A Numerical Study of Geometrical Effects on the Strouhal Number of a Circular Cylinder

2008 ◽  
Author(s):  
Farzan Kazemifar ◽  
Mehdi Molai ◽  
Bahar Firoozabadi ◽  
Goodarz Ahmadi
Keyword(s):  
Circular Cylinder ◽  
Strouhal Number ◽  
Vortex Shedding ◽  
Numerical Study ◽  
Circular Cylinders ◽  
Percentage Reduction ◽  
Blade Angle ◽  
Vortex Shedding Frequency ◽  
Shedding Frequency ◽  
Geometrical Effects

In this paper, reducing the Strouhal number of a circular cylinder is studied numerically. Two-dimensional numerical simulations of flow over a normal circular cylinder and various modified circular cylinders are carried out using FLUENT® soft ware. Two small blades are attached to a circular cylinder and the effects of variation of the blades length and the blade angle are studied numerically. The blade angle is chosen 2α = 0°, 30°, 90°, 120° and 150°. The blades length is chosen l/d = 0.125, 0.25, 0.375. Effects of blade angles and blade lengths were studied for both 2α = 0° and 150°. Results show that increasing in blade lengths decreases the Strouhal number. Moreover, as the blade angle was increased from zero to 90°, the percentage reduction in Strouhal number decreased; however, as the blade angle was further increased from 90° to 150°, the percentage reduction in Strouhal number increased. Although the modifications studied here decrease the vortex shedding frequency they make the vortices shed from the cylinder farther and stronger hence increasing the magnitude of the fluctuating forces.

scholarly journals FLOW AROUND MODIFIED CIRCULAR CILYNDERS

2004 ◽  
Vol 3 (1) ◽  
Author(s):  
R. L. Ferreira ◽  
E. D. R. Vieira
Keyword(s):  
Circular Cylinder ◽  
Strouhal Number ◽  
Vortex Shedding ◽  
Reynolds Numbers ◽  
Continuous Mode ◽  
Longitudinal Slit ◽  
Vortex Shedding Frequency ◽  
Shedding Frequency ◽  
Hot Film ◽  
The Relationship

The flow around a circular cylinder has awaken the attention of different researchers since the historic Strouhal's work of 1878. Ever since, many experimental and numeric works have been carried out in order to determine the relationship between the vortex shedding frequency and the flow regime. Recently, a number of studies have been developed using several small modifications in circular cylinder. In this work a circular cylinder modified with a longitudinal concave notch, has been tested in order to determine the relationship between the non-dimensional vortex shedding frequency (Strouhal number) and the Reynolds number has been determined to Reynolds up to 600. Additionally a modified circular cylinder with a longitudinal slit also has been tested in order to determine the Strouhal-Reynolds relationship in several attack angle configurations. The experiments have been carried out in a vertical low turbulence hydrodynamic tunnel with 146x146x500 mm of test section operating in continuous mode. Flow visualization by direct liquid dye injection has been utilized in order to produce vortex images. These images have been captured in still chemical photography for different Reynolds numbers. A hot-film probe has been adequately positioned in the vortex wake to determine the vortex shedding frequency and consequently the Strouhal number.

Investigation of Wall Induced Modifications to Vortex Shedding From a Circular Cylinder

1982 ◽  
Vol 104 (4) ◽  
pp. 518-522 ◽  
Author(s):  
F. Angrilli ◽  
S. Bergamaschi ◽  
V. Cossalter
Keyword(s):  
Velocity Field ◽  
Circular Cylinder ◽  
Vortex Shedding ◽  
Reynolds Numbers ◽  
Circular Cylinders ◽  
Frequency Measurements ◽  
Geometrical Pattern ◽  
Cylinder Diameter ◽  
Vortex Shedding Frequency ◽  
Shedding Frequency

In this paper the influence of a wall on vortex shedding frequency, geometrical pattern, and velocity field are investigated. Frequency measurements were carried out with three circular cylinders at Reynolds numbers of 2860, 3820, and 7640. Mean and fluctuating velocities at several traverses were also measured at Re = 3820 both for an isolated cylinder and for an arrangement with a gap from the wall equal to one cylinder diameter. The modifications of the wake pattern are shown in several figures. It is also shown that the proximity of the wall induces a slight increase of vortex shedding frequency.

