Flow Over A Circular Cylinder.docx

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Flow Over A Circular CylinderVijay Guvvada,Vidur Paliwal,Vishnu Vardhan,Gaurav Vaibhav,Mofeez Alam

B. Tech. Aerospace, Indian Institute of Space Science and Technology

AbstractWhen a fluid flows around a stationary cylinder or when a cylinder moves through a stationary fluid, the fluid exerts a force on thecylinder. The Component of this force in the direction of free stream velocity is called Drag force. Drag force depends on many factors likeshape, orientation etc.In this experiment the drag force on a circular cylinder is found by equating torque on the cylinder and a dead weight

connected to it with the help of the deflection of dead weight. This experiment presents the variation of Cdwith Reynolds Number over the range1.687*104-1.04681*105. The Cd decreases with Reynolds number over this range.

Keywords-Drag force, Reynolds number, Torque, Flow seperation, Drag coefficent

I. INTRODUCTION

Drag, or more specifically air drag, is a phenomenon that occurs as an object passes through a fluid.There are a few factors that

determine the drag force that an object experiences. Some of the moreobvious factors are shape, speed, fluid medium, and surface

of the object. Thus we have 2 kinds of bodies depending upon the shape- bluff body and streamline body. When the drag force ismore due to pressure it is called bluff body and if the contribution of skin friction drag is high-then it is called streamline body. In

some instances these factors are manipulated in order to either minimize or maximize drag.

The drag coefficient over a cylinder decreases with increase in Reynolds number for the laminar, subcritical and supercritical

regimes and in supercritical regime there is a phenomenon called drag crisis-which is an intense decrease in Cd due the movement

of the point of separation downstream to 120~130 degree from 80 degree. This phenomenon is seen because the boundary layer

becomes turbulent and as it has more kinetic energy -fluid particles can travel further in the adverse pressure gradient.

After the supercritical regime the Cd increases as the shear forces keeps on increasing due to lage pressure gradient in the normal

direction of wall.

The parameters such as roughness, freestream turbulence and spanwise flow has not been considered in this experiment. These

parameters have an important effect on the flow over the cylinder.

In other cases the drag forces must simply be known in order to design for other parameters possibly such as engine horsepower,structural strength, etc. Regardless of the need for finding the drag force, the need for an accurate calculation of this force persists.

With this in mind, we experiment with shapes, speeds, and methods in order to draw insight on the ability to predict drag

Figure 1- Ideal flow over a cylinder Figure 2- Real flow over a cylinder

In this experiment we find the drag force on the cylinder by connecting it to a dead weight and with the help of the deflection on

the dead weight we find the drag force on the cylinder. A relation was established between the angle of deflection and the drag

force on the body. The body is held by a rod on the top of it so that it is pivoted for free rotation. We got the results for a Reynold

number range of of 1.687*104-1.04681*105


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II. EXPERIMENTAL APPARATUS

The Experiment requires the following equipments

Wind Tunnel

Cylinder, Dead weight, connecting rods

Manometer

Fig.1 (experimental apparatus) Fig. 2 (experimental apparatus)

III. THEORY

The sources of this drag are: (a) friction between the fluid and the surface of the cylinder, and (b) a non-uniform pressure

distribution.

The cylinder in the fluid stream presents a certain area perpendicular to the direction of fluid motion. This is called the planeform

area of the cylinder (length x width (diameter)) the fluid moves toward and is deflected around the cylinder, some of its

momentum is transferred to the cylinder in the form of pressure on the projected area facing the flow.

If the flow follows the contour of the cylinder, the pressure on the side facing the flow is balanced by the pressure on the reverse

side in which case the pressure drag is very small or zero. (see Figure 1). This condition is described by potential theory where the

fluid is ideal and is realized in real fluids at very low Reynolds numbers. At high Reynolds numbers, the flow does not follow the

contour of the cylinder, i.e., the boundary layer grows more rapidly for an adverse pressure gradient and if the pressure gradient is

large enough, separation may occur, and turbulent eddies form in the wake of the cylinder.

