An Experimental Investigation of Staggered Circular Cylinders in a Uniform Planar Shear Flow

2002 ◽  
Author(s):  
O. O. Akosile ◽  
D. Sumner
Keyword(s):  
Shear Flow ◽  
Strouhal Number ◽  
Vortex Shedding ◽  
Bluff Body ◽  
Incidence Angle ◽  
Lift Coefficient ◽  
Circular Cylinders ◽  
Aerodynamic Forces ◽  
Planar Shear ◽  
Critical Incidence

Two circular cylinders of equal diameter, arranged in staggered configurations of P/D = 1.125 and 1.25, were immersed in a uniform planar shear flow, at Re = 5.0×104 and a dimensionless shear parameter of K = 0.05. The mean aerodynamic forces and the vortex shedding frequencies were measured for the upstream and downstream cylinders at each P/D. Under uniform, no-shear flow conditions, K = 0, the flow field of the cylinder group is similar to a single bluff body. As the incidence angle is varied from α = 0° to 90°, the forces on each cylinder undergo discontinuous changes, or attain local minimum or maximum values, at several critical incidence angles. At small α, the Strouhal number is greater than that of a single, isolated circular cylinder, whereas at high α the Strouhal number is lower than the single-cylinder value. The effects of shear, K = 0.05, on the aerodynamic forces were different depending on whether the downstream cylinder was situated at a higher or lower centreline velocity compared to the upstream cylinder. The planar shear flow had its greatest influence when the cylinders were in a nearly side-by-side arrangement. This indicated that the effect of shear was mostly on the flow through the gap between the cylinders. The lift coefficient data were mostly unchanged by the shear flow, the drag coefficient data were lowered, and there were shifts in the critical incidence angles. The influence of shear on vortex shedding was less pronounced, but there was a small reduction in Strouhal number compared to the no-shear case.

Vortex Shedding From Two Circular Cylinders in a Staggered Arrangement

2003 ◽  
Author(s):  
D. Sumner ◽  
M. D. Richards
Keyword(s):  
Strouhal Number ◽  
Vortex Shedding ◽  
Bluff Body ◽  
Incidence Angle ◽  
Power Spectra ◽  
Circular Cylinders ◽  
Aerodynamic Forces ◽  
Staggered Arrangement ◽  
Pitch Ratio ◽  
Small Increments

Vortex shedding from two circular cylinders of equal diameter in a staggered configuration was studied experimentally in the subcritical Reynolds number regime, for Re = 3.2×104–7.4×104. The dimensionless centre-to-centre pitch ratio of the staggered cylinders was ranged from P/D = 1.125–4.0, and the incidence angle was varied in small increments from α = 0°–90°. The behaviour of the Strouhal number measurements was broadly classified according to whether the cylinders were closely, moderately, or widely spaced, corresponding to P/D < 1.5, 1.5 ≤ P/D ≤ 2.5, and P/D > 2.5, respectively. For closely spaced staggered configurations, the flow around the cylinders is similar to a single bluff body, and only a single Strouhal number is measured. For moderately spaced cylinders, two distinct Strouhal numbers are measured when α > 30°, but there is considerable scatter in the Strouhal data when α < 30°. For widely spaced cylinders, the Strouhal numbers remain close to that of a single circular cylinder, in contrast to the behaviour of the aerodynamic forces. Evidence of the outer lift peak is seen in the power spectra for the downstream cylinder.

