Stability analysis of experimental flow fields behind a porous cylinder for the investigation of the large-scale wake vortices

2013 ◽  
Vol 715 ◽  
pp. 499-536 ◽  
Author(s):  
Simone Camarri ◽  
Bengt E. G. Fallenius ◽  
Jens H. M. Fransson
Keyword(s):  
Stability Analysis ◽  
Circular Cylinder ◽  
Vortex Shedding ◽  
Large Scale ◽  
Flow Fields ◽  
Velocity Fields ◽  
Global Mode ◽  
Experimental Database ◽  
Wake Vortices ◽  
Suction And Blowing

AbstractWhen the linear stability analysis is applied to the time-averaged flow past a circular cylinder after the primary instability of the wake, a nearly marginally stable global mode is predicted with a frequency in time equal to that of the saturated vortex shedding. This behaviour has recently been shown to hold up to Reynolds number $\mathit{Re}= 600$ by direct numerical simulations. In the present work we verify that the global stability analysis provides reasonable estimation also when applied to experimental velocity fields measured in the wake past a porous circular cylinder at $\mathit{Re}\simeq 3. 5\ensuremath{\times} 1{0}^{3} $. Different intensities of continuous suction and blowing through the entire surface of the cylinder are considered. The global direct and adjoint stability modes, derived from the experimental data, are used to sort the random instantaneous snapshots of the velocity field in phase. The proposed method is remarkable, sorting the snapshots in phase with respect to the vortex shedding, allowing phase-averaged velocity fields to be extracted from the experimental database. The phase-averaged flow fields are analysed in order to study the effect of the transpiration on the kinematical characteristics of the large-scale wake vortices.

Tomographic PIV investigation on 3D wake structures for flow over a wall-mounted short cylinder

2017 ◽  
Vol 831 ◽  
pp. 743-778 ◽  
Author(s):  
Hang-Yu Zhu ◽  
Cheng-Yue Wang ◽  
Hong-Ping Wang ◽  
Jin-Jun Wang
Keyword(s):  
Aspect Ratio ◽  
Circular Cylinder ◽  
Dynamic Characteristics ◽  
Large Scale ◽  
Low Frequency ◽  
Streamwise Vortex ◽  
Flow Fields ◽  
Vortex Structures ◽  
Wake Flow ◽  
Wake Field

Tomographic particle image velocimetry (TPIV) measurement with six high-resolution charge-coupled device (CCD) cameras is conducted to investigate flow structures over a finite circular cylinder with an aspect ratio of 2 ($h/d=2$). This short wall-mounted cylinder is fully immersed in a thick turbulent boundary layer ($\unicode[STIX]{x1D6FF}/h=1.025$). Focus is placed on the three-dimensional instantaneous vortex structures and their dynamic characteristics in the wake flow fields. Based on the present results, a refined topological model of the mean wake field behind the finite circular cylinder is proposed, where the spatial locations of the typical vortex structures and their interactions are described in more detail. Among the reported typical vortex structures (i.e. the horseshoe, tip, base, trailing and arch vortex), emphasis is laid on discussion of the tip and arch vortex. The instantaneous 3D M-shape arch vortex and an alternating large-scale streamwise vortex are first found in the present experiment, and their developments are also discussed. Therefore, it is suggested that the instantaneous finite-cylinder wake is dominated by the arch vortex system and the large-scale streamwise vortices. Moreover, in the instantaneous volumetric flow fields, both the antisymmetric and the symmetric wake behaviours are observed. With proper orthogonal decomposition (POD) analysis, the dynamic characteristics of the wake field are clarified. Different from the flow around an infinite cylinder without control, the third and fourth POD modes are characterized by low-frequency symmetric shedding. The low-frequency feature shown in the second mode pair is observed and associated with the occurrence of instantaneous symmetric 3D wake behaviour triggered by the low-aspect-ratio effect and the extension of the separated shear layer. The low frequency seems be attributed to the flapping phenomenon, i.e. oscillation of the recirculation in the backward-facing step flow. It is found that the flapping motion has a modulating effect on the occurrence of the antisymmetric shedding vortex and thus the large-scale streamwise vortex.

