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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
B.Tech, Trivandrum, Kerala, 695547
and
Mridul6
B.Tech, Trivandrum, Kerala, 695547
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)
Level of manometer corresponding total P in Pitot - static (h1) = 9.7 cm
Level of manometer corresponding P in Pitot - static (h2) = 12.7 cm
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)
Level of manometer corresponding total P in Pitot - static (h1) = 9.7 cm
Level of manometer corresponding P in Pitot - static (h2) = 12.3 cm
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/