The Influence of Boundary Layer Velocity Gradients and Bed Proximity on Vortex Shedding From Free Spanning Pipelines

1984 ◽  
Vol 106 (1) ◽  
pp. 70-78 ◽  
Author(s):  
A. J. Grass ◽  
P. W. J. Raven ◽  
R. J. Stuart ◽  
J. A. Bray
Keyword(s):  
Boundary Layer ◽  
Strouhal Number ◽  
Vortex Shedding ◽  
Combined Effects ◽  
Maximum Increase ◽  
Velocity Gradients ◽  
Large Velocity ◽  
Layer Velocity ◽  
Vortex Shedding Frequency ◽  
Shedding Frequency

The paper summarizes the results of a laboratory study of the separate and combined effects of bed proximity and large velocity gradients on the frequency of vortex shedding from pipeline spans immersed in the thick boundary layers of tidal currents. This investigation forms part of a wider project concerned with the assessment of span stability. The measurements show that in the case of both sheared and uniform approach flows, with and without velocity gradients, respectively, the Strouhal number defining the vortex shedding frequency progressively increases as the gap between the pipe base and the bed is reduced below two pipe diameters. The maximum increase in vortex shedding Strouhal number, recorded close to the bed in an approach flow with large velocity gradients, was of the order of 25 percent.

Applicability of The Equivalent Diameter Approach to Estimate Vortex Shedding Frequency and Acoustic Resonance Excitation From Different Finned Cylinders In Cross-Flow

2021 ◽  
Author(s):  
Mohammed Alziadeh ◽  
Atef Mohany
Keyword(s):  
Strouhal Number ◽  
Vortex Shedding ◽  
Interaction Mechanism ◽  
Acoustic Resonance ◽  
Resonance Excitation ◽  
Equivalent Diameter ◽  
Effective Diameter ◽  
Finned Tubes ◽  
Vortex Shedding Frequency ◽  
Shedding Frequency

Abstract This article explores the applicability of utilizing different equivalent diameter (Deq) equations to estimate the vortex shedding frequency and onset of self-excited acoustic resonance for various types of finned cylinders. The focus is on three finned cylinder types that are commonly used in industrial heat exchangers: straight, twist-serrated, and crimped spirally finned cylinders. Within each type of fins, at least three different finned cylinders are investigated. The results indicate that at off-resonance conditions, utilizing the appropriate equivalent diameter collapses the Strouhal number data within the typical Strouhal number variations of an equivalent diameter circular, bare cylinder. However, when acoustic resonance is initiated, the onset and the peak of resonance excitation in all of the finned cylinder cases generally occurred at a reduced flow velocity earlier than that observed from their equivalent diameter bare cylinders. This suggests that although utilizing the appropriate equivalent diameter can reasonably estimate the vortex shedding frequency away from acoustic resonance excitation, it cannot be used to predict the onset of acoustic resonance in finned tubes. The findings of this study indicate that the effective diameter approach is not sufficient to capture the intrinsic changes in the flow-sound interaction mechanism as a result of adding fins to a bare cylinder. Thus, a revision of the acoustic Strouhal number charts is required for finned tubes of different types and arrangements.

Influence of slip on the flow past superhydrophobic circular cylinders

2011 ◽  
Vol 680 ◽  
pp. 459-476 ◽  
Author(s):  
PRANESH MURALIDHAR ◽  
NANGELIE FERRER ◽  
ROBERT DANIELLO ◽  
JONATHAN P. ROTHSTEIN
Keyword(s):  
Circular Cylinder ◽  
Vortex Shedding ◽  
Flow Direction ◽  
Superhydrophobic Surfaces ◽  
Separation Point ◽  
Circular Cylinders ◽  
Recirculation Region ◽  
Small Scale ◽  
Flow Past ◽  
Shedding Frequency

Superhydrophobic surfaces have been shown to produce significant drag reduction for both laminar and turbulent flows of water through large- and small-scale channels. In this paper, a series of experiments were performed which investigated the effect of superhydrophobic-induced slip on the flow past a circular cylinder. In these experiments, circular cylinders were coated with a series of superhydrophobic surfaces fabricated from polydimethylsiloxane with well-defined micron-sized patterns of surface roughness. The presence of the superhydrophobic surface was found to have a significant effect on the vortex shedding dynamics in the wake of the circular cylinder. When compared to a smooth, no-slip cylinder, cylinders coated with superhydrophobic surfaces were found to delay the onset of vortex shedding and increase the length of the recirculation region in the wake of the cylinder. For superhydrophobic surfaces with ridges aligned in the flow direction, the separation point was found to move further upstream towards the front stagnation point of the cylinder and the vortex shedding frequency was found to increase. For superhydrophobic surfaces with ridges running normal to the flow direction, the separation point and shedding frequency trends were reversed. Thus, in this paper we demonstrate that vortex shedding dynamics is very sensitive to changes of feature spacing, size and orientation along superhydrophobic surfaces.