Influence of Oscillating Boundary Layer Suction on Aerodynamic Performance in Highly-Loaded Compressor Cascades

2018 ◽  
Author(s):  
Xu Hao ◽  
Liu Bao ◽  
Cai Le ◽  
Zhou Xun ◽  
Wang Songtao ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Vortex Shedding ◽  
Large Scale ◽  
Reverse Flow ◽  
Separation Bubble ◽  
Flow Region ◽  
Flow Fields ◽  
Separation Flow ◽  
Oscillating Boundary ◽  
Pod Analysis

Vortex structures of the separation flow fields in compressor cascades controlled by the boundary layer oscillating suction (BLOS) are numerically investigated. The proper orthogonal decomposition (POD) method is adopted to present the variation of characteristics owned by large-scale vortices. It is found that unsteady perturbation re-organizes the aspirated flow fields and, if in a proper situation, reduces the loss furthermore. Through POD analysis, variations of vortical structures are described. The results turn out that the periodic perturbation leads to a vortex shedding process with the same frequency as the excitation. The reason of loss reduction could be summarized by actuated vortices enhancing the momentum of the stagnated fluid in the reverse flow region as well as decreasing the frequencies of vortex shedding. Finally, 3-D numerical results turn out that the oscillation can transform the stable corner separation bubble to vortex rings shedding downstream and hence improve cascade performance.

Some effects of intense turbulence on the aerodynamics of a circular cylinder at subcritical Reynolds number

1972 ◽  
Vol 52 (3) ◽  
pp. 543-563 ◽  
Author(s):  
David Surry
Keyword(s):  
Reynolds Number ◽  
Circular Cylinder ◽  
Vortex Shedding ◽  
Large Scale ◽  
Central Element ◽  
Structural Response ◽  
Describing Function ◽  
Free Stream Turbulence ◽  
Drag Forces ◽  
Unsteady Lift

The effect, of high intensity large-scale free-stream turbulence on the flow past a rigid circular cylinder has been studied experimentally a t subcritical Reynolds numbers. Grids were used to produce homogeneous turbulence fields with longitudinal scales ranging from 0·36 to 4·40 cylinder diameters and with longitudinal intensities greater than 10%. Power and cross-spectra of the turbulence components (the ‘system input’) have been measured in order to carefully define the turbulence characteristics.In the response experiments, a special model measured arbitrary two-point pressure correlations. Subsequent integrations yielded the specbral properties of the unsteady lift and drag. Measurements of mean drag and Strouhal frequency indicate that to some extent even severe large-scale turbulence can be considered to be qualitatively equivalent to an increase in the effective Reynolds number. Vortex shedding is not seriously disrupted by severe turbulence, but is affected more by low than by high frequencies. The unsteady lift response is still dominated by the vortex shedding, whereas the unsteady drag becomes primarily a response to turbulence. The cross-spectra of the drag forces for the one turbulence case examined overlay well when plotted against lateral separation divided by wavelength. This has enabled a ‘describing function’ for the drag response to turbulence to be derived. This describing function is the central element needed for the calculation of the structural response of such cylinders in the drag direction.