Closely Spaced Circular Cylinders in Cross-Flow and a Universal Wake Number

2004 ◽  
Vol 126 (2) ◽  
pp. 245-249 ◽  
Author(s):  
David Sumner
Keyword(s):  
Vortex Shedding ◽  
Bluff Body ◽  
Incidence Angle ◽  
Cross Flow ◽  
Base Pressure ◽  
Circular Cylinders ◽  
Bluff Bodies ◽  
Aerodynamic Forces ◽  
Single Vortex ◽  
Total Drag

To investigate the effectiveness of a universal wake number for groups of closely spaced bluff bodes, staggered cylinder configurations with center-to-center pitch ratios of P/D=1.125 and 1.25, and incidence angles from α=0 deg–90 deg, were tested in the subcritical Reynolds number regime. The aerodynamic forces, base pressure, and vortex shedding frequencies were measured for the upstream and downstream cylinders, and were found to be strongly dependent on the incidence angle and small changes in the flow pattern. The Griffin number was found to be an appropriate universal wake number for the closely spaced staggered cylinders, based on the total drag force acting on the two cylinders, and the average base pressure for the two cylinders. The results suggest that the single vortex wake of a pair of closely spaced staggered cylinders is broadly comparable to the wake of a solitary bluff body, and that the universal wake number concept can be extended to groups of closely spaced bluff bodies.

Influence of incidence angle on the aerodynamic characteristics of square cylinders with rounded corners

2016 ◽  
Vol 26 (1) ◽  
pp. 269-283 ◽  
Author(s):  
Sajjad Miran ◽  
Chang Hyun Sohn
Keyword(s):  
Incidence Angle ◽  
Lift Coefficient ◽  
Square Cylinder ◽  
Aerodynamic Forces ◽  
Content Type ◽  
Minimum Drag ◽  
Rounded Corner ◽  
Drag And Lift ◽  
Drag And Lift Coefficient ◽  
Critical Incidence

Purpose – The purpose of this paper is to focus on the variation of wake structures and aerodynamic forces with changes in the cylinder corner radius and orientation. Design/methodology/approach – Numerical simulations were performed for flow past a square cylinder with different corner radii placed at an angle to the incoming flow. In the present study, the rounded corner ratio R/D=0 (square cylinder), 0.1, 0.2, 0.3, and 0.4 (where R is the corner radius and D is the characteristic dimension of the body) and the angle of incidence α in the range of 0°-45° were considered. Findings – The numerical model was validated by comparing the present results with results in the available literature, and they were found to be in good agreement. The critical incidence angle for the rounded corner cylinder – corresponding to the minimum mean drag coefficient (C D ), the minimum root mean square value of the lift coefficient C L,RMS), and the maximum Strouhal number – shifted to a lower incidence angle compared with the sharp corner square cylinder. The minimum drag and lift coefficient at R/D=0 were observed for the critical incidence angle αcri=12°, whereas for R/D=0.1-0.4, the minimum drag and lift coefficient were found to be within the range of 5°-10° for α. Originality/value – The presented results shows the importance of the incidence angle and rounded corners of the square cylinder for reduction of aerodynamic forces. The two parameters support the shear layer flow reattachment on the lateral surface of the cylinder, have a strong correlation with the reduction of the wake width, and hence reduced the values of C D and C L .

An Experimental Investigation of Aspect Ratio and Incidence Angle Effects for the Flow Around Surface-Mounted Finite-Height Square Prisms

2014 ◽  
Vol 136 (8) ◽  
Author(s):  
J. F. McClean ◽  
D. Sumner
Keyword(s):  
Aspect Ratio ◽  
Drag Coefficient ◽  
Strouhal Number ◽  
Vortex Shedding ◽  
Incidence Angle ◽  
Lift Coefficient ◽  
Ground Plane ◽  
Aspect Ratios ◽  
Square Prism ◽  
The Mean

