calibration of a wind tunnel

8/10/2019 Calibration of a Wind Tunnel

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Calibration of wind tunnel and sensitivity of a pitot-static

tube with angle of attack

Madhu1and Kush2

B.Tech, Trivandrum, Kerala, 695547

Manas3, Maneesh4andMohit5


and

Mridul6


An experiment was done in aerodynamics lab in IIST to calibrate a subsonic suctiontype wind tunnel with the help of a pitot tube and the sensitivity of pitot-static tube with

the angle of attack was quantified. Mean flow speed in the test section was measured with

the variation of the fan rpm. The test section velocity profile was drawn and observed to

be almost constant.

Nomenclature

Po = stagnation pressure

P = static pressure

Re = Reynolds number

= density

v = velocity of flow = coefficient of viscosity

I.IntroductionA wind tunnel is a machine which can simulate the movement of air around an aircraft in flight. In the wind tunnel,

we can control the conditions that effect the forces and motion of the aircraft. Wind tunnels can be subsonic,

transonic or supersonic, depending on the flow Mach number in the test section. Wind tunnels can also be

classified as closed or open type wind tunnels. In open type wind tunnel the atmospheric air is taken in and

exhausted to the atmosphere. As the kinetic energy of the air discharged from the diffuser is usually only a smallpercentage of the kinetic energy of the air in the test section, the power required by an open-circuit tunnel may be

less than that required by a closed-circuit tunnel of the same aerodynamic design, because no power is wasted in

the drag of the corner vanes.

The major components of a wind tunnel are:

Settling Chamber

1Madhu Kumar, B.Tech, Aerospace, IIST2Kush Sreen, B.Tech, Aerospace, IIST3Manas Maraal, B.Tech, Aerospace, IIST4

Maneesh Kumar, B.Tech, Aerospace, IIST5Mohit Singh Malik, B.Tech, Aerospace, IIST6Mridul Songara, B.Tech, Aerospace, IIST


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The primary function of the settling chamber is to make the air flow unidirectional and uniform as it

enters the wind tunnel and passes to the next section. A honeycomb-shaped structure and wire mesh

screens remove turbulence and reduces the speed of air to approximately zero, so that for theoretical

purpose pressure P measured in this chamber is taken to be stagnation pressure Po. The settling chamber

reduces fluctuations in velocity.

Contraction Cone

The function of contraction cone is to increase the velocity of flow with decrease in static pressure i.e. to

convert pressure energy of flow to its kinetic energy. It is basically used to accelerate the flow, which is

usually done with a fan which consumes more power for same flow speed to be obtained in the test

section.

Test Section

The test section houses the test object, along with sensors that measure its response to wind forces. One

force measured in the test section is lift, which causes objects to rise. Another is drag, which is a measure

of an object's resistance to air as it moves through the atmosphere.

Diffuser

The diffuser slows the high velocity of air in the wind tunnel before it exits the system and increases the

static pressure P. The recovery of pressure from kinetic energy reduces the power needed to drive the

tunnel.

Drive Section

The drive section is responsible for pulling air through the entire tunnel. Normally, large exhaust fans

draw air through the tunnel.

To ascertain that results obtained from the wind tunnel testing are applicable to real-life situations, dynamic

similarity has to be satisfied. The criteria to ensure that two flows are dynamic similar is as follows:

Bodies and any other boundaries are geometrically similar for both flows. Similarity parameters (usually Re, M and for given body) are same for both flows.


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Pitot-Static tubes, which are also called Prandtl tubes, are used

on aircraft as speedometers. Several small holes are drilled

around the outside of the tube and a centre hole is drilled down

the axis of the tube. Both the centre hole and outside hole are

connected to a manometer, which gives the difference in static

and stagnation P (dynamic P) as difference in levels of the

manometric fluid.

II. Theory

The experiment was done on a typical suction type wind tunnel with maximum speed capacity of 40m/s.A Pitot

tube measures both static and stagnation pressures and can be used to measure the speed of the flow in the test

section. Using Bernoulli's equation:

Stagnation pressure = static pressure + dynamic pressure, which can also be written

= + Solving for the velocity we get

= 2 ( ) Where:

is fluid velocity is stagnation pressure

is stagnation pressure

is fluid densityThe Bernoullis equation is applicable only for incompressible, inviscid and steady flows, which is true in our

case since the maximum velocity achievable in the wind tunnel is 40 m/s, which is smaller than Mach 0.3 and

flow through the settling chamber is uniform and unidirectional and effects of viscosity is neglected.

