Near-wake structure of an oscillating cylinder: effect of controlled shear-layer vortices

1996 ◽  
Vol 322 ◽  
pp. 21-49 ◽  
Author(s):  
C. K. Chyu ◽  
D. Rockwell
Keyword(s):  
Shear Layer ◽  
Vortex Formation ◽  
Small Scale ◽  
Asymptotic State ◽  
Near Wake ◽  
Wake Structure ◽  
Image Velocimetry ◽  
Image Density ◽  
Karman Vortex ◽  
Limiting Case

The instantaneous structure of the near wake of a cylinder subjected to small-amplitude perturbations is characterized using high-image-density particle image velocimetry. Emphasis is on control of the small-scale shear-layer vortices, which feed into the Kármán vortices. Modifications of the Kármán vortex formation are classified according to patterns of modulated and locked-on shear-layer vortices. The formation length of the Kármán vortices can be dramatically shortened and, in the limiting case, occur adjacent to the base of the cylinder when it is perturbed at the inherent instability frequency of the shear layer and its subharmonics. Moreover, the induced shear-layer vortices can lead to large-amplitude transverse undulations of the entire near-wake region during formation of the Kármán vortices.These variations of the near-wake structure are further elucidated by considering the transient response of the wake, induced by abrupt cessation and onset of periodic motion of the cylinder. Distinctive intermediate states of the wake arise during relaxation to its asymptotic state; such relaxation requires a very large number of periods of the inherent instability of the shear layer.

Effects of a geometrical surface disturbance on flow past a circular cylinder: a large-scale spanwise wire

2010 ◽  
Vol 665 ◽  
pp. 120-157 ◽  
Author(s):  
A. EKMEKCI ◽  
D. ROCKWELL
Keyword(s):  
Circular Cylinder ◽  
Shear Layer ◽  
Large Scale ◽  
Angular Position ◽  
Near Wake ◽  
Surface Disturbance ◽  
Significant Extension ◽  
Image Density ◽  
Karman Vortex ◽  
The Wire

Flow control induced by a single wire that is attached on the outer surface and parallel to the span of a stationary circular cylinder is investigated experimentally. The Reynolds number has a value of 10 000 and the wire diameter is nearly two orders of magnitude smaller than the cylinder diameter, while being larger than the thickness of the unperturbed boundary layer forming around the cylinder. A technique of high-image-density particle image velocimetry is used to characterize mean and unsteady structures of the separating shear layer and the near wake. Only one of the shear layers is directly perturbed by the surface wire. This disturbance, however, has important global consequences over the entire near wake, provided that the wire is located within a certain range of angular positions with respect to the approach flow. Over this range, there are two angles that can be defined as critical on the basis of the streamwise extent of the near-wake structure. In a simplified sense, these critical angles are associated with significant extension and contraction of the near wake, relative to the wake in the absence of the effect of a surface disturbance. The critical angle of the wire that yields the most significant extension of the near wake is also found to lead to bistable oscillations of the separating shear layer at irregular time intervals, much longer than the time scale associated with the classical Kármán vortex shedding. The foregoing two critical states of extension and contraction of the near wake are, respectively, linked to attenuation or enhancement of the Kármán instability. Moreover, the onset of the shear-layer instability, Reynolds stress, Strouhal number and the transverse extent of shear-layer flapping are all shown to depend on the angular position of the wire within the defined range of angles.

