The vortex-shedding process behind two-dimensional bluff bodies

1982 ◽  
Vol 116 ◽  
pp. 77-90 ◽  
Author(s):  
A. E. Perry ◽  
M. S. Chong ◽  
T. T. Lim
Keyword(s):  
Circular Cylinder ◽  
Flow Visualization ◽  
Vortex Shedding ◽  
Bluff Bodies ◽  
Two Dimensional ◽  
Closed Cavity ◽  
Time Exposure ◽  
Starting Flow ◽  
Visualization Techniques ◽  
Insight Into

Using a variety of flow-visualization techniques, the flow behind a circular cylinder has been studied. The results obtained have provided a new insight into the vortex-shedding process. Using time-exposure photography of the motion of aluminium particles, a sequence of instantaneous streamline patterns of the flow behind a cylinder has been obtained. These streamline patterns show that during the starting flow the cavity behind the cylinder is closed. However, once the vortex-shedding process begins, this so-called ‘closed’ cavity becomes open, and instantaneous ‘alleyways’ of fluid are formed which penetrate the cavity. In addition, dye experiments also show how layers of dye and hence vorticity are convected into the cavity behind the cylinder, and how they are eventually squeezed out.

scholarly journals Effect of Cylinder Gap Ratio on The Wake of a Circular Cylinder Enclosed by Various Perforated Shrouds

CFD letters ◽  
2021 ◽  
Vol 13 (4) ◽  
pp. 51-68
Author(s):  
Nurul Azihan Ramli ◽  
Azlin Mohd Azmi ◽  
Ahmad Hussein Abdul Hamid ◽  
Zainal Abidin Kamarul Baharin ◽  
Tongming Zhou
Keyword(s):  
Reynolds Number ◽  
Circular Cylinder ◽  
Vortex Shedding ◽  
Control Method ◽  
Lift Coefficient ◽  
Base Case ◽  
Bluff Bodies ◽  
Ansys Fluent ◽  
Gap Ratio ◽  
Spacing Ratio

Flow over bluff bodies produces vortex shedding in their wake regions, leading to structural failure from the flow-induced forces. In this study, a passive flow control method was explored to suppress the vortex shedding from a circular cylinder that causes many problems in engineering applications. Perforated shrouds were used to control the vortex shedding of a circular cylinder at Reynolds number, Re = 200. The shrouds were of non-uniform and uniform holes with 67% porosity. The spacing gap ratio between the shroud and the cylinder was set at 1.2, 1.5, 2, and 2.2. The analysis was conducted using ANSYS Fluent using a viscous laminar model. The outcomes of the simulation of the base case were validated with existing studies. The drag coefficient, Cd, lift coefficient, Cl and the Strouhal number, St, as well as vorticity contours, velocity contours, and pressure contours were examined. Vortex shedding behind the shrouded cylinders was observed to be suppressed and delayed farther downstream with increasing gap ratio. The effect was significant for spacing ratio greater than 2.0. The effect of hole types: uniform and non-uniform holes, was also effective at these spacing ratios for the chosen Reynolds number of 200. Specifically, a spacing ratio of 1.2 enhanced further the vortex intensity and should be avoided.

Suppression of Vortex Shedding Downstream of a Circular Cylinder Using Permeable Cylinders in Shallow Water

2013 ◽  
Author(s):  
Göktürk Memduh Özkan ◽  
Hüseyin Akıllı
Keyword(s):  
Circular Cylinder ◽  
Flow Control ◽  
Shallow Water ◽  
Flow Structure ◽  
Vortex Shedding ◽  
Outer Cylinder ◽  
Vortex Formation ◽  
Stream Velocity ◽  
Bluff Bodies ◽  
Different Diameters

