final report ecah!!

8/3/2019 Final Report Ecah!!

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Contents

1.0 TITTLE ...................................................................................................................................................... 2

2.0 OBJECTIVES ............................................................................................................................................. 2

3.0 INTRODUCTION ....................................................................................................................................... 3

4.0 APPARATUS ............................................................................................................................................ 7

5.0 EXPERIMENTAL PROCEDURES ................................................................................................................. 8

6.0 RESULT .................................................................................................................................................... 9

7.0 DISCUSSION ........................................................................................................................................... 15

8.0 CONCLUSION. ........................................................................................................................................ 17

9.0 RECCOMENDATION. .............................................................................................................................. 17

10.0 REFERENCES. ....................................................................................................................................... 18


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1.0 TITTLE

In this experiment, we are studying the behaviour of incompressible flow over a circular

cylinder.

2.0 OBJECTIVE

We are trying to study the characteristics of incompressible flow over a circular cylinder

in this experiment which compromises of the velocity field as well as pressure field.

Specifically, when we are doing the experiment, the objectives are as follow;

I. To gain a better understanding of the characteristics of incompressible flow over

a circular cylinder.

II. To study the pressure profile and flow characteristics for flow around a circular

cylinder

III. To measure the pressure distribution around the circular cylinder when subjected

to an incompressible flow.

IV. To estimate the pressure drag of the cylinder and determine its drag coefficientbased on the pressure profile for flow around the cylinder.


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3.0 INTRODUCTION

3.1 Background

The objective of this lab is to investigate the interactions between a

flowing fluid (air) and a solid object (cylinder). External flows past objects have

been studied extensively because of their many practical applications. For

example, airfoils are made into streamline shapes in order to increase the lifts,

and at the same time, reducing the aerodynamic drags exerted on the wings. On

the other hand, flow past a blunt body, such as a circular cylinder, usually

experiences boundary layer separation and very strong flow oscillations in the

wake region behind the body.

Figure 3.1.1 viscous flow around a stationary cylinder,

As a fluid stream such as air flows around a blunt object like a

cylinder, it tends to adhere to the surface or a portion of the length of the body

due to the fluid viscosity. As a result, the friction drag will be generated on theinterface between air and cylinder. In order to calculate the friction drag and the

drag coefficient, the pressure along the cylinder surface will be measured using

the pressure tap (a hole in the cylinder) and an inclined manometer. Due to the

dramatic change in the cylinder curvature, the flow cannot follow the change and

thus separates from the surface at a certain point. This causes a turbulent wake

to be formed behind the cylinder.


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3.2 Theory

Figure 3.2.1 Boundary layer separation

At the front stagnation point A (figure 3.2.1) , the static pressure isgiven by the equation 3.2.1 below;

Recognize that the appropriate nondimensional pressure is the pressure

coefficient defined as shown in equation 3.2.2 below;

Therefore, at the front stagnation point A, the pressure coefficient;

For any point on the cylinder surface,

( ) ( )


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Since and ,

Thus,

Whereby is the pressure coefficient while is the differential manometerheight and is the free-stream velocity of the flow.

When a body is subjected to a certain amount of force over a

concentrated area as shown in the figure 3.2.2., it experiences pressure as

P=F/A and for the case of a cylinder, the force exerted on the specific area of its

surface is as follow;

Since the drag force of the cylinder is defined as the total force exerted on the

specific area of its surface, therefore;

Figure 3.2.2 Definition of symbols used in the calculation of pressure drag


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From the equation 3.2.2, .Therefore, the drag force of the

cylinder,

.

( )

Since () thus .Therefore, the equation for the dragforce of the cylinder is as follow;

( )

Then the coefficient of drag for the cylinder can be defined as;

The measurements obtained in the lab will be analyzed using the Bernoulli

equation. The following conditions must be satisfied for the equation to be valid:

1) The equation is applied along a streamline.

2) The flow is steady.

3) The flow is incompressible; = constant

4) The flow is inviscid (frictionless).


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4.0 APPARATUS

The apparatus needed to conduct the experiment are ;

I. Wind tunnel to provide air flow over the circular cylinder

II. 2-inch diameter circular cylinder

III. Multi-tube manometer to measure the pressure at each location on the cylinder

surface.

IV. Rheostat to adjust the velocity of the air flow in the wind tunnel.

V. Inclined gage to indicate the velocity of the air flow in the wind tunnel.

Figure 4.1 Wind tunnel

Figure 4.2 Circular cylinder Figure 4.3 Multi-tube manometer

Rheostat

Inclined gage


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5.0 EXPERIMENTAL PROCEDURE

In order to fulfil the objectives of the experiment, we are required to follow the proper

experimental procedures in order to obtain an adequate result. Before proceeding with the

experimental procedures, we are first adequately supervised and briefed by the lecturer andtechnician about the theory and the experimental procedures that we have to follow. After

shortly briefed by the lecturer and technician, we noticed that the apparatus has been setup

at the wind tunnel as shown in figure 4.1 and the test apparatus consist of 2-inch diameter

circular cylinder as shown in figure 4.2.