On vortex formation from a cylinder. Part 1. The initial instability

1988 ◽  
Vol 190 ◽  
pp. 491-512 ◽  
Author(s):  
M. F. Unal ◽  
D. Rockwell
Keyword(s):  
Reynolds Number ◽  
Kinetic Energy ◽  
Circular Cylinder ◽  
Shear Layer ◽  
Vortex Shedding ◽  
Vortex Formation ◽  
Absolute Instability ◽  
Near Wake ◽  
Lower Range ◽  
Fluctuation Amplitude

Vortex shedding from a circular cylinder is examined over a tenfold range of Reynolds number, 440 ≤ Re ≤ 5040. The shear layer separating from the cylinder shows, to varying degrees, an exponential variation of fluctuating kinetic energy with distance downstream of the cylinder. The characteristics of this unsteady shear layer are interpreted within the context of an absolute instability of the near wake. At the trailing-end of the cylinder, the fluctuation amplitude of the instability correlates well with previously measured values of mean base pressure. Moreover, this amplitude follows the visualized vortex formation length as Reynolds number varies. There is a drastic decrease in this near-wake fluctuation amplitude in the lower range of Reynolds number and a rapid increase at higher Reynolds number. These trends are addressed relative to the present, as well as previous, observations.

Vortex Formation in the Wake of a Vibrating, Flexible Cable

1974 ◽  
Vol 96 (4) ◽  
pp. 317-322 ◽  
Author(s):  
S. E. Ramberg ◽  
O. M. Griffin
Keyword(s):  
Vortex Shedding ◽  
Vortex Formation ◽  
Reynolds Numbers ◽  
Hot Wire ◽  
Near Wake ◽  
Formation Region ◽  
Vortex Streets ◽  
Region Length ◽  
Karman Vortex ◽  
Flexible Cables

The von Karman vortex streets formed in the wakes of vibrating, flexible cables were studied using a hot-wire anemometer. All the experiments took place in the flow regime where the vibration and vortex-shedding frequencies lock together, or synchronize, to control the wake formation. Detailed measurements were made of the vortex formation flow for Reynolds numbers between 230 and 650. As in the case of vibrating cylinders, the formation-region length is dependent on a shedding parameter St* related to the natural Strouhal number and the vibrational conditions. Furthermore, the near wake configuration is found to be dependent on the local amplitude of vibration suggesting that the vibrating cylinder rseults are directly applicable in that region.

Flow Structures in a U-Shaped Fuel Cell Flow Channel: Quantitative Visualization Using Particle Image Velocimetry

2004 ◽  
Vol 2 (1) ◽  
pp. 70-80 ◽  
Author(s):  
J. Martin ◽  
P. Oshkai ◽  
N. Djilali
Keyword(s):  
Particle Image Velocimetry ◽  
Fuel Cell ◽  
Cross Sections ◽  
Particle Image ◽  
Fluid Dynamic ◽  
Vortex Formation ◽  
Image Velocimetry ◽  
Quantitative Visualization ◽  
Image Density ◽  
Transport Channels

Flow through an experimental model of a U-shaped fuel cell channel is used to investigate the fluid dynamic phenomena that occur within serpentine reactant transport channels of fuel cells. Achieving effective mixing within these channels can significantly improve the performance of the fuel cell and proper understanding and characterization of the underlying fluid dynamics is required. Classes of vortex formation within a U-shaped channel of square cross section are characterized using high-image-density particle image velocimetry. A range of Reynolds numbers, 109⩽Re⩽872, corresponding to flow rates encountered in a fuel cell operating at low to medium current densities is investigated. The flow fields corresponding to two perpendicular cross sections of the channel are characterized in terms of the instantaneous and time-averaged representations of the velocity, streamline topology, and vorticity contours. The critical Reynolds number necessary for the onset of instability is determined, and the two perpendicular flow planes are compared in terms of absolute and averaged velocity values as well as Reynolds stress correlations. Generally, the flow undergoes a transition to a different regime when two recirculation zones, which originally develop in the U-bend region, merge into one separation region. This transition corresponds to generation of additional vortices in the secondary flow plane.