In this case the pressure on the reverse side fails to recover (see Figure 2) leading to an unbalanced pressure distribution and

pressure drag. Ordinarily, it is not practical to separate the viscous and pressure drag forces, and indeed, it is usually their sum inwhich we are interested.

Therefore, the usual practice is to characterize their combined effects with two dimensionless parameters, the drag coefficient

-------- (1)


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0.5*1.204*(25.43)^2=388.378 Pa

Cd=Fd/(Fref *S)=1.42/(388.378*2.25*10^-3)=1.56

VI. OBSERVATIONS

Fig 3. (Cd vs Re)

Fig 4. (Fdvs Re)

0

0.5

1

1.5

2

2.5

3

0.00E+00 2.00E+04 4.00E+04 6.00E+04 8.00E+04 1.00E+05 1.20E+05

CoefficientofDrag

Reynolds Number, based on Frontal Area

Cd Vs Re

Cd Vs Re

0

0.5

1

1.5

2

2.5

0.00E+00 2.00E+04 4.00E+04 6.00E+04 8.00E+04 1.00E+05 1.20E+05

CoefficnetofDrag

Reynold Number

dragVs Reynolds number

dragVs Reynolds number


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VII.RESULT AND CONCLUSION

In the experiment flow over a cylinder following points were figured out.

1.

the variation in the Cd is not accurately measured for low reynolds number 1.64*10^4- 5.6*10^4 because the error due tofluctuation of the measurement of Deflection was of the order of the reading itself.

2.

After Re greater than 5.6*10^4 , the amplitude of oscillation was small and we got fairly steady trend of Cd.

3.

On increasing the Reynolds no. in the above mentioned range the drag force increases.

The explanation why we got such trend in the plot Re vs Cd is that,

If ReD is less than about 100,000 the boundary layer remains laminar from the stagnation point at the front of the cylinderto the point where it separates. The resulting flow pattern shown in fig. termed sub-critical, is associated with a high dragon the cylinder. The laminar boundary layer separates just upstream of the maximum thickness. Separation occurs becausethe boundary layer anticipates the deceleration of the flow (and therefore positive pressure gradient) that would otherwiseoccur on the rearward face of the cylinder. Downstream of separation the flow quickly becomes turbulent and a broad wakeis formed. The wake as a whole is unstable and rolls up into vortices that are shed antisymmetrically at regular intervalsfrom the cylinder. This type of wake is called a von Krmn vortex street. Because of separation the pressure remains lowand approximately constant over the rearward face of the cylinder. This causes a net imbalance of pressure forces on thecylinder, usually referred to as the pressure drag. Pressure drag accounts for about 90% of the total drag on the cylinder in

this regime. The remaining 10% is due to skin-friction drag - friction between the flow and the cylinder. Most skin-frictiondrag is produced on the forward-face of the cylinder where the boundary layer is thin and velocity gradients at the cylinder

surface are large.

As observed in above graphs (fig.4 , 3), as reynols no. increases Cd decreases but Fd increases because the 0.5**V^2increases more rapidly than the increase in force with velocity, as reference pressure is in denominator so it compensate theincrease in numerator, overall we get decrease in Cd. The obtained graph can be compared with the available data withinthe range(50000-100000).

VIII.ACKNOWLEDGEMENT

We are very much thankful to all who contributed directly or indirectly in performing the experiment Flow over a symmetric air

foil on a low speed wind tunnel in aerodynamic lab IIST. We are specially thankful to Dr. Vinoth B.R. and Dr. Satyeesh who gave

their maximum in performing this experiment. We are also thankful to all lab assistants and our friends who helped us.

IX. REFERENCES

1. Pope, Alan, Wind Tunnel Testing, John Wiley & Sons, Inc., New York, 1954

2.http://mars.uta.edu/

3.http://www.eng.fsu.edu

4. https://www.google.co.in/search?q=Cd+vs+Re&source=lnms
http://mars.uta.edu/http://www.eng.fsu.edu/http://www.eng.fsu.edu/http://mars.uta.edu/


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X.APENDIX

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Flow Over A Circular Cylinder.docx