The characteristics of the flow around a 50mm circular cylinder surrounded by a permeable outer cylinder were investigated by Particle Image Velocimetry (PIV) and flow visualization techniques in order to control the unsteady flow structure downstream of the cylinder in shallow water. The effect of outer permeable cylinder with a porosity of β = 0.4 on the flow control was studied using five different diameters; D = 60, 70, 80, 90, 100mm. Depth-averaged free stream velocity was kept constant as U = 170mm/s corresponding to a Reynolds number of Re = 8500 and the water height was adjusted to hw = 25mm throughout the study. The results clearly showed that the outer permeable cylinder significantly affects the flow structure of the inner cylinder. It was found that by the existence of outer cylinder, the frequency of unsteady vortex shedding is reduced, vortex formation region is elongated and fluctuations are attenuated which are good indications of effective flow control. Owing to the results, optimum parameters were defined and suggested for the suppression of vortex-induced vibrations on bluff bodies.

Properties of the mean recirculation region in the wakes of two-dimensional bluff bodies

1997 ◽  
Vol 351 ◽  
pp. 167-199 ◽  
Author(s):  
S. BALACHANDAR ◽  
R. MITTAL ◽  
F. M. NAJJAR
Keyword(s):  
Reynolds Number ◽  
Shear Stress ◽  
Reynolds Stress ◽  
Vortex Shedding ◽  
Reynolds Numbers ◽  
Recirculation Region ◽  
Bluff Bodies ◽  
Two Dimensional ◽  
Stress Components ◽  
The Mean

The properties of the time- and span-averaged mean wake recirculation region are investigated in separated flows over several different two-dimensional bluff bodies. Ten different cases are considered and they divide into two groups: cylindrical geometries of circular, elliptic and square cross-sections and the normal plate. A wide Reynolds number range from 250 to 140000 is considered, but in all the cases the attached portion of the boundary layer remains laminar until separation. The lower Reynolds number data are from direct numerical simulations, while the data at the higher Reynolds number are obtained from large-eddy simulation and the experimental work of Cantwell & Coles (1983), Krothapalli (1996, personal communication), Leder (1991) and Lyn et al. (1995). Unlike supersonic and subsonic separations with a splitter plate in the wake, in all the cases considered here there is strong interaction between the shear layers resulting in Kármán vortex shedding. The impact of this fundamental difference on the distribution of Reynolds stress components and pressure in relation to the mean wake recirculation region (wake bubble) is considered. It is observed that in all cases the contribution from Reynolds normal stress to the force balance of the wake bubble is significant. In fact, in the cylinder geometries this contribution can outweigh the net force from the shear stress, so that the net pressure force tends to push the bubble away from the body. In contrast, in the case of normal plate, owing to the longer wake, the net contribution from shear stress outweighs that from the normal stress. At higher Reynolds numbers, separation of the Reynolds stress components into incoherent contributions provides more insight. The behaviour of the coherent contribution, arising from the dominant vortex shedding, is similar to that at lower Reynolds numbers. The incoherent contribution to Reynolds stress, arising from small-scale activity, is compared with that of a canonical free shear layer. Based on these observations a simple extension of the wake model (Sychev 1982; Roshko 1993a, b) is proposed.

Inviscid Model of Two-Dimensional Vortex Shedding by a Circular Cylinder

AIAA Journal ◽  
1979 ◽  
Vol 17 (11) ◽  
pp. 1193-1200 ◽  
Author(s):  
Turgut Sarpkaya ◽  
Ray L. Schoaff
Keyword(s):  
Circular Cylinder ◽  
Vortex Shedding ◽  
Two Dimensional

Flow Visualization and Leakage Measurements of Stepped Labyrinth Seals: Part 2—Sloping Surfaces

1997 ◽  
Vol 119 (4) ◽  
pp. 844-848 ◽  
Author(s):  
D. L. Rhode ◽  
J. S. Younger ◽  
M. D. Wernig
Keyword(s):  
Flow Visualization ◽  
Leading Edge ◽  
Test Facility ◽  
Variable Geometry ◽  
Two Dimensional ◽  
Labyrinth Seals ◽  
Leakage Resistance ◽  
Water Test ◽  
Sloping Surface ◽  
Visualization Techniques