Wake Flow Behind a Cylinder With a Detached Splitter Plate

2005 ◽  
Author(s):  
Minter Cheng
Keyword(s):  
Strouhal Number ◽  
Vortex Shedding ◽  
Formation Process ◽  
Vortex Formation ◽  
Splitter Plate ◽  
Wake Flow ◽  
Plate Length ◽  
Splitter Plates ◽  
Vortex Shedding Frequency ◽  
Shedding Frequency

Fluid flow across a bluff body can induce a series alternating vortices in the downstream flow field. The vortex flow can produce adverse effects on many engineering applications. A number of studies have shown that the wake splitter plate is one of the means to stabilize the vortex formation process. However, most of the previous studies are confined to cylinders with attached splitter plates. Very few studies investigate the effects of the spacing between the cylinder and the splitter plate on the formation of wake vortices. In the present study, the effects of the splitter plate length as well as the gap distance between the splitter plate and the cylinder on the wake flow behind a cylinder have been studied experimentally for low Reynolds number of 400. Both circular and square cylinders are studied in this research. Four splitter plates with different length, 1 ≤ L/D ≤ 4, have been used and a range of cylinder and splitter plate gap distance, 0 < G/D < 6, have been studied. By using flow visualization technique and hot-film anemometer measurement, detailed measurements of the velocity distribution, the vortex shedding frequency, the wake width, and the wake formation length are carried out in order to get a clear understanding of the flow interference behavior. The experimental results indicate that splitter plates alter the vortex formation process in the wake causing a decrease in vortex shedding frequency. The Strouhal number decreases with increasing the splitter plate length as well as the gap distance between the cylinder and the splitter plate. It is shown that a jump in Strouhal number occurs at G/D of 3 to 6. The jump is splitter plate length dependent, and generally the gap distance at which jump takes place increases as the splitter plate length increases.

Vortex-Induced Vibration Characteristics of an Elastic Square Cylinder on Fixed Supports

2004 ◽  
Vol 127 (2) ◽  
pp. 241-249 ◽  
Author(s):  
Z. J. Wang ◽  
Y. Zhou
Keyword(s):  
Circular Cylinder ◽  
Vortex Shedding ◽  
Flow Separation ◽  
Natural Frequencies ◽  
Separation Point ◽  
Square Cylinder ◽  
Vortex Induced Vibration ◽  
The Third ◽  
Vortex Shedding Frequency ◽  
Shedding Frequency

The vortex-induced structural vibration of an elastic square cylinder, on fixed supports at both ends, in a uniform cross flow was measured using fiber-optic Bragg grating sensors. The measurements are compared to those obtained for an elastic circular cylinder of the same hydraulic diameter in an effort to understand the effect of the nature (fixed or oscillating) of the flow separation point on the vortex-induced vibration. It is found that a violent vibration occurs at the third-mode resonance when the vortex-shedding frequency coincides with the third-mode natural frequency of the fluid-structure system, irrespective of the cross-sectional geometry of the cylinder. This is in distinct contrast to previous reports of flexibly supported rigid cylinders, where the first-mode vibration dominates, thus giving little information on the vibration of other modes. The resonance behavior is neither affected by the incidence angle (α) of the free stream, nor by the nature of the flow separation point. However, the vibration amplitude of the square cylinder is about twice that of the circular cylinder even though the flexural rigidity of the former is larger. This is ascribed to a difference in the nature of the flow separation point between the two types of structures. The characteristics of the effective modal damping ratios, defined as the sum of structural and fluid damping ratios, and the system natural frequencies are also investigated. The damping ratios and the system natural frequencies vary little with the reduced velocity at α=0deg, but appreciable at α⩾15deg; they further experience a sharp variation, dictated by the vortex-shedding frequency, near resonance.

The Spanwise Dependence of Vortex-Shedding From Yawed Circular Cylinders

2010 ◽  
Vol 132 (3) ◽  
Author(s):  
James D. Hogan ◽  
Joseph W. Hall
Keyword(s):  
Vortex Shedding ◽  
Flow Direction ◽  
Rapid Decrease ◽  
Circular Cylinders ◽  
Mean Flow ◽  
Independence Principle ◽  
Simultaneous Measurements ◽  
Vortex Shedding Frequency ◽  
The Mean ◽  
Shedding Frequency

Simultaneous measurements of the fluctuating wall pressure along the cylinder span were used to examine the spanwise characteristics of the vortex-shedding for yaw angles varying from α=60 deg to α=90 deg. The Reynolds number based on the diameter of the cylinder was 56,100. The results indicate that yawing the cylinder to the mean flow direction causes the vortex-shedding in the wake to become more disorderly. This disorder is initiated at the upstream end of the cylinder and results in a rapid decrease in correlation length, from 3.3D for α=90 deg to 1.1D for α=60 deg. The commonly used independence principle was shown to predict the vortex-shedding frequency reasonably well along the entire cylinder span for α>70 deg, but did not work as well for α=60 deg.

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