The flow around a surface-mounted finite-height square prism was investigated using a low-speed wind tunnel. The experiments were conducted at a Reynolds number of Re = 7.3 × 104 for prism aspect ratios of AR = 3, 5, 7, 9, and 11 and incidence angles from α = 0 deg to 45 deg. The thickness of the boundary layer on the ground plane relative to the side length was δ/D = 1.5. Measurements of the vortex shedding frequency were made with a single-component hot-wire probe, and measurements of the mean drag and lift forces were obtained with a force balance. For all aspect ratios and incidence angles, the mean drag coefficient and Strouhal number were lower than those of an infinite prism, while the mean lift coefficient was of nearly similar magnitude. As the aspect ratio was increased from AR = 3 to 11, the force coefficients and Strouhal number slowly approached the infinite-square-prism data. The mean drag coefficient and Strouhal number for the finite prism were less sensitive to changes in incidence angle compared to the infinite square prism. The critical incidence angle, corresponding to minimum mean drag coefficient, minimum (most negative) mean lift coefficient, and maximum Strouhal number, shifted to a higher incidence angle compared to the infinite square prism, with values ranging from αcritical = 15 deg to 18 deg; this shift was greatest for the prisms of higher aspect ratio. The behavior of the force coefficients and Strouhal number for the prism of AR = 3 was distinct from the other prisms (with lower values of mean drag coefficient and mean lift coefficient magnitude, and a different Strouhal number trend), suggesting the critical aspect ratio was between AR = 5 and AR = 3 in these experiments. In the wall-normal direction, the power spectra for AR = 11 and 9 tended to have weaker and/or more broad-banded vortex shedding peaks near the ground plane and near the free end at α = 0 deg and 15 deg. For AR = 7 to 3, well-defined vortex shedding peaks were detected along the entire height of the prisms. For AR = 11 and 9, at α = 30 deg and 45 deg, vortex shedding peaks were absent in the power spectra in the upper part of the wake.

Aerodynamic Forces and Vortex Shedding for Surface-Mounted Finite Square Prisms and the Effects of Aspect Ratio and Incidence Angle

2012 ◽  
Author(s):  
John F. McClean ◽  
David Sumner
Keyword(s):  
Aspect Ratio ◽  
Drag Coefficient ◽  
Strouhal Number ◽  
Vortex Shedding ◽  
Incidence Angle ◽  
Lift Coefficient ◽  
Ground Plane ◽  
Aspect Ratios ◽  
Square Prism ◽  
The Mean

The flow around a surface-mounted square prism of finite height was investigated experimentally using a low-speed wind tunnel. Of interest were the effects of aspect ratio and incidence angle on the mean aerodynamic forces and vortex shedding. Compared to the case of the “infinite” (or two-dimensional) square prism, the flow around the finite square prism has not been extensively studied. The experiments were conducted at a Reynolds number of Re = 7.2 × 104 for aspect ratios of AR = 3, 5, 7, 9, and 11 and incidence angles of α = 0°, 15°, 30° and 45°. The thickness of the boundary layer on the ground plane relative to the side length was δ/D = 1.5. Measurements of the vortex shedding frequency were made with a single-component hot-wire probe in the wake, and measurements of the mean drag and lift forces were obtained with a force balance. For all aspect ratios and incidence angles, the Strouhal number and the mean drag coefficient were lower than those of an infinite prism, while the mean lift coefficient was of nearly similar magnitude. As the aspect ratio was increased from AR = 3 to 11, the force coefficients and Strouhal number slowly approached the infinite-square-prism data. The behaviours of the mean drag coefficient and Strouhal number with incidence angle were less sensitive compared to the case of the infinite square prism, although a minimum mean drag coefficient, minimum (most negative) mean lift coefficient, and maximum Strouhal number were found at α = 15°. The reduced sensitivity to incidence angle is attributed to the complex three-dimensional flow over the free end of the prism and the downwash flow that enters the near wake. The behaviour of the force coefficients and Strouhal number for the prism of AR = 3 was distinct from the other prisms (with lower values of drag coefficient and lift coefficient magnitude, and a different Strouhal number trend), suggesting the critical aspect ratio was between AR = 5 and AR = 3 in these experiments. In the wall-normal direction, the power spectra for AR = 11 and 9 tended to have weaker and/or more broad-banded vortex shedding peaks near the ground plane and near the free end at α = 0° and 15°. For AR = 7 to 3, well-defined vortex shedding peaks were detected along the entire height of the prisms. For AR = 11 and 9, at α = 30° and 45°, vortex shedding peaks were absent in the power spectra in the upper part of the wake.

