Vortex synchronization in the cylinder wake due to harmonic and non-harmonic perturbations

2016 ◽  
Vol 804 ◽  
pp. 248-277 ◽  
Author(s):  
Efstathios Konstantinidis ◽  
Demetri Bouris
Keyword(s):  
Relative Velocity ◽  
Numerical Study ◽  
Vortex Formation ◽  
Phase Portraits ◽  
Cylinder Wake ◽  
Frequency Space ◽  
Flow Past A Cylinder ◽  
Harmonic Perturbation ◽  
Perturbation Frequency ◽  
Vortex Patterns

This paper reports a numerical study of two-dimensional periodically perturbed flow past a cylinder. Both harmonic and non-harmonic perturbation waveforms of the inflow velocity are considered for a maximum instantaneous Reynolds number of 180. Phase portraits of the lift force are employed to identify the dynamical state of the cylinder wake and determine the range of kinematical parameters for which primary synchronization occurs, that is the regime where vortex formation is phase-locked to the subharmonic of the perturbation frequency. The effect of different perturbation waveforms on the synchronization range and on patterns of vortex formation is examined in detail over the normalized amplitude–frequency space. It is shown that systematic shifts of the synchronization range, towards either higher or lower frequencies, can be attained by imposing different perturbation waveforms. As a consequence, in certain regions of the parameter space it is possible to obtain multiple periodic and/or quasi-periodic wake states for waveforms of exactly the same amplitude and frequency. For the range of parameters where synchronization occurs, different vortex patterns are attained in the wake involving the shedding of solitary and binary vortices, or mixtures thereof, which can be controlled by the perturbation waveform. The phenomenology of these patterns, which result from modification of the basic Kármán mode in the unperturbed wake, is discussed and explained in terms of the generation of circulation that is induced by perturbations in the relative velocity. Vortex patterns from cylinders oscillating either in line with or transverse to a free stream are recast in the framework of the relative velocity.

Numerical study of an oscillating cylinder in uniform flow and in the wake of an upstream cylinder

1992 ◽  
Vol 237 ◽  
pp. 457-478 ◽  
Author(s):  
Jing Li ◽  
Jiong Sun ◽  
Bernard Roux
Keyword(s):  
Numerical Study ◽  
Vortex Formation ◽  
Uniform Flow ◽  
Driving Frequency ◽  
Cylinder Wake ◽  
Oscillating Cylinder ◽  
Vortex Shedding Frequency ◽  
Shedding Frequency ◽  
Response State ◽  
Lock In

Direct numerical simulation is carried out to study the response of an oscillating cylinder in uniform flow and in the wake of an upstream cylinder. It is found that the response of the cylinder wake is either a periodic (lock-in) or a quasi-periodic (non-lock-in) state. In the lock-in state, the vortex shedding frequency equals the forcing frequency. In the non-lock-in state, the shedding frequency shows a smooth variation with the driving frequency. For a cylinder oscillating in uniform flow, a lock-in diagram of different forcing amplitude is computed. However, no clear chaotic behaviour is detected near the lock-in boundary. For a cylinder oscillating in the wake of an upstream cylinder, the response state is strongly influenced by the distance between the two cylinders. By changing cylinder spacing, two different flow regimes are identified. In the ‘vortex formation regime’, found at large spacings, the vortex street develops behind both the upstream and downstream cylinders. The strength of the naturally produced oscillation upstream of the second cylinder becomes important compared to the forced oscillation and dominates the flow, leading to a very small or even indistinguishable zone of synchronization. However, in the ‘vortex suppression regime’, observed at small spacings, the oncoming flow to the downstream cylinder becomes so weak that it hardly affects its vortex wake, and therefore a large zone of synchronization is obtained. The numerical results are in good agreement with available experimental data.

Three-dimensional vortex structures in a cylinder wake

1996 ◽  
Vol 312 ◽  
pp. 201-222 ◽  
Author(s):  
J. Wu ◽  
J. Sheridan ◽  
M. C. Welsh ◽  
K. Hourigan
Keyword(s):  
Particle Image ◽  
Vortex Formation ◽  
Three Dimensional ◽  
Longitudinal Vortex ◽  
Cylinder Wake ◽  
Vorticity Field ◽  
Longitudinal Vortices ◽  
The Past ◽  
Image Velocimetry ◽  
Vortex Patterns

The three-dimensionality of the velocity field in the wake of a circular cylinder has excited considerable interest and debate over the past decade. Presented here are experimental results that characterize the underlying vorticity field of such wakes. Using particle image velocimetry (PIV), instantaneous velocity fields were measured and from these the vorticity of the longitudinal vortices lying in the region between Kármán vortices was found. Near the saddle point, induced by the stretching of the Kármán vortices, the vorticity of the longitudinal vortices was found to be greater than the Kármán vortices themselves. Their circulation was of the order of 10% of the Kármán vortices. The high levels of vorticity result from the stretching of the longitudinal vortices, as evident in the topology of the vortices. It is shown that the longitudinal vortices are locked in phase to the KármánK vortices, effectively riding on their backs in the braid region. While only one mode of longitudinal vortex formation was observed, evidence was found of a step change in the vorticity levels at a Reynolds number of approximately 200. This is consistent with the transition point between the two modes of vortex shedding shown to exist by Williamson (1988). It had previously been proposed that the observed vortex patterns were consistent with the evolution of the longitudinal vortices from perturbations of vortex lines in the separating shear layer which experience self-induction and stretching from the Kármán vortices. Evidence is presented that supports this model.