III. Experimental setup

The experiment was done on a typical suction type wind tunnel with speed maximum speed capacity of 40m/s

in aerodynamic lab at IIST. The set up mainly consists of an axial flow fan, converging duct, 300 x 400 x 600

test section. The bell mouth section is fitted with honeycomb mesh to reduce the turbulence and impurities with

incoming air. The fan was driven by a three phase motor with 5 HP power capacities. Multi-tube manometer

with variable inclination is used to measure the pressure at various cross section of the wind tunnel. The

manometric fluid used is ethyl alcohol in order to measure even small changes in pressure. The test section

houses two Pitot - static tubes in order to measure the static and total P at different locations and different

orientations in the test section.


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IV. Procedure

The Pitot tube is placed in the centre of the test section and reading is measured from multitube manometer for

varying fan rpm. These readings are used to calibrate the wind tunnel. To measure the variation of velocity

across the cross section of the test section, the manometer reading is measured for different heights of the Pitottube. Using these readings we get velocity distribution (transverse section) in the test section. Another Pitot

tube is placed near the wall of the test section, so that its inclination can be varied. For variation of inclination

from -30oto 30omanometer readings are noted. These readings can be used to quantify the sensitivity of the

Pitot tube with angle of attack. The Pitot tube is rotated by 90 o along its own axis and again, for various

inclinations, manometer readings are noted, so that effect of orientation of the pitot static tube can be observed.

V. Sample Calculation

Calibration of subsonic wind tunnel:

For reading #4 @ 800 rpm

Initial level of manometer = 8.7 cm

Density of ethanol ( e) = 789 kg/m3

Density of air () = 1.16 kg/m3

Level of manometer corresponding total P in Pitot - static (h5) = 9.1 cm

Level of manometer corresponding P in Pitot - static (h4) = 11.0 cm

Level of manometer corresponding total P in settling chamber (h30) = 9.2 cm

Level of manometer corresponding P at the inlet of test section (h30) = 11.1 cm

Total P in Pitot - static tube (Po2) = P atm+ eg (0.087-h5)/2= P atm+ e g (0.087-0.091)/2

Static P in Pitot - static tube (P) = P atm+ e g (0.087-h4)/2= P atm+ e g (0.087-0.11)/2

Po2 - P = e g (0.11-0.091)/2=789*9.81*0.019/2=104.00 N/m2

Assuming flow to be incompressible and isentropic, Po2P = 0.5* ava2

va = 2(Po2 P)a

va = 2(147.06)1.167


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va= 13.35 m/sRe =

For unit Re, d=1m

Unit Re = = ..8.98 = 933953.10136 m-1

Using continuity equation,

Ats va= A30v30

Ats= Cross-sectional area of the test section

A30= Cross-sectional area of the settling chamber

v30 = velocity of air in the settling chamber.

v30=30

From specifications,30= 1:6

v30=113.35

6 v30= 2.225 m/s

Pressure loss in the section from inlet to wire mesh = P atm- Ps30

= P atm - (P30+ 0.5* av302)

= P atm(P atm+ e g (0.087-h30)2+ 0.5* av302)= e g (h30-0.087)/2- 0.5* av302= 789*9.81*0.005/2-0.5*1.167*2.2252= 24.45 N/m2

Pressure loss in the section from inlet to contraction = P atmPs29

= P atm - (P29+ 0.5* av292)

= P atm(P atm+ e g (0.087-h29)/2+ 0.5* av292)= e g (h29-0.087)/2- 0.5* av292= 789*9.81*0.024/2-0.5*1.167*13.352= 27.36 N/m2

(Assuming velocity to be same at contraction and in the test section).