On the Spacing of Karman Vortices

1969 ◽  
Vol 36 (2) ◽  
pp. 370-372 ◽  
Author(s):  
D. W. Sallet
Keyword(s):  
Strouhal Number ◽  
Cross Section ◽  
Vortex Shedding ◽  
Bluff Body ◽  
Slenderness Ratio ◽  
The Body ◽  
Vortex Street ◽  
Circular Cylinders ◽  
Flat Plates ◽  
Uniform Cross Section

Equations for the absolute dimensions of the Karman vortex street are developed in terms of the coefficient of drag and the Strouhal number of the vortex shedding bluff body. The body is assumed to be of large slenderness ratio and of uniform cross section. The predicted vortex spacings are compared with the experimental results of other investigators for circular cylinders, flat plates, and a wedge.

Aerodynamics of a two-dimensional bluff body with the cross-section of a train

2020 ◽  
Vol 23 (12) ◽  
pp. 2679-2693 ◽  
Author(s):  
Huan Li ◽  
Xuhui He ◽  
Hanfeng Wang ◽  
Si Peng ◽  
Shuwei Zhou ◽  
...  
Keyword(s):  
Reynolds Number ◽  
Vortex Shedding ◽  
Large Amplitude ◽  
High Speed ◽  
Bluff Body ◽  
Mean Amplitude ◽  
Two Dimensional ◽  
Aerodynamic Forces ◽  
Moderate Reynolds Number ◽  
Amplitude Mode

Experiments on the aerodynamics of a two-dimensional bluff body simplified from a China high-speed train in crosswinds were carried out in a wind tunnel. Effects of wind angle of attack α varying in [−20°, 20°] were investigated at a moderate Reynolds number Re = 9.35 × 104 (based on the height of the model). Four typical behaviors of aerodynamics were identified. These behaviors are attributed to the flow structure around the upper and lower halves of the model changing from full to intermittent reattachment, and to full separation with a variation in α. An alternate transition phenomenon, characterized by an alteration between large- and small-amplitude aerodynamic fluctuations, was detected. The frequency of this alteration is about 1/10 of the predominant vortex shedding. In the intervals of the large-amplitude behavior, aerodynamic forces fluctuate periodically with a strong span-wise coherence, which are caused by the anti-symmetric vortex shedding along the stream-wise direction. On the contrary, the aerodynamic forces fluctuating at small amplitudes correspond to a weak span-wise coherence, which are ascribed to the symmetric vortex shedding from the upper and lower halves of the model. Generally, the mean amplitude of the large-amplitude mode is 3 times larger than that of the small one. Finally, the effects of Reynolds number were examined within Re = [9.35 × 104, 2.49 × 105]. Strong Reynolds number dependence was observed on the model with two rounded upper corners.

Vortex shedding from bluff bodies in a shear flow

1973 ◽  
Vol 60 (2) ◽  
pp. 401-409 ◽  
Author(s):  
D. J. Maull ◽  
R. A. Young
Keyword(s):  
Shear Flow ◽  
Vortex Shedding ◽  
Bluff Body ◽  
Pressure Coefficient ◽  
Uniform Flow ◽  
Base Pressure ◽  
Longitudinal Vortex ◽  
Bluff Bodies ◽  
Stream Direction

Experiments are described in which the vortex shedding from a bluff body and the base pressure coefficient have been measured in a shear flow. It is shown that the shedding breaks down into a number of spanwise cells in each of which the frequency is constant. The division between the cells is thought to be marked by a longitudinal vortex in the stream direction and this is supported by evidence from experiments where a longitudinal vortex was generated in an otherwise uniform flow.

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