Tonal and Silent Wake Modes of a Square Rod at Incidence

2016 ◽  
Vol 102 (3) ◽  
pp. 419-422 ◽  
Author(s):  
J. F. Dorneanu ◽  
A. Mueller ◽  
P. Rambaud ◽  
E. T. A. v. d. Weide ◽  
A. Hirschberg
Keyword(s):  
Wake Modes

scholarly journals Wake modes behind a streamwisely oscillating cylinder at constant and ramping frequencies

2019 ◽  
Vol 22 (3) ◽  
pp. 505-527
Author(s):  
Zhanwei Hu ◽  
Jinsheng Liu ◽  
Lian Gan ◽  
Shengjin Xu
Keyword(s):  
Oscillating Cylinder ◽  
Wake Modes

Cylinder Wake Dynamics in the Presence of Stream-Wise Harmonic Forcing

2006 ◽  
Author(s):  
M. Rodriguez ◽  
N. W. Mureithi
Keyword(s):  
Perturbation Parameter ◽  
Wake Flow ◽  
Forced Oscillations ◽  
Vortex Wake ◽  
Cylinder Wake ◽  
Drag Forces ◽  
Wake Patterns ◽  
Spatio Temporal ◽  
Wake Modes ◽  
Lift And Drag

The vortex wake flow dynamics downstream of a cylinder undergoing streamwise harmonic (fe/fs=1) forced oscillations has been investigated numerically using a CFD code for Re=1000. The steady-state of the wake flow has been analysed considering the amplitude of oscillations as a perturbation parameter. The resulting dynamics of the fluid lift and drag forces acting on the cylinder have been linked to the different vortex wake modes observed downstream of the cylinder. Forced oscillations lead to periodic, quasi-periodic and chaotic responses depending on the amplitude of oscillation of the cylinder. Different vortex wake patterns or modes (including 2S, P+S and S modes) have also been identified and described. Symmetry related bifurcations both in the computed fluid force dynamics as well as in the vortex wake patterns were identified. The key role played by spatio-temporal symmetry in the interaction between the wake flow and the oscillating cylinder has been elucidated by a Proper Orthogonal Decomposition (POD) of the wake velocity field. Symmetric and antisymmetric spatio-temporal modes were identified and bifurcations in the wake flow were explained in terms of mode interactions in the wake.

Modes of vortex formation and frequency response of a freely vibrating cylinder

2000 ◽  
Vol 420 ◽  
pp. 85-130 ◽  
Author(s):  
R. GOVARDHAN ◽  
C. H. K. WILLIAMSON
Keyword(s):  
Natural Frequency ◽  
Vibration Frequency ◽  
Critical Mass ◽  
Flow Speed ◽  
Total Force ◽  
Mode Transition ◽  
Lower Branch ◽  
Lower Amplitude ◽  
Total Phase ◽  
Wake Modes

In this paper, we study the transverse vortex-induced vibrations of an elastically mounted rigid cylinder in a fluid flow. We use simultaneous force, displacement and vorticity measurements (using DPIV) for the first time in free vibrations. There exist two distinct types of response in such systems, depending on whether one has a high or low combined mass–damping parameter (m*ζ). In the classical high-(m*ζ) case, an ‘initial’ and ‘lower’ amplitude branch are separated by a discontinuous mode transition, whereas in the case of low (m*ζ), a further higher-amplitude ‘upper’ branch of response appears, and there exist two mode transitions.To understand the existence of more than one mode transition for low (m*ζ), we employ two distinct formulations of the equation of motion, one of which uses the ‘total force’, while the other uses the ‘vortex force’, which is related only to the dynamics of vorticity. The first mode transition involves a jump in ‘vortex phase’ (between vortex force and displacement), ϕvortex, at which point the frequency of oscillation (f) passes through the natural frequency of the system in the fluid, f ∼ fNwater. This transition is associated with a jump between 2S [harr ] 2P vortex wake modes, and a corresponding switch in vortex shedding timing. Across the second mode transition, there is a jump in ‘total phase’, phis;total , at which point f ∼ fNvacuum. In this case, there is no jump in ϕvortex, since both branches are associated with the 2P mode, and there is therefore no switch in timing of shedding, contrary to previous assumptions. Interestingly, for the high-(m*ζ) case, the vibration frequency jumps across both fNwater and fNvacuum, corresponding to the simultaneous jumps in ϕvortex and ϕtotal. This causes a switch in the timing of shedding, coincident with the ‘total phase’ jump, in agreement with previous assumptions.For large mass ratios, m* = O(100), the vibration frequency for synchronization lies close to the natural frequency (f* = f/fN ≈ 1.0), but as mass is reduced to m* = O(1), f* can reach remarkably large values. We deduce an expression for the frequency of the lower-branch vibration, as follows:formula herewhich agrees very well with a wide set of experimental data. This frequency equation uncovers the existence of a critical mass ratio, where the frequency f* becomes large: m*crit = 0.54. When m* < m*crit, the lower branch can never be reached and it ceases to exist. The upper-branch large-amplitude vibrations persist for all velocities, no matter how high, and the frequency increases indefinitely with flow velocity. Experiments at m* < m*crit show that the upper-branch vibrations continue to the limits (in flow speed) of our facility.

