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ISSN 0974-5904, Volume 10, No. 01 SPL
March 2017, P.P. 18-23
Development and Performance Investigation of Supersonic
Blow-down Wind Tunnel
KALAKANDA ALFRED SUNNY1, NALLAPANENI MANOJ KUMAR2, HARITHRA. M1,
AND RAJAKUMAR S. RAI3
2
1
Department of Aerospace Engineering, Karunya University, Coimbatore-641 114, India
Department of Electrical & Electronics Engineering, Bharat Institute of Engineering and Technology,
Mangalpally, Ibrahimpatnam, Ranga Reddy-501 510, India
3
Department of Mechanical Engineering, Karunya University, Coimbatore-641 114, India
Email: alfredsunny@karunya.edu, nallapanenichow@gmail.com, harithram@karunya.edu.in,
rajakumars@karunya.edu
Abstract:
Wind tunnel is an apparatus used to investigate the aerodynamic behavior of flow over the solid bodies. The
importance of this facility is its ability to produce an airstream that simulates real-time condition and its accurate
means for aerodynamic research. A supersonic wind tunnel is a type of wind tunnel providing high-speed flow
regimes up to Mach number 5, which is helpful in analyzing the flight conditions of space vehicles. In this regard, to
analyze the flight conditions in a laboratory level, a supersonic blow-down wind tunnel is developed in Department
of Aerospace Engineering, Karunya University. But the major concern is its performance. The performance of this
supersonic blow-down wind tunnel is analyzed theoretically and experimentally. Standard methodologies were
followed while developing the wind tunnel facility and a brief description is given about its configuration in this
paper. Runtime based test was adopted for the theoretical and experimental performance investigation of this
facility. Results shows that the developed supersonic blow-down wind tunnel performs better and it is observed that
there is no much difference between the experimental and theoretical values.
Keywords: wind tunnel, aerodynamic research, supersonic blow down a wind tunnel, Mach number, runtime, mass
flow rate.
1. Introduction:
The wind tunnel is an experimental setup designed for
aerodynamic studies of reduced scaled models, based on
achieving geometric, kinematic and dynamic
similarities, over full scaled objects. Wind tunnel
consists of a tubular passage with the object under test,
mounted in the middle. Air is made to move past the
object by a powerful fan system or other means. The
test object is instrumented with suitable sensors to
measure aerodynamic forces, pressure distribution, or
other aerodynamic-related characteristics. Application
of wind tunnel ranges from aeronautical, automotive,
aeroacoustic, aquadynamic to wind study. Wind tunnel
is indispensable to the development of modern aircraft.
The advances in computational fluid dynamics (CFD)
modeling on high-speed digital computers has reduced
the demand for wind tunnel testing.
However, CFD results are still not completely reliable
and wind tunnels are used to verify these predictions
[1]. There are several different ways to classify wind
tunnels, but they are most often use the ratio of the
speed of the fluid, or of any other object, and the speed
of sound designated by Mach number in order to
emphasize relative importance of compressibility. Each
type has its advantages and disadvantages, making
certain wind tunnels more suitable for some purposes
than others. Wind tunnel that operates at the Mach range
of 1.5-5 is called as a supersonic wind tunnel. The
power to drive a low-speed wind tunnel varies as the
cube of the velocity in the wind tunnel. Although this
rule does not hold into the high-speed regime, the
implication of rapidly increasing power requirements
with test speed is correct. Because of the power
requirements, the high-speed wind tunnels are often of
the intermittent type, in which the energy is stored in the
form of pressure or vacuum and is allowed to drive the
tunnel only a few seconds out of each pumping hour.
There is a choice in the type of intermittent tunnel to be
used at the higher Mach numbers. The operation of the
wind tunnel with atmospheric inlet pressure is the in
draft tunnel or with atmospheric discharge pressure is
the blow-down tunnel. Based on the discussion and
available resources, the objective of this paper was to
develop a supersonic blow-down type wind tunnel to
#021001SPL04 Copyright © 2017 CAFET-INNOVA TECHNICAL SOCIETY. All rights reserved.
KALAKANDA ALFRED SUNNY, NALLAPANENI MANOJ KUMAR,
HARITHRA. M, AND RAJAKUMAR S. RAI
serve as an educational and research tool [2] to analyze
basic flow principles.
2. Design Methodology of Supersonic Wind Tunnel
The intermittent blow-down tunnel usually consists of a
basic circuit of the compressor, air storage tank, valves,
settling chamber, test section, and exhaust or diffuser.
The sizing of components and the selection and