SYMMETRIC LARGE-SCALE VELOCITY FIELDS IN TWO-DIMENSIONAL THERMAL CONVECTION

1994 ◽  
Vol 04 (05) ◽  
pp. 1369-1374 ◽  
Author(s):  
J. PRAT ◽  
I. MERCADER ◽  
J.M. MASSAGUER
Keyword(s):  
Stability Analysis ◽  
Velocity Field ◽  
Velocity Profile ◽  
Numerical Simulations ◽  
Linear Stability Analysis ◽  
Thermal Convection ◽  
Vertical Profile ◽  
Large Scale ◽  
Velocity Fields ◽  
Two Dimensional

Recent experiments on thermal convection in finite containers [Krishnamurti & Howard, 1981; Howard & Krishnamurti, 1986] show the presence of flows spanning the largest dimension of the container. Numerical simulations of 2D thermal convection showing large-scale flows of this kind have been presented elsewhere [Prat et al., 1993a, 1993b]. In every known example the large scale velocity field has been found to display a vertical profile either antisymmetric or showing rather small departures from antisymmetry. In contrast, theoretical group arguments support the existence of symmetric velocity profiles. In the present paper it will be shown that large-scale velocity fields with vertically symmetric velocity profile do exist. In spite of these flows not being dominant in the range of parameters explored, their geometry and dynamics will be discussed on the basis of a linear stability analysis.

On the Effect of Various Shrouds on the Wake of a Circular Cylinder

2014 ◽  
Author(s):  
Azlin Mohd Azmi ◽  
Tongming Zhou ◽  
Liang Cheng
Keyword(s):  
Circular Cylinder ◽  
Vortex Shedding ◽  
Coherent Structures ◽  
Large Scale ◽  
Great Distance ◽  
Measured Velocity ◽  
Wire Probe ◽  
Vortex Shedding Frequency ◽  
Shedding Frequency ◽  
Porous Screen

The wake of a circular cylinder enclosed in various shrouds is experimentally investigated in a wind tunnel at Reynolds number of 7000. The aim of the present work is to understand the effect of different shroud types on the vortex shedding frequency and vortex structures from the shrouded cylinders. The tested shrouds are porous screen cylinders and a circular-holed shroud at various porosities of 37%, 48% and 40%, respectively, with the diameter ratio between the shroud and the inner cylinder of 1.3. Phase-averaged analysis is used to examined the large-scale coherent structures with one hot-wire probe moving across the wake in the y-direction to measure the velocity components and another fixed at y/d=1 to 2 from the wake centerline to provide a phase reference to the measured velocity signals. It was found that the vortex shedding persists to some great distance downstream in the wake of the tested shrouds. While the strength of the coherent structures in the wakes of the bare cylinder and tested shrouds are comparable, those in the circular-holed shroud and screen shroud of 48% porosity are 40% higher than the former two at x/d=10.

Manipulation of the Flow Around a Circular Cylinder by Utilizing the Flow Receptivity

2007 ◽  
Author(s):  
Yasuaki Kozato ◽  
Satoshi Kikuchi ◽  
Shigeki Imao
Keyword(s):  
Circular Cylinder ◽  
Shear Layer ◽  
Vortex Shedding ◽  
Velocity Fields ◽  
Shear Layer Instability ◽  
Hot Wire ◽  
Acoustic Disturbance ◽  
Wire Probe ◽  
Separated Shear Layer ◽  
Karman Vortex

An attempt to control the flow around a circular cylinder by utilizing the receptivity to the external acoustic disturbance was carried out and its mechanism was also studied. The velocity fields around the cylinder vicinity are carefully investigated with an X-type hot-wire probe. When the disturbance of a higher frequency related to the separated shear layer instability is added, the development of turbulence and the spreading of the shear layer are restrained. And, the amplification of the fluctuating velocity component of the Karman vortex shedding is delayed and its degree is reduced. Furthermore, the process of the gradual scale modification of the shear layer instability that appears prior to the transition of the flow is suppressed.

