney, Stavroula BalabaniAbstract:Time-resolved Particle-Image Velocimetry (PIV) has been used to study mode competition and transient behaviour in the wake of a Cylinder experiencing Vortex-Induced Vibrations (VIV) in the streamwise direction. The Cylinder response regime contained two branches, occurring above and below the onset of synchronisation between the wake and the Cylinder Motion (lock-in). During the first branch, the wake exhibited both the S-I mode (in which two vortices are shed simultaneously per vibration cycle) and the alternate A-II mode (similar to the well known von Karman vortex street). An extended PIV data set acquired in this region revealed mode switching between the S-I and A-II modes. A criterion based on Proper-Orthogonal Decomposition was developed to identify which mode was dominant as a function of time. The A-II mode was found to be dominant for over 90% of the instantaneous fields examined, while the S-I mode appeared to be more unstable. Symmetrically shed vortices were found to rearrange downstream into an alternate structure in which the wake was no longer synchronised to the Cylinder Motion. The dominant frequency of transverse velocity fluctuations was measured throughout the wake in order to study the effects of this breakdown in more detail. For the majority of the wake, the fluctuations occurred at the Strouhal frequency, while in a region in the near wake the fluctuations occurred at the frequency of the Cylinder Motion. It is thought that during the first response branch vortices are formed at the Cylinder response frequency, but tend to quickly rearrange downstream into an alternate structure which is no longer synchronised to the Cylinder Motion. As a result, the fluctuating drag will be synchronised to the structural Motion, and is capable of providing positive energy transfer in the apparent absence of lock-in. Finally, the spatial dependence of the frequency of velocity fluctuations throughout the wake is used to explain some of the conflicting results in the literature regarding streamwise VIV, and the implications for the general study of VIV are discussed.
Neil Cagney - One of the best experts on this subject based on the ideXlab platform.
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wake modes of a Cylinder undergoing free streamwise vortex induced vibrations
Journal of Fluids and Structures, 2013Co-Authors: Neil Cagney, Stavroula BalabaniAbstract:Abstract Simultaneous measurements of the response of a circular Cylinder experiencing vortex-induced vibrations (VIVs) in the streamwise direction and the resulting wake field were performed for a range of reduced velocities using time-resolved Particle-Image Velocimetry in the Reynolds number range 450–3700. The dominant vortex shedding mode was identified using phase-averaged vorticity fields. The Cylinder response amplitude was characterised by two response branches, separated by a low amplitude region at resonance, as has been previously reported in the literature. During the first response branch the wake exhibited not only the symmetric S-I mode, but also the alternate A-II mode at slightly higher reduced velocities. For both modes, the vortices were observed to be shed at the Cylinder response frequency, but rearranged downstream into a more stable structure in which the velocity fluctuations were no longer synchronised to the Cylinder Motion. A special case of the A-II mode, referred to as the SA mode, was found to dominate in the second response branch and the low amplitude region, while the far wake and the Cylinder Motion were synchronised (lock-in). A change in the timing of the vortex shedding with respect to the Cylinder Motion was observed between the low amplitude region and the second response branch. This is likely to correspond to a change in the fluid forcing and levels of excitation, and may explain the variation in the Cylinder amplitude observed in this region. Lock-in and the second response branch were found to coincide with a contraction of the wake and an increase in strength of the shed vortices. This work reveals the inherent differences between the extensively studied case of transverse-only VIV and the streamwise-only case, which is crucial if the wealth of information available on transverse VIV is to be extended to the more practical two degree-of-freedom case.