scholarly journals Numerical Study of Flow Characteristics Around Confined Cylinder using OpenFOAM

2018 ◽  
Vol 7 (4.35) ◽  
pp. 617
Author(s):  
P. Mathupriya ◽  
L. Chan ◽  
H. Hasini ◽  
A. Ooi
Keyword(s):  
Reynolds Number ◽  
Vortex Shedding ◽  
Numerical Study ◽  
Vortex Formation ◽  
Plane Channel ◽  
Flow Characteristics ◽  
Strong Interactions ◽  
Two Dimensional ◽  
Computational Fluid Dynamics Cfd ◽  
Shedding Frequency

The numerical study of the flow over a two-dimensional cylinder which is symmetrically confined in a plane channel is presented to study the characteristics of vortex shedding. The numerical model has been established using direct numerical simulation (DNS) based on the open source computational fluid dynamics (CFD) code named OpenFOAM. In the present study, the flow fields have been computed at blockage ratio, β of 0.5 and at Reynolds number, Re of 200 and 300. Two-dimensional simulations investigated on the effects of Reynolds number based on the vortex formation and shedding frequency. It was observed that the presence of two distinct shedding frequencies appear at higher Reynolds number due to the confinement effects where there is strong interactions between boundary layer, shear layer and the wake of the cylinder. The range of simulations conducted here has shown to produce results consistent with that available in the open literature. Therefore, OpenFOAM is found to be able to accurately capture the complex physics of the flow.

A Combined Experimental and Numerical Study of Flow Past a Single Step Cylinder

2010 ◽  
Author(s):  
Chris R. Morton ◽  
Serhiy Yarusevych
Keyword(s):  
Vortex Shedding ◽  
Numerical Study ◽  
Experimental Tests ◽  
Numerical Approach ◽  
Single Step ◽  
Wake Flow ◽  
Cylinder Wake ◽  
Flow Topology ◽  
Single Vortex ◽  
Flow Past

The current study investigates flow past a step cylinder for ReD = 1050 and D/d = 2 using both experimental and numerical methods. The focus of the study is on the vortex shedding and vortex interactions occurring in the step cylinder wake. Flow visualization with hydrogen bubble technique and planar Laser Induced Fluorescence has shown that three distinct spanwise vortex cells form: a single vortex shedding cell in the wake of the small cylinder and two vortex shedding cells in the wake of the large cylinder. Vortex connections form between the spanwise vortices in these cells downstream of the step, and vortex dislocations occur at cell boundaries. Complementary to the experimental tests, an LES-RANS hybrid numerical simulation is used to model the flow development. A comparison of the experimental and numerical results indicates that the numerical approach adequately models vortex dynamics in the wake of a step cylinder and, thus, may be used to analyze time dependent, three-dimensional flow topology which is difficult to characterize quantitatively using experimental methods.

The vortex-street wakes of vibrating cylinders

1974 ◽  
Vol 66 (3) ◽  
pp. 553-576 ◽  
Author(s):  
Owen M. Griffin ◽  
Steven E. Ramberg
Keyword(s):  
Vortex Formation ◽  
The Body ◽  
Vortex Street ◽  
Longitudinal Vortex ◽  
Cylinder Wake ◽  
Hot Wire ◽  
Formation Region ◽  
Lateral Vibrations ◽  
Vorticity Generation ◽  
Shedding Frequency

The strength (initial circulation) and spacing of vortices in the wake of a circular cylinder have been obtained for conditions under which the body undergoes lateral vibrations. The vibrations of the cylinder were at all times synchronized with those in the wake, thereby suppressing the natural Strouhal frequency in favour of a common synchronized or ‘locked-in’ frequency for the body-wake system. All experiments were performed at a Reynolds number of 144 or 190. An inverse relation between the initial circulation K and the length lF of the vortex formation region was obtained for cylinder oscillations of up to 50% of a diameter, at vibration frequencies both above and below the Strouhal shedding frequency. The initial circulation K of the vortices was increased by as much as 65%, at lF = 1·6 diameters, from the stationary-cylinder value of K corresponding to lF = 3·2d. An increase in the rate A of vorticity generation of 80% from the stationary-cylinder wake value was obtained with the cylinder vibrating at 30% of a diameter and 110% of the Strouhal frequency. Both flow-visualization and hot-wire results show that the lateral spacing of the vortex street decreases as the vibration amplitude of the cylinder is increased, but that the longitudinal vortex spacing is independent of changes in amplitude. The longitudinal spacing, however, varies inversely with the vibration frequency. The street approaches a single line of vortices of alternating sign as the amplitude of vibration approaches values near a full cylinder diameter, and secondary vortex formation at these large amplitudes is associated with the vanishing lateral spacing of the street. Observation of the wake has elucidated the mechanism of vortex formation; the entrainment processes in the formation region have been observed at small intervals over a cycle of the cylinder's motion.