An experimental investigation was conducted to determine the geometry–leakage relationship for advanced, stepped labyrinth seals. A unique, variable-geometry water test facility was constructed and used to acquire leakage resistance measurements for two-dimensional, planar models. Flow visualization techniques were also used to assist in identifying and understanding the turbulence generating flow patterns. It was found that contoured surfaces and restrictor tooth leading-edge shapes of proper dimensions can be incorporated into the cavity geometry to reduce seal leakage. Specifically, the combination of a sloping surface and a curved surface on the rotor within the labyrinth cavity gave significant improvement.

Vortex shedding by a circular cylinder in lock-exchange density current 

2021 ◽  
Author(s):  
Ana M Ricardo ◽  
Giovanni Di Lollo ◽  
Moisés Brito ◽  
Claudia Adduce ◽  
Rui M.L. Ferreira
Keyword(s):  
Circular Cylinder ◽  
Fresh Water ◽  
Vortex Shedding ◽  
Gravity Current ◽  
Shear Instability ◽  
Density Current ◽  
Bluff Bodies ◽  
Density Currents ◽  
Ambient Fluid ◽  
Dense Fluid

<p>Flow around bluff bodies have been attracting the interest of the research community for more than a century. The physical mechanisms associated with the vortex shedding in the wake of bluff bodies is still of fundamental research interest. However, flow-structure interaction in density currents has not received enough attention. The transient nature of the interaction between the density driven flow and a stationary object constitutes the motivation for the present laboratory study aiming at investigating the vortex generation and fate on the wake of a circular cylinder in a density current.</p><p>The experiments were conducted in a horizontal and rectangular cross-section channel with 3.0 m long, 0.175 m wide and 0.4 m deep. The gravity current was generated using the classic lock-exchange configuration. A sliding stainless-steel gate with 1 mm thickness, sealed by PVC board glued in the sidewall, was positioned at 0.3 m from the left hand side of the channel. The experiment starts when the gate is suddenly removed, leaving the dense fluid to flow along the bottom of the channel, while the ambient fluid moves above in the opposite direction. The dense fluid consists in a mixture of fresh water and salt while the ambient fluid is a solution fresh water and ethanol (96%). The amount of salt and alcohol added in each mixture was determined in order to obtain a given density difference and to ensure the same refractive index in both fluids. Two different currents were tested with reduced gravity equal to 0.06 ms<sup>-2</sup> and 0.24 ms<sup>-2</sup>. For each test ten repetitions were carried out. Instantaneous velocity maps were acquired with a Particle Image Velocimetry system at 15 Hz. Polyamide seeding particles of density equal to 1.03 were added in both dense and ambient fluids.</p><p> The Reynolds number varied between 1500 and 4000. The results show that vortex shedding varies as the current reaches and overtakes the cylinder. Boundary layer detachment and shear instability is initiated shortly before the snout reaches the cylinder. A pattern of well-defined symmetrical vortexes is formed as a result of the initial shear instability. As the head of the current engulfs the cylinder, stronger turbulence diffusion contributes to reduce vortex coherence. Vortexes are smaller and detach sooner, while is not clear if shedding is alternate or simply random. The formation length is smaller than that of a steady flow with the same Re. When the back of the current passes, the formation length is increased and vortex shedding becomes periodical again. A striking feature is that the Von Kármán street is frequently symmetrical rather than exhibiting a pattern of alternate vortices.</p><p>This research was funded by national funds through Portuguese Foundation for Science and Technology (FCT) project PTDC/CTA-OHR/30561/2017 (WinTherface).</p>

Two-dimensional vortex shedding of a circular cylinder

2001 ◽  
Vol 13 (3) ◽  
pp. 557-560 ◽  
Author(s):  
C.-Y. Wen ◽  
C.-Y. Lin
Keyword(s):  
Circular Cylinder ◽  
Vortex Shedding ◽  
Two Dimensional
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