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Sensitivity of Pitot - static tube:

For reading #3 @ an angle of attack of 20o(when the holes are oriented in horizontal plane)



Pitot P (Po) = P atm+ e g (0.087-h1)/2= P atm+ e g (0.087-0.097)2

Static P (P) = P atm+ e g (0.087-h2)2= P atm+ e g (0.087-0.127)/2

Using principle of pitot static tube,

PoP = 0.5* ava2

va = 2 (Po P)a

va= 2232.201.167

va= 16.77 m/s

For reading #3 @ an angle of attack of 20o(when the holes are oriented in vertical plane)



Pitot P (Po) = P atm+ e g (0.087-h1)/2= P atm+ e g (0.087-0.097)/2

Static P (P) = P atm+ e g (0.087-h2)/2= P atm+ e g (0.087-0.123)/2

Using principle of pitot static tube,

PoP = 0.5* ava2

va = 2 (Po P)

a


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va= 2142.301.167 va= 13.13 m/s

VI. Result

Fig 1. Variation of flow speed with fan speed Fig 2. Variation of unit Re with fan speed

Fig3. Pitot static tube in horizontal position. Fig 4. Pitot static tube in vertical position.

0

200000

400000

600000

800000

1000000

1200000

500 600 700 800 900 1000

R

eynold'sno.

fan speed (rpm)

Unit Reynold's no. vs. fan speed

0

2

4

6

8

10

12

14

16

18

20

-40 -20 0 20 40

velocity(m/s)

inclination of pitot tube(in deg)

Chart Title

y = 1.8034x + 6.2489

0

2

46

8

10

12

14

16

18

500 600 700 800 900 1000

mean

flow

speed(m/s)

fan speed (rpm)

mean flow speed vs. fan speed

actual plot

Linear (actual

plot)

0

2

4

6

8

10

12

14

16

18

-40 -20 0 20 40

velocity(m/s)

inclination of pitot tube(in deg)


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Fig 5. Pitot P vs Inclination when holes Fig 6. Pitot P Vs Inclination when holes are

Oriented in horizontal plane aligned in vertical plane

Fig 7. Static P vs Inclination when holes Fig 8. Static P vs Inclination when holes

Oriented in horizontal plane Oriented in vertical plane

99900

99910

99920

99930

99940

99950

99960

99970

99980

-40 -20 0 20 40

PitotPressure

Inclination of Pitot Static tube

99900

99910

99920

99930

99940

99950

99960

99970

-40 -20 0 20 40

Pitot

Pressure

Inclination of Pitot Static tube

99750

99755

99760

99765

99770

99775

99780

99785

99790

99795

99800

99805

-40 -20 0 20 40

StaticPressure

Inclination of Pitot static tube

99797

99798

99799

99800

99801

99802

99803

99804

-40 -20 0 20 40

StaticPressure

Inclination of Pitot static tube


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Fig 9. Estimated power to run the tunnel Fig 10. Velocity distribution in transverse

Direction of the test section

The velocity profile in the test section of the subsonic wind tunnel and the empirical correlation between mean

flow speed and the fan speed has been obtained in graph 1 and it can also be inferred that the velocity in the test

section is approximately uniform. The Pitot pressure and static pressure readings are quite close at different

orientations as long as the orientation angles are smaller than 20. For larger angles the variation in themeasurements are much larger.

VII. Conclusion

Pressure loss in the tunnel can be attributed to friction since the friction effects cannot be completely eliminated

hence there are pressure drop for every section considered in the wind tunnel. The wind tunnel test section being

a constant area duct also shows a pressure drop across it owing due to friction which continuously increases with

flow velocity. The static pressure measured by the tube varies by large amounts with the increase in its inclination

because the duration in which measurements are made, static probe remains no longer perpendicular to the flow

but takes into account the dynamic pressure as flow would take some time to change according to wall condition.

Also the stagnation pressure values are close to each other for a large range of inclinations as it is the total pressurei.e. the sum of static and dynamic pressure. The variation in mean flow speed in the test section can be attributed

to errors such as fluctuation in levels of manometric fluid, parallax error assumption that the flow is unidirectional

and uniform while entering the test section which is not the case in practical.