Circular cylinder vortex-synchronization control with a synthetic jet positioned at the rear stagnation point

2010 ◽  
Vol 662 ◽  
pp. 232-259 ◽  
Author(s):  
LI HAO FENG ◽  
JIN JUN WANG
Keyword(s):  
Circular Cylinder ◽  
Stagnation Point ◽  
Large Scale ◽  
Water Channel ◽  
Excitation Frequency ◽  
Synthetic Jet ◽  
Reynolds Stresses ◽  
Flow Topology ◽  
Wake Vortices ◽  
Rear Stagnation Point

The flow over a circular cylinder controlled by a two-dimensional synthetic jet positioned at the mean rear stagnation point has been experimentally investigated in a water channel at the cylinder Reynolds number Re = 950. This is an innovative arrangement and the particle-image-velocimetry measurement indicates that it can lead to a novel and interesting phenomenon. The synthetic-jet vortex pairs induced near the exit convect downstream and interact with the vorticity shear layers behind both sides of the cylinder, resulting in the formation of new induced wake vortices. The present vortex synchronization occurs when the excitation frequency of the synthetic jet is between 1.67 and 5.00 times the natural shedding frequency at the dimensionless stroke length 99.5. However, it is suggested that the strength of the synthetic-jet vortex pair plays a more essential role in the occurrence of vortex synchronization than the excitation frequency. In addition, the wake-vortex shedding is converted into a symmetric mode from its original antisymmetric mode. The symmetric shedding mode weakens the interaction between the upper and lower wake vortices, resulting in a decrease in the turbulent kinetic energy produced by them. It also has a significant influence on the global flow field, including the velocity fluctuations, Reynolds stresses and flow topology. However, their distributions are still dominated by the large-scale coherent structures.

Simulation of Vortex-Shedding Flow About a Circular Cylinder at High Reynolds Numbers

1990 ◽  
Vol 112 (2) ◽  
pp. 155-161 ◽  
Author(s):  
Charles C. S. Song ◽  
Mingshun Yuan
Keyword(s):  
Circular Cylinder ◽  
Vortex Shedding ◽  
Large Scale ◽  
Flow Equations ◽  
Large Scale Vortex ◽  
Scale Turbulence ◽  
High Reynolds ◽  
Lift And Drag ◽  
Secondary Vortices ◽  
Volume Method

Vortex shedding over a circular cylinder is modeled based on the weakly compressible flow equations with a simple subgrid scale turbulence model and a simple hybrid boundary condition. An explicit finite volume method is used. A subcritical and a supercritical case are computed. It is shown that the large-scale vortex-shedding phenomenon, the primary vortices, and the related oscillatory lift and drag can be calculated fairly well with a grid system coarser than the boundary layer thickness. The secondary vortices and the related higher frequency oscillations are also calculated by using somewhat finer grids.

A numerical study of global frequency selection in the time-mean wake of a circular cylinder

2010 ◽  
Vol 645 ◽  
pp. 435-446 ◽  
Author(s):  
J. S. LEONTINI ◽  
M. C. THOMPSON ◽  
K. HOURIGAN
Keyword(s):  
Stability Analysis ◽  
Circular Cylinder ◽  
Saddle Point ◽  
Numerical Study ◽  
Streamwise Velocity ◽  
Three Dimensions ◽  
Cylinder Wake ◽  
Mean Flow ◽  
Global Mode ◽  
The Mean

A series of direct numerical simulations, both in two- and three-dimensions, of the flow past a circular cylinder for Reynolds numbers Re ≤ 600 has been conducted. From these simulations, the time-mean (and, for the three-dimensional simulations, the spanwise spatial-mean) flow has been calculated. A global linear stability analysis has been conducted on these mean flows, showing that the mean cylinder wake for Re ≤ 600 is marginally stable and the eigenfrequency of the leading global mode closely predicts the eventual saturated vortex shedding frequency. A local stability analysis has also been conducted. For this, a series of streamwise velocity profiles has been extracted from the mean wake and the stability of these profiles has been analysed using the Rayleigh stability equation. The real and imaginary instability frequencies gained from these profiles have then been used to find the global frequency selected by the flow using a saddle-point criterion. The results confirm the success of the saddle-point criterion when the mean flow is quasi-parallel in the vicinity of the saddle point; however, the limitations of the method when the mean flow exhibits higher curvature are also elucidated.

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