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Mode competition in streamwise-only vortex induced vibrations
Journal of Fluids and Structures, 2013Co-Authors: Neil Cagney, Stavroula BalabaniAbstract:Time-resolved Particle-Image Velocimetry (PIV) has been used to study mode competition and transient behaviour in the wake of a Cylinder experiencing Vortex-Induced Vibrations (VIV) in the streamwise direction. The Cylinder response regime contained two branches, occurring above and below the onset of synchronisation between the wake and the Cylinder Motion (lock-in). During the first branch, the wake exhibited both the S-I mode (in which two vortices are shed simultaneously per vibration cycle) and the alternate A-II mode (similar to the well known von Karman vortex street). An extended PIV data set acquired in this region revealed mode switching between the S-I and A-II modes. A criterion based on Proper-Orthogonal Decomposition was developed to identify which mode was dominant as a function of time. The A-II mode was found to be dominant for over 90% of the instantaneous fields examined, while the S-I mode appeared to be more unstable. Symmetrically shed vortices were found to rearrange downstream into an alternate structure in which the wake was no longer synchronised to the Cylinder Motion. The dominant frequency of transverse velocity fluctuations was measured throughout the wake in order to study the effects of this breakdown in more detail. For the majority of the wake, the fluctuations occurred at the Strouhal frequency, while in a region in the near wake the fluctuations occurred at the frequency of the Cylinder Motion. It is thought that during the first response branch vortices are formed at the Cylinder response frequency, but tend to quickly rearrange downstream into an alternate structure which is no longer synchronised to the Cylinder Motion. As a result, the fluctuating drag will be synchronised to the structural Motion, and is capable of providing positive energy transfer in the apparent absence of lock-in. Finally, the spatial dependence of the frequency of velocity fluctuations throughout the wake is used to explain some of the conflicting results in the literature regarding streamwise VIV, and the implications for the general study of VIV are discussed.
Wolfgang Rodi - One of the best experts on this subject based on the ideXlab platform.
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NUMERICAL INVESTIGATION OF LAMINAR VORTEX-SHEDDING FLOW PAST A SQUARE Cylinder OSCILLATING IN LINE WITH THE MEAN FLOW
Journal of Fluids and Structures, 1994Co-Authors: S. Minewitsch, R. Franke, Wolfgang RodiAbstract:Abstract Numerical calculations of the flow around a square Cylinder (prism) oscillating in line with the mean flow are reported for a Reynolds number Re = 200. The unsteady forces on the Cylinder are evaluated dependent on the frequency and amplitude of the Cylinder Motion. The interaction of vortex shedding and forced Cylinder Motion is examined, in particular the occurrence of the lock-in phenomenon. The numerical results lead to the postulation of three regimes for antisymmetric vortex shedding, which are discussed in some detail for exemplary cases.
Zhihua Chen - One of the best experts on this subject based on the ideXlab platform.
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Suppression mechanism of two-degree-of-freedom vortex-induced vibration by Lorentz forces in the uniform flow
Computers & Fluids, 2017Co-Authors: Hui Zhang, Meng-ke Liu, Yang Han, Ming-yue Gui, Zhihua ChenAbstract:Abstract In this paper, the control of 2DOF VIV (two-degree-of-freedom vortex-induced vibration) by Lorentz force has been investigated numerically based on the derivation of stream function-vorticity equations together with the initial and boundary conditions in exponential-polar coordinates attached on a moving Cylinder, hydrodynamics forces and the Cylinder Motion equation. From the derivations of force components, the lift/drag induced by the inertial force only depend on the Motion along the corresponding direction, while the lift/drag induced by flow field is affected by the Cylinder Motion along the two directions. Based on the calculation results, the displacement variation of 2DOF VIV along the transverse direction is similar with that of 1DOF VIV (one-degree-of-freedom vortex-induced vibration). However, the secondary vortex is strengthened with the effect of the pressure side and weakened with the effect of the suction side. With the application of symmetrical Lorentz force, the effects of the pressure/suction side and vortex shedding are weakened, which lead to the suppression of 2DOF VIV. Moreover, the Cylinder vibration is fully suppressed and the drag is negative due to the net thrust generated if Lorentz force is large enough, which means the final position of Cylinder is at the upstream of the initial position. Furthermore, the fluid-structure interactions from the quiescent Cylinder to the steady vibration and then vibration control by Lorentz force are investigated. The shear layers and secondary vortexes grow with the increase of Cylinder amplitude and decay with the decrease of Cylinder amplitude due to the suppression of Cylinder vibration controlled by Lorentz force in the whole process.