Vortex shedding and heat transfer in rotationally oscillating cylinders

2014 ◽  
Vol 748 ◽  
pp. 549-579 ◽  
Author(s):  
Prabu Sellappan ◽  
Tait Pottebaum
Keyword(s):  
Heat Transfer ◽  
Convective Heat Transfer ◽  
Heat Transfer Rate ◽  
Convective Heat ◽  
Parameter Space ◽  
Vortex Shedding ◽  
Transfer Rate ◽  
Oscillation Amplitude ◽  
Tangential Velocity ◽  
Wake Modes

AbstractWake formation and heat transfer from a rotationally oscillating circular cylinder in cross-flow at $\mathit{Re}= 750$ are studied. Two aspects, the effect of cylinder forcing on vortex shedding and the effect of the wake structures on convective heat transfer, are studied. Cylinder forcing conditions range between $0.09 \leq \theta _{PP} \leq 2.09$, where $\theta _{PP}$ is the peak-to-peak oscillation amplitude in radians and $0.70 \leq F_{R} \leq 3.16$, where $F_{R}$ is the ratio of forcing frequency to natural shedding frequency. Digital particle image velocimetry (DPIV) is used to obtain quantitative wake structure information. Wake modes, and regions of the parameter space in which they occur, are identified for both heated and unheated cylinders. For the heated cylinder, cylinder forcing is found to affect the convective heat-transfer rate. Certain wake modes, including newly discovered wake modes synchronized over multiple oscillation cycles, are found to correlate with significant heat-transfer enhancement. Cylinder tangential velocity is also found to affect the heat-transfer rate in certain regions of the parameter space.

The effect of Reynolds number on the dynamics and wakes of freely rising and falling spheres

2010 ◽  
Vol 651 ◽  
pp. 251-294 ◽  
Author(s):  
M. HOROWITZ ◽  
C. H. K. WILLIAMSON
Keyword(s):  
Reynolds Number ◽  
Critical Mass ◽  
Vortex Formation ◽  
Vertical Plane ◽  
Vortex Rings ◽  
Mass Ratio ◽  
Periodic Sequences ◽  
Wide Range ◽  
Falling Spheres ◽  
Wake Modes

In this paper, we study the effect of the Reynolds number (Re) on the dynamics and vortex formation modes of spheres rising or falling freely through a fluid (where Re = 100–15000). Since the oscillation of freely falling spheres was first reported by Newton (University of California Press, 3rd edn, 1726, translated in 1999), the fundamental question of whether a sphere will vibrate, as it rises or falls, has been the subject of a number of investigations, and it is clear that the mass ratio m* (defined as the relative density of the sphere compared to the fluid) is an important parameter to define when vibration occurs. Although all rising spheres (m* < 1) were previously found to oscillate, either chaotically or in a periodic zigzag motion or even to follow helical trajectories, there is no consensus regarding precise values of the mass ratio (m*crit) separating vibrating and rectilinear regimes. There is also a large scatter in measurements of sphere drag in both the vibrating and rectilinear regimes.In our experiments, we employ spheres with 133 combinations of m* and Re, to provide a comprehensive study of the sphere dynamics and vortex wakes occurring over a wide range of Reynolds numbers. We find that falling spheres (m* > 1) always move without vibration. However, in contrast with previous studies, we discover that a whole regime of buoyant spheres rise through a fluid without vibration. It is only when one passes below a critical value of the mass ratio, that the sphere suddenly begins to vibrate periodically and vigorously in a zigzag trajectory within a vertical plane. The critical mass is nearly constant over two ranges of Reynolds number (m*crit ≈ 0.4 for Re = 260–1550 and m*crit ≈ 0.6 for Re > 1550). We do not observe helical or spiral trajectories, or indeed chaotic types of trajectory, unless the experiments are conducted in disturbed background fluid. The wakes for spheres moving rectilinearly are comparable with wakes of non-vibrating spheres. We find that these wakes comprise single-sided and double-sided periodic sequences of vortex rings, which we define as the ‘R’ and ‘2R’ modes. However, in the zigzag regime, we discover a new ‘4R’ mode, in which four vortex rings are created per cycle of oscillation. We find a number of changes to occur at a Reynolds number of 1550, and we suggest the possibility of a resonance between the shear layer instability and the vortex shedding (loop) instability. From this study, ensuring minimal background disturbances, we have been able to present a new regime map of dynamics and vortex wake modes as a function of the mass ratio and Reynolds number {m*, Re}, as well as a reasonable collapse of the drag measurements, as a function of Re, onto principally two curves, one for the vibrating regime and one for the rectilinear trajectories.