Twenty pressure tapping holes are drilled at equidistance over half of the

circumference of the cylinder in order to measure the pressure distribution around the

cylinder. Then, these holes are connected to the multi-tube manometer to measure the

pressure at each holes location and the circular cylinder is placed at across test section of the wind tunnel.

Next, the wind tunnel is closed and the blower fan that provided air flow in the wind

tunnel is switched on.Then, the velocity of air flow in the wind tunnel is adjusted to by using a rheostat and indicated by the inclined gage. After that, the pressure

measurements corresponding to each pressure tapping location were taken by reading the

multi tube manometer.

Last but not least,the velocity of air flow in the wind tunnel is adjusted once again

to by using a rheostat and the pressure measurements corresponding to eachpressure tapping location were taken once again by reading the multi tube manometer.

Then, all the data collected from both tests were tabulated in the table 6.1 and table 6.2 in

the result section.


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6.0 EXPERIMENTAL RESULTS

6.1 Data tabulated from the experiments

Test 1

Manometer height (tube 20),

Location Angle, ( ) () ()

1 0 230 0 1

12 10 230 0 1

0.9848083 20 232 2 0.74017

0.6955324 30 234 4 0.48034

0.4159875 40 238 8 -0.0393

-0.030116 50 242 12 -0.559

-0.359327 60 244 14 -0.8188

-0.40948 70 246 16 -1.0787

-0.368949 80 246 16 -1.0787

-0.1873110 90 246 16 -1.0787

011 100 246 16 -1.0787

0.18731412 110 246 16 -1.0787

0.36893713 120 246 16 -1.0787

0.5393514 130 246 16 -1.0787

0.69337515 140 246 16 -1.0787

0.826332

16 150 246 16 -1.0787 0.93418217 160 246 16 -1.0787

1.01364618 170 246 16 -1.0787

1.06231219 180 246 16 -1.0787

1.0787 8.4454

Table 6.1 Data collected from test 1


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Test 2

Manometer height (tube 20),

Location Angle, ( ) () ()

1 0 194 0 11

2 10 194 0 10.984808

3 20 200 6 0.8051256760.756571

4 30 212 18 0.4153770270.359727

5 40 226 32 -0.03932973-0.03013

6 50 242 48 -0.55899459-0.35931

7 60 256 62 -1.01370135-0.50685

8 70 264 70 -1.27353378-0.43557

9 80 262 68 -1.20857568-0.20987

10 90 258 64 -1.078659460

11 100 258 64 -1.07865946 0.18730712 110 258 64 -1.07865946

0.36892313 120 260 66 -1.14361757

0.57180914 130 262 68 -1.20857568

0.77685715 140 262 68 -1.20857568

0.92582316 150 260 66 -1.14361757

0.99040217 160 260 66 -1.14361757

1.07464918 170 260 66 -1.14361757

1.12624319 180 258 64 -1.07865946

1.078659 8.660044

Table 6.2 Data collected from test 2


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6.2 Graph of Coefficient of Pressure, Cp vs. Angle,

Graph 6.1 Graph of Coefficient of Pressure, Cp vs. Angle,

-1.5

-1

-0.5

0

0.5

1

1.5

0 50 100 150 200

CoefficientofPressure

,Cp

Angle , ()

Coefficient of Pressure, Cp vs. Angle,

Poly. (v=10 m/s)

Poly. (v=20 m/s)


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6.3 Sample Calculation

Given that ;

TEST 1

The calculation done is based on result for test 1, where

At ,

and

Differential manometer height, ()

=

Coefficient of Pressure, Cp

Pressure drag on the cylinder,

( )

Whereby,

And


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Thus, ( )

()

Drag Coefficient,

()

TEST 2

The calculation done is based on result for test 2, where

At ,

and

Differential manometer height, ()

=

Coefficient of Pressure, Cp

0.756571


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Pressure drag on the cylinder,

( )

Whereby,

And Thus,

( )

()

Drag Coefficient,

()

6.4 Result Summary

Free-Stream velocity (m/s) Pressure drag, (N) Drag Coefficient,

10 20


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7.0 DISCUSSION

From the experimental result, it can be seen that at the front stagnation point (at

=0),the pressure coefficient is unity and this point also have the highest pressure in the

entire flow field. In physical variables, static pressure is equal to .In other

words, the full dynamic pressure of the oncoming fluid is felt as a static pressure on the nose

of the body as the fluid is decelerated to zero speed at the stagnation point.