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Numerical study of the suppression mechanism of vortex-induced vibration by symmetric Lorentz forces
Journal of Fluids and Structures, 2014Co-Authors: Hui Zhang, Baochun Fan, Zhihua ChenAbstract:Abstract In this paper, the electro-magnetic control of vortex-induced vibration (VIV) of a circular Cylinder is investigated numerically based on the stream function–vorticity equations in the exponential–polar coordinates attached on the moving Cylinder for Re=150. The effects of the instantaneous wake geometries and the corresponding Cylinder Motion on the hydrodynamic forces for one entire period of vortex shedding are discussed using a drag–lift phase diagram. The drag–lift diagram is composed of the upper and lower closed curves due to the contributions of the vortex shedding but is magnified, translated and turned under the action of the Cylinder Motion. The Lorentz force for controlling the vibration Cylinder is classified into the field Lorentz force and the wall Lorentz force. The symmetric field Lorentz force will symmetrize the flow passing over the Cylinder and decreases the lift oscillation, which, in turn, suppresses the VIV, whereas the wall Lorentz force has no effect on the lift. The Cylinder vibration increases as the work performed by the lift dominates the energy transfer. Otherwise, the Cylinder vibration decreases. If the net transferred energy per Motion is equal to zero, the Cylinder will vibrate steadily or be fixed.
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An in-depth study on vortex-induced vibration of a circular Cylinder with shear flow
Computers & Fluids, 2014Co-Authors: Hui Zhang, Baochun Fan, Zhihua ChenAbstract:Abstract To investigate the in-depth mechanism of vortex-induced vibration of a circular Cylinder with shear flow, in this paper, with the use of exponential-polar coordinate attached on the moving Cylinder, the stream function-vorticity equations of vortex-induced vibration, the initial/boundary conditions and distribution of hydrodynamic force together with Cylinder Motion equation in shear flow are deduced, the hydrodynamic force consists of inertial force, the vortex-induced force and viscous damping force. Similarly, the Cylinder Motion equation with virtual mass is induced where the virtual mass consists of the Cylinder mass, the potential added mass and the apparent added mass induced by viscosity. Our numerical results revealed that there are three factors affecting fluid-structure interactions from the fixed Cylinder to its steady vibration: The first is the vortex shedding where one side shear layer of Cylinder strengthens with the effect of the dominated vortex. The second is the vibration of Cylinder which pushes the fluid on the pressure side and pumps that on the suction side. The third is the vortexes strengthen in one side and weaken in the other side together with the shift of front stagnation point with the effect of background vortex which is generated by shear flow. The character of vortex-induced vibration in shear flow are affected by the above three factors.
S. Minewitsch - One of the best experts on this subject based on the ideXlab platform.
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NUMERICAL INVESTIGATION OF LAMINAR VORTEX-SHEDDING FLOW PAST A SQUARE Cylinder OSCILLATING IN LINE WITH THE MEAN FLOW
Journal of Fluids and Structures, 1994Co-Authors: S. Minewitsch, R. Franke, Wolfgang RodiAbstract:Abstract Numerical calculations of the flow around a square Cylinder (prism) oscillating in line with the mean flow are reported for a Reynolds number Re = 200. The unsteady forces on the Cylinder are evaluated dependent on the frequency and amplitude of the Cylinder Motion. The interaction of vortex shedding and forced Cylinder Motion is examined, in particular the occurrence of the lock-in phenomenon. The numerical results lead to the postulation of three regimes for antisymmetric vortex shedding, which are discussed in some detail for exemplary cases.
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Numerical Investigation of Square-Section Cylinder Vibration in Line With an Incident Uniform Flow
Bluff-Body Wakes Dynamics and Instabilities, 1993Co-Authors: S. MinewitschAbstract:Numerical work on the flow around an in line oscillating square Cylinder is reported. The unsteady forces on the Cylinder were calculated for the laminar two-dimensional case at Reynolds number of Re = 200. The influence of the frequency and amplitude of the Cylinder Motion was studied with respect to the occurence of the “lock-in” effect and different mechanisms of vortex shedding.