Effects of Axis Ratio on the Vortex-Induced Vibration and Energy Harvesting of Rhombus Cylinder

2014 ◽  
Author(s):  
Li Zhang ◽  
Heng Li ◽  
Lin Ding
Keyword(s):  
Wide Band ◽  
Reynolds Numbers ◽  
Mechanical Power ◽  
Navier Stokes ◽  
Axis Ratio ◽  
Vortex Induced Vibrations ◽  
Mechanical Power Output ◽  
Wake Patterns ◽  
Wake Modes ◽  
High Reynolds

The vortex-induced vibrations of a rhombus cylinder are investigated using two-dimensional unsteady Reynolds-Averaged Navier-Stokes simulations at high Reynolds numbers ranging from 10,000 to 120,000. The rhombus cylinder is constrained to oscillate in the transverse direction, which is perpendicular to the flow velocity direction. Three rhombus cylinders with different axis ratio (AR=0.5, 1.0, 1.5) are considered for comparison. The simulation results indicate that the vibration response and the wake modes are dependent on the axis ratio of the rhombus cylinder. The amplitude ratios are functions of the Reynolds numbers. And as the AR increases, higher peak amplitudes can be made over a significant wide band of Re. On the other hand, a narrow lock-in area is observed for AR=0.5 and AR=1.5 when 30,000<Re<50,000, but the frequency ratio of AR=1.0 monotonically increases at a nearly constant slope in the whole Re range. The vortex shedding mode is always 2S mode in the whole Re range for AR=0.5. However, the wake patterns become diverse with the increasing of Re for AR=1.0 and 1.5. In addition, the mechanical power output of each oscillating rhombus cylinder is calculated to evaluate the efficiency of energy transfer in this paper. The theoretical mechanical power P between water and a transversely oscillating cylinder is achieved. On the base of analysis and comparison, the rhombus cylinder with AR=1.0 is more suitable for harvesting energy from fluid.

scholarly journals A parametric study of energy extraction from vortex-induced vibrations

2018 ◽  
Vol 42 (4) ◽  
pp. 359-369
Author(s):  
Olivier Paré-Lambert ◽  
Mathieu Olivier
Keyword(s):  
Reynolds Number ◽  
Vortex Shedding ◽  
Maximum Efficiency ◽  
Mass Ratio ◽  
Parametric Space ◽  
Turbine Performance ◽  
Vortex Induced Vibrations ◽  
Mass Ratios ◽  
Low Mass ◽  
Wake Modes

This paper presents a parametric investigation of an oscillating-cylinder turbine concept based on vortex-induced vibrations. The parametric space includes four parameters: the Reynolds number, the mass ratio, the dimensionless stiffness, and the dimensionless damping. The damping–stiffness space is explored for four different mass ratios at a fixed Reynolds number of 200. Also, the influence of the parameters on the amplitude of cylinder displacement and on the efficiency of power harnessing is discussed. Vortex-shedding patterns observed within the parametric space are investigated. The 2S, 2P, and C(2S) wake modes are observed and are related to turbine performance. Preliminary results show a maximum efficiency of 10.6%, which is obtained with low mass ratios.

Dynamics of Flow Around a Cylinder Oscillating In-Line for Low Reynolds Numbers

2010 ◽  
Author(s):  
L. Baranyi ◽  
K. Huynh ◽  
N. W. Mureithi
Keyword(s):  
Reynolds Number ◽  
Oscillation Amplitude ◽  
Wake Flow ◽  
Computational Domain ◽  
Cylinder Wake ◽  
Mean Values ◽  
Reynolds Number Flow ◽  
Lower Amplitude ◽  
Low Reynolds ◽  
Wake Modes

This study builds on an earlier study of low-Reynolds number flow about a cylinder forced to oscillate in-line with the main flow, which found vortex switches at some oscillation amplitude values. Here we extend the Reynolds number domain to Re = 60–350, utilize a computational domain characterized by R2/R1 = 360, and do computations at two frequency ratios of f/St0 = 0.8 and 0.9. Computations were carried out using a thoroughly tested finite-difference code. Some results were compared with those obtained by Ansys CFX, and good agreement was found. When plotted against oscillation amplitude, rms and time-mean values of force coefficients revealed a shift toward lower amplitude with higher Re. Findings for the effect of frequency ratio are similar. Where vortex switches occurred, a pre- and post-jump analysis is carried out. POD analysis of the cylinder wake flow field is employed to reveal the detailed wake dynamics as the forcing parameters are varied. The analysis provides further details on the transition of the dominant wake modes in response to the symmetry breaking bifurcation underlying the vortex switches observed in the simulations.

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