Lock-On Vortex Shedding Patterns and Bifurcation Analysis of the Forced Streamwise Oscillation of the Cylinder Wake

2015 ◽  
Vol 25 (09) ◽  
pp. 1530022
Author(s):  
N. Nabatian ◽  
N. W. Mureithi
Keyword(s):  
Frequency Ratio ◽  
Transverse Velocity ◽  
Lift Coefficient ◽  
Oscillation Amplitude ◽  
Vortex Formation ◽  
Excitation Frequency ◽  
Immersed Boundary ◽  
Cylinder Wake ◽  
Single Vortex ◽  
Frequency Ratios

The two-dimensional numerical simulation of the flow over a cylinder forced to oscillate in the streamwise direction for Re = 200 is performed in CFX ANSYS. The controlled-vibration comprises of prescribed inline vibration from displacement amplitude-to-cylinder diameter A/D = 0.05 up to 0.5 with the excitation frequency ratios of 1, 1.5 and 2 including the harmonic and superharmonic excitation regions. The immersed boundary method is used to model the cylinder oscillation. Modal decomposition of the transverse velocity field via the proper orthogonal decomposition (POD) method is applied to uncover the interaction of symmetric and antisymmetric modes of the near wake. A model using the first two POD modes is developed based on symmetry group equivariance. The model predicts the mode interactions and bifurcated solution branches for all cases, and is shown to be in good agreement with numerical as well as previous experimental results. Lock-on is determined for a range of values of the oscillation amplitudes and frequency ratios. It is shown that the lock-on range widens with increasing nondimensional oscillation amplitude. The asymmetric 2S, P + S and symmetric pattern S with symbol S for a single vortex and P for a vortex pair shed per cycle, as well as a regime in which vortex formation is not synchronized with cylinder motion are observed in the cylinder wake depending on the combination of oscillation amplitude and frequency ratio. The frequency ratio variation from 1 to 2 leads to the switching from asymmetric to symmetric modes. The symmetric flow pattern corresponds to a near zero lift coefficient on the cylinder.

scholarly journals Upstream vortex and elastic wave in the viscoelastic flow around a confined cylinder

2019 ◽  
Vol 864 ◽  
Author(s):  
Boyang Qin ◽  
Paul F. Salipante ◽  
Steven D. Hudson ◽  
Paulo E. Arratia
Keyword(s):  
Elastic Wave ◽  
Wave Speed ◽  
Three Dimensional ◽  
Weissenberg Number ◽  
Viscoelastic Flow ◽  
Absolute Instability ◽  
Elastic Instability ◽  
Cylinder Wake ◽  
Cylinder Axis ◽  
Flow Past A Cylinder

Viscoelastic flow past a cylinder is a classic benchmark problem that is not completely understood. Using novel three-dimensional (3D) holographic particle velocimetry, we report three main discoveries of the elastic instability upstream of a single cylinder in viscoelastic channel flow. First, we observe that upstream vortices initiate at the corner between the cylinder and the wall, and grow with increasing flow rate. Second, beyond a critical Weissenberg number, the flow upstream becomes unsteady and switches between two bistable configurations, leading to symmetry breaking in the cylinder axis direction that is highly 3D in nature. Lastly, we find that the disturbance of the elastic instability propagates relatively far upstream via an elastic wave, and is weakly correlated with that in the cylinder wake. The wave speed and the extent of the instability increase with Weissenberg number, indicating an absolute instability in viscoelastic fluids.

Friction-driven vibro-impact system for percussive-rotary drilling: A numerical study of the system dynamics

2008 ◽  
Vol 222 (10) ◽  
pp. 1925-1934 ◽  
Author(s):  
A D L Batako ◽  
P T Piiroinen
Keyword(s):  
Detrimental Effect ◽  
Numerical Study ◽  
Drill String ◽  
Phase Portraits ◽  
Rotary Drilling ◽  
Stick Slip ◽  
Impact System ◽  
Drilling System ◽  
Numerical Bifurcation ◽  
Further Development

Stick—slip-induced vibration in drilling has a detrimental effect on the drilling system and may lead to the failure of the drill string. This study is a further development of a friction-driven vibro-impact system which was investigated previously. The system used the stick—slip properties to generate a vibratory motion of a hammer that collides with the bit. The previous study focused on the influence of the friction on the response of the system without impacts. This paper investigates the full dynamic response of the model including friction and impact. Numerical bifurcation analysis of the system is undertaken to establish various motions and dynamical changes. This study focuses on the system performance outside the stable interval identified in the earlier investigation. The response of the system is illustrated along with the phase portraits.

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