The wake from a cylinder subjected to amplitude-modulated excitation

1993 ◽  
Vol 247 ◽  
pp. 79-110 ◽  
Author(s):  
M. Nakano ◽  
D. Rockwell
Keyword(s):  
Reynolds Number ◽  
Modulation Frequency ◽  
Low Reynolds Number ◽  
Vortex Formation ◽  
Near Wake ◽  
Proper Selection ◽  
Wake Structure ◽  
Far Wake ◽  
Stream Direction ◽  
Selection Of

Controlled, amplitude-modulated excitation of a cylinder at low Reynolds number (Re equals; 136) in the cross-stream direction generates several states of response of the near wake including: a locked-in wake structure, which is periodic at the modulation frequency; a period-doubled wake structure, which is periodic at a frequency half the modulation frequency; and a destabilized structure of the wake, which is periodic at the modulation frequency, but involves substantial phase modulations of the vortex formation relative to the cylinder displacement. The occurrence of each of these states depends upon the dimensionless modulation frequency, as well as the nominal frequency and amplitude of excitation. Transition through states of increasing disorder can be attained by either decreasing the modulation frequency or increasing the amplitude of excitation at a constant value of nominal frequency. These states of response in the near wake are crucial in determining whether the far wake is highly organized or incoherent. Both of these extremes are attainable by proper selection of the parameters of excitation.

Vortex formation out of two-dimensional orifices

2010 ◽  
Vol 655 ◽  
pp. 198-216 ◽  
Author(s):  
GIANNI PEDRIZZETTI
Keyword(s):  
Shear Layer ◽  
Formation Process ◽  
Vortex Formation ◽  
Formation Time ◽  
Inviscid Limit ◽  
Small Scale ◽  
Two Dimensional ◽  
Single Vortex ◽  
Vortex Model ◽  
Vortex Pairs

The understanding of the vortex formation process is currently driving a novel attempt to evaluate the performance of fluid dynamics in biological systems. The concept of formation time, developed for axially symmetric orifices, is here studied in two-dimensional flows for the generation of vortex pairs. The early stage of the formation process is studied with the single vortex model in the inviscid limit. Within this framework, the equation can be written in a universal form in terms of the formation time. The single vortex model properly represents the initial circular spiralling vortex sheet and its acceleration for self-induced motion. Then, an analysis is performed by numerical simulation of the two-dimensional Navier–Stokes equations to cope with the spatially extended vortex structure. The results do not show the pinch-off phenomenon previously reported for vortex rings. The two-dimensional vortex pair tends to a stably growing structure such that, while it translates and extends longitudinally, it remains connected to the sharp edge by a shear layer whose velocity is always about twice that of the leading vortex. At larger values of the Reynolds number the instability of the shear layer develops small-scale vortices capable of destabilizing the coherent vortex growth. The absence of a critical formation number for two-dimensional vortex pairs suggests further considerations for the development of concepts of optimal vortex formation from orifices with variable curvature or of a tapered shape.

An Experimental Study of the Near-Wake Structure of a Cylinder Undergoing Combined Translational and Rotational Oscillatory Motions

2010 ◽  
Author(s):  
Mehdi Nazarinia ◽  
Mark C. Thompson ◽  
John Sheridan ◽  
David Lo Jacono
Keyword(s):  
Vortex Shedding ◽  
Near Wake ◽  
Double Row ◽  
Wake Structure ◽  
Single Row ◽  
Rotational Oscillation ◽  
Medial Plane ◽  
Image Velocimetry ◽  
Phase Angles ◽  
Phase Differences

The experimental research reported here uses particle image velocimetry to extend the study of Nazarinia et al. [1], recording detailed vorticity fields in the near-wake of a circular cylinder undergoing combined translational and rotational oscillatory motions. The focus of the present study is to examine the effect of the ratio between the translational and rotational velocities of the cylinder on the synchronization of the near-wake structures. The frequencies are fixed close to that of the natural frequency of vortex shedding. The results are presented for a fixed amplitude of rotational oscillation of 1 radian and a range of ratios between the translational and rotational velocities (VR) = [0.25, 0.5, 1.0, 1.5]. In particular, it was found that varying the VR value changed the near-wake structure. The results show that at lower values of VR = 0.25, for all of the phase differences examined, the vortices are shed in a single-row 2S mode aligned in the medial plane with a slight offset from the centerline and also synchronized with the combined oscillatory motion. As VR increases the vortex shedding mode changes from a 2S single-row to a 2S double-row structure and eventually back to the single-row (at VR = 0.5). Increasing VR further resulted in the appearance of unlocked-on regimes over the range of negative phase angles and a transition from 2S to P + S mode at the in-phase case There was transition back to the 2S with a further decrease of Φ. For a higher VR the range of desynchronization increased.

