Reduction in drag and vortex shedding frequency through porous sheath around a circular cylinder

2011 ◽  
Vol 65 (6) ◽  
pp. 683-698 ◽  
Author(s):  
S. Bhattacharyya ◽  
A. K. Singh
Keyword(s):  
Circular Cylinder ◽  
Vortex Shedding ◽  
Vortex Shedding Frequency ◽  
Shedding Frequency

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.

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.

Vortex shedding from a rectangular prism and a circular cylinder placed vertically in a turbulent boundary layer

1983 ◽  
Vol 126 ◽  
pp. 147-165 ◽  
Author(s):  
Hiroshi Sakamoto ◽  
Mikio Arie
Keyword(s):  
Boundary Layer ◽  
Aspect Ratio ◽  
Circular Cylinder ◽  
Turbulent Boundary Layer ◽  
Vortex Shedding ◽  
Rectangular Prism ◽  
Stream Velocity ◽  
Vortex Shedding Frequency ◽  
Shedding Frequency ◽  
Two Bodies

Measurements of the vortex-shedding frequency behind a vertical rectangular prism and a vertical circular cylinder attached to a plane wall are correlated with the characteristics of the smooth-wall turbulent boundary layer in which they are immersed. Experimental data were collected to investigate the effects of (i) the aspect ratio of these bodies and (ii) the boundary-layer characteristics on the vortex-shedding frequency. The Strouhal number for the rectangular prism and the circular cylinder, defined by S = fcw/U0 and fcd/U0 respectively, was found to be expressed by a power function of the aspect ratio h/w (or h/d). Here fc is the vortex-shedding frequency, U0 is the free-stream velocity, h is the height, w is the width and d is the diameter. As the aspect ratio is reduced, the type of vortex shedding behind each of the two bodies was found to change from the Karman-type vortex to the arch-type vortex at the aspect ratio of 2·0 for the rectangular prism and 2·5 for the circular cylinder.

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.

A Numerical Investigation of the Effects of Flow Pulsations on the Drag Force Over a Cylinder

2011 ◽  
Author(s):  
Eric D’herde ◽  
Laila Guessous
Keyword(s):  
Reynolds Number ◽  
Circular Cylinder ◽  
Drag Coefficient ◽  
Natural Frequency ◽  
Vortex Shedding ◽  
Free Stream ◽  
Stream Velocity ◽  
Vortex Shedding Frequency ◽  
The Mean ◽  
Shedding Frequency

Flow over a cylinder is a fundamental fluid mechanics problem that involves a simple geometry, yet increasingly complex flow patterns as the Reynolds number is increased, most notably the development of a Karman vortex with a natural vortex shedding frequency fs when the Reynolds number exceeds a value of about 40. The goal of this ongoing study is to numerically investigate the effect of an incoming free-stream velocity pulsation with a mean Reynolds number of 100 on the drag force over and vorticity dynamics behind a circular cylinder. This paper reports on initial results involving unsteady, laminar and incompressible flows over a circular cylinder. Sinusoidal free-stream pulsations with amplitudes Av varying between 25% and 75% of the mean free-stream velocity and frequencies f varying between 0.25 and 5 times the natural shedding frequency were considered. Of particular interest to us is the interaction between the pulsating frequency and natural vortex shedding frequency and the resulting effects on drag. Interestingly, at frequencies close to the natural frequency, and to twice the natural frequency, a sudden drop in the mean value of the drag coefficient is observed. This drop in the drag coefficient is also accompanied by a change in the flow and vortex shedding patterns observed behind the cylinder.

A Numerical Investigation of the Effects of Flow Pulsations on the Vortex Shedding, Drag and Lift Forces Over a Cylinder

2011 ◽  
Author(s):  
Eric D’herde ◽  
Laila Guessous
Keyword(s):  
Reynolds Number ◽  
Circular Cylinder ◽  
Vortex Shedding ◽  
Free Stream ◽  
Lift Coefficient ◽  
Stream Velocity ◽  
Vortex Shedding Frequency ◽  
The Mean ◽  
Shedding Frequency ◽  
Drag And Lift

Flow over a cylinder is a fundamental fluid mechanics problem that involves a simple geometry, yet increasingly complex flow patterns as the Reynolds number is increased, most notably the development of a Karman vortex with a natural vortex shedding frequency when the Reynolds number exceeds a value of about 40. The goal of this ongoing study is to numerically investigate the effect of an incoming free-stream velocity pulsation with a mean Reynolds number of 100 on the drag and lift forces over and vorticity dynamics behind a circular cylinder. This paper reports on initial results involving unsteady, laminar and incompressible flows over a circular cylinder. Sinusoidal free-stream pulsations with amplitudes Av varying between 25% and 75% of the mean free-stream velocity and frequencies varying between 0.25 and 5 times the natural shedding frequency fs were considered. Of particular interest to us is the interaction between the pulsating frequency and natural vortex shedding frequency and the resulting effects on drag. Interestingly, at frequencies close to the natural frequency, and to twice the natural frequency, a sudden drop in the mean value of the drag coefficient is observed. The first drop in the drag coefficient, i.e. near f = fs, is also accompanied by a change in the flow and vortex shedding patterns observed behind the cylinder. This change in vortex shedding pattern manifests itself as a departure from symmetrical shedding, and in a non-zero mean lift coefficient value. The second drop, i.e. near f = 2 fs, has similar characteristics, except that the mean lift coefficient remains at zero.

scholarly journals FLOW AROUND MODIFIED CIRCULAR CILYNDERS

2004 ◽  
Vol 3 (1) ◽  
pp. 62
Author(s):  
R. L. Ferreira ◽  
E. D. R. Vieira
Keyword(s):  
Circular Cylinder ◽  
Vortex Shedding ◽  
Reynolds Numbers ◽  
Vortex Wake ◽  
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.

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.

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