The experimental data for the flow over the surface of a circular cylinder is plotted in

the Graph of Coefficient of Pressure, Cp vs. Angle, as shown in the graph 6.1 in the result

section. The graph shows that the flow separation started to occur slightly before 90 and for

air flows at 20m/s, the flow separation occur slightly farther downstream than the air f lows at10m/s.Separation occurs because the boundary layer anticipates the deceleration of the

flow (and therefore positive pressure gradient) that would otherwise occur on the rearward

face of the cylinder. Downstream of separation the flow quickly becomes turbulent and a

broad wake is formed.

For inviscid (Ideal) flow around a stationary cylinder as shown in the figure 7.1 below,

the impingement of flow on the cylinder creates a stagnation point on the approaching

surface. The departure of the flow away from the cylinder creates another stagnation point

on the trailing surface .In the idealized situation where viscosity is neglected, the no-slip

condition at the surface of the cylinder does not apply. Also in the absence of vorticity

(inviscid flow) flow separation cannot occur.

Figure 7.1 inviscid flow around a stationary cylinder


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On the other hand, for viscous flow around a stationary cylinder,one stagnation point

is created in front of the cylinder. Because of the viscosity, a no-slip condition exists

everywhere on the surface of the cylinder, i.e., the velocity must vanish everywhere on the

surface. Consequently a boundary layer is created where the velocity transitions from a

value of zero at the surface to the free stream value some distance away from the cylinder

surface. The inertia of the fluid as it rounds the top and bottom of the trailing surface causes

the flow to separate at these locations. This creates a disturbed wake (Von-Karman vortex

street) downstream from the cylinder.

As for the pressure drag, of the cylinder, there are not much different between theair flows at 10m/s and air flows at 20m/s. For the air flows at 10m/s, it has pressure drag of

which is slightly smaller than the air flow at 20m/s which has pressure drag of

.Consequently, the drag coefficient, for the air flow at 10m/s also not muchdifferent than the air flow at 20m/s whereas the drag coefficient, for the air flow at 10m/sand 20m/s are and respectively.

A body moving through a fluid experiences a drag force, which is usually divided into

two components: frictional drag, and pressure drag. Frictional drag comes from friction

between the fluid and the surfaces over which it is flowing. On the other hand,Pressure drag

comes from the eddying motions that are set up in the fluid by the passage of the body. The

boundary layer and its interaction with the local pressure gradient plays a major role inaffecting the flow over a cylinder. In particular, near the shoulder, the pressure gradient

changes from being negative (decreasing pressure) to positive (increasing pressure). The

force due to pressure differences changes sign from being an accelerating force to being a

retarding force. In response, the flow slows down. However, the fluid in the boundary layer

has already given up some momentum because of viscous losses and viscous friction, and it

does not have enough momentum to overcome the retarding force. Some fluid near the wall

actually reverses direction, and the flow separates.


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8.0 CONCLUSION

From the experimental result, it can be said that the objectives of the experiment was

not successfully achieved. The drag coefficient, of the cylinder obtained from theexperiment is quite different from the theoretical value. As for the experimental value, the

drag coefficient, of the cylinder obtained are and for air flow at 10m/sand 20m/s respectively while the theoretical value of drag coefficient, of the cylinder isbetween 1.0 to 1.1depending on the ratio of the cylinders length and diameter (L/D).As

for the cylinder used in this experiment;

Therefore, the theoretical value of drag coefficient, of the cylinder is 0.9475 whichmake the experimental value obtained have large discrepancies than the theoretical

value which are due to errors that have occurred during the experiment.

9.0 RECCOMENDATION

In order to achieve more accurate result, some precaution measurement should me

implied for reducing the possibility for errors to occur while conducting the experiment.

One of the way to reduce the errors is by ensuring that the test section of the wind tunnel

is fully sealed to ensure that the air in the wind tunnel flowing consistently.

Besides that, the error which occur while taking the pressure reading can be reduced

by replacing the multi-tube manometer with a digital manometer which capable of giving

more accurate reading than the conventional manometer.


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10.0 REFERENCES

1. Fluid Mechanics Fundamentals and Applications 1ST Edition in SI units; Yunus A.

engel and John M. Cimbala, Mc Graw Hill International Edition 2006

2. Bruce R. Munson, Donald F. Young, Theodore H. Okiishi, Fundementals of Fluid

Mechanics, 5th Edition, John Wiley & Sons, Asia,2006.

3. Subsonic Wind Tunnel Test By Katelyn Pierson,Aaron Klapheck,Bryan

Mark;sacramento State; Nov. 07, 2008

4. http://www.eng.fsu.edu/~shih/succeed/cylinder/cylinder.htm#Flow

Separation/Wake

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