Wavelet Analysis of Reynolds Number Effect on 3-D Vorticity in a Turbulent Near Wake

2003 ◽  
Author(s):  
M. W. Yiu ◽  
H. Li ◽  
Y. Zhou
Keyword(s):  
Reynolds Number ◽  
Large Scale ◽  
Lift Coefficient ◽  
Pressure Coefficient ◽  
Vortex Formation ◽  
Small Scale ◽  
Stream Velocity ◽  
Near Wake ◽  
Scale Structures ◽  
Three Components

When Reynolds number, Re (≡U∞d/v, where U∞ is the free stream velocity, d is the cylinder diameter and v is the kinematic viscosity of the fluid), is in the range of 103 to 104, there is a large variation in the near-wake formation region in terms of the base pressure coefficient, the fluctuating lift coefficient, the vortex formation length, which have previously been connected to the generation of small-scale Kelvin-Helmholtz vortices. This work aims to investigate how this Re variation affects the three components of vorticity in terms of time-averaged and small-scale structures and also to provide a relatively complete set of 3-D vorticity data. All three components of vorticity data were simultaneously measured in the intermediate region of the turbulent wake using a multi-wire vorticity probe. It is observed that the root-mean-square (rms) values of the three vorticity components increase with Re, especially the streamwise component, which shows a large jump from Re = 5×103 to 104. At the central frequencies of f0 and 2f0, the contributions from the large-scale and intermediate-scale structures of ωzi2/(ωz2)max decreases 13% and 16% respectively as the Re. increases. However, at the central frequency of 16f0, the contribution of the small-scale structure of ωzi2/(ωz2)max dramatic suddenly 7% increase at Re = 5×103 to 104. The result suggest the generation of small-scale Kelvin-Helmholtz vortices in the spanwise structure. The effect of Re on vorticity signals, spectra, contributions from the wavelet components to the vorticity variances are also examined.

scholarly journals An investigation of the near-wake flow structure of a cylinder with guiding plates

2018 ◽  
Vol 180 ◽  
pp. 02024
Author(s):  
Firat Ekinci ◽  
Erhan Firat ◽  
Göktürk M. Özkan ◽  
Hüseyin Akilli
Keyword(s):  
Large Scale ◽  
Transverse Velocity ◽  
Vortex Formation ◽  
Angular Position ◽  
Wake Flow ◽  
Circular Cylinders ◽  
Near Wake ◽  
Image Velocimetry ◽  
Large Scale Vortex ◽  
Viv Suppression

In this study, the flow behind a circular cylinder with a pair of outer identical guiding plates was investigated using particle image velocimetry (PIV) for various angular positions of the plates (i.e. α=±70°, ±100°, and ±130°). The gaps between these plates and cylinder are equal and are 0.3D. Experiments were carried out at a subcritical Reynolds (Re=ρ·U∞·D/μ) number of 7500, based on the cylinder diameter (D) and the flow velocity (U∞). The features of the near-wake with and without the guiding plates were interpreted in terms of patterns of time-averaged vorticity and streamlines, time-averaged and fluctuating velocity components. The spectral analysis was also carried out to determine the time-dependent variation of the transverse velocity at given locations in the near-wake. Two-dimensional computations of flow around circular cylinders with and without guiding plates have also been performed to predict the timeaveraged and root-mean-square of force coefficients of the various models. It was seen that the guiding plates at an appropriate angular position can lead to substantial attenuation, or retardation, of the process of large-scale vortex formation in the near-wake, thus can lead to vortex-induced vibration (VIV) suppression without any increase in drag.

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