0

20

40

60

80

100

120

0 500 1000 1500

power

(W)

fan speed (rpm)

0

2

4

6

8

1012

14

16

18

20

0 3 6 9 12 15 18 21 24

velocity(m

/s)

height (from top)(cm)


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Appendix

Observation tables:

a) Calibration of wind tunnel:

Fan

Speed

h4 h5 h30 h29 Dynamic

pressure(po-ps)

Flow

velocityin test

section

(va)

Reynold's

number

Velocity

insettling

chamber

(v30)

Pressure

loss frominlet to

wire mesh

Pressure

loss frominlet to

contraction

rpm (cm) (cm) (cm) (cm) (N/m2) (m/s) Re (m/s) (N/m2) (N/m2)

500 9.6 8.9 8.9 9.6 38.31149 8.102967 528572.55 1.350494 9.8819321 10.946140

600 10 9 9 10.1 54.73070 9.684898 631765.04 1.614149 14.898913 21.892280

700 10.5 9 9.1 10.5 82.09605 11.86152 773750.99 1.976921 19.611834 16.419210

800 11 9.1 9.2 11.1 103.9883 13.34971 870828.01 2.224951 24.476785 27.365350

900 11.7 9.2 9.3 11.7 136.8267 15.31316 998908.24 2.552194 29.037677 27.365350

1000 12.5 9.4 9.4 12.5 169.6651 17.05202 1112337.1 2.842003 33.598569 38.311490

Height h4 h5 Dynamic

pressure

Velocity

(cm) cm cm (N/m2) (m/s)

0 12.5 9.3 175.1382 17.3249

3 12.5 9.3 175.1382 17.3249

6 12.5 9.3 175.1382 17.3249

9 12.5 9.3 175.1382 17.3249

12 12.5 9.4 169.6652 17.0520

15 12.5 9.4 169.6652 17.0520

18 12.5 9.4 169.6652 17.0520

21 12.5 9.4 169.6652 17.0520

24 12.5 9.4 169.6652 17.0520


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b) Sensitivity of Pitot-static tube :

Holes oriented in horizontal plane (rpm=1000):

Pitot tube

inclination

h1 h2 Dynamic

pressure

Velocity Pitot P Static P

cm cm (m/s) (N/m2) (N/m2)

0 9.4 12.4 164.1921 16.7747 99961.7 99797.5

10 9.4 12.4 164.1921 16.7747 99961.7 99797.5

20 9.7 12.7 164.1921 16.7747 99945.3 99781.1

30 10.5 13.2 147.7729 15.9139 99901.5 99753.7

-10 9.4 12.5 169.6652 17.0520 99961.7 99792.0

-20 9.5 12.9 186.0844 17.8581 99956.2 99770.1

-30 10.1 13.2 169.6652 17.0520 99923.4 99753.7

Holes oriented in vertical plane (rpm=1000):

Pitot tube

inclination

h1 h2 Dynamic

pressure

Velocity Pitot P Static P

cm cm (m/s) (N/m2) (N/m2)

0 9.3 12.3 164.1921 16.7747 99967.2 99803.0

10 9.4 12.3 158.7190 16.4928 99961.7 99803.0

20 9.7 12.3 142.2998 15.6164 99945.3 99803.0

30 10.4 12.4 109.4614 13.6965 99907.0 99797.5

-10 9.4 12.3 158.7190 16.4928 99961.7 99803.0

-20 9.6 12.3 147.7729 15.9139 99950.7 99803.0

-30 10.3 12.4 114.9345 14.0348 99912.4 99797.5

Specifications of wind tunnel:

Test Section Size Cross Section: 300mmx500mm

Length: 600mm

Maximum Speed 40 m/sec

Axial Flow fan of Diameter: 900mm

Maximum rpm: 1400

Hub Diameter: 500mm

Contraction Ratio 6: 1

Contraction length 750mm

Settling chamber 740mm x 740mm

Honey Comb Size 12mm x 12mm x 96mm


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Error Analysis

Least count of the manometer (dh): 1 mm

Error in calculation of flow velocity: =

For reading #3: = .9 = .Acknowledgments

We would like to acknowledge all the persons involved directly or indirectly in completion of this experiment.

We would like to express our gratitude towards our lab supervisor Dr.B.R.Vinoth & Dr. C Prathap for their

guidance and clearing our doubts. We would also like to thank our instructor Ms. Athira for familiarizing us with

the experiment and lab assistant for providing us the needful for conducting the experiment.

References

Web sites1www.google.co.in2

www.grc.nasa.gov3www.academia.edu

4www.ehow.com

Books

3Fundamentals of Aerodynamics, John D Anderson Jr.
http://www.grc.nasa.gov/http://www.grc.nasa.gov/http://www.grc.nasa.gov/

calibration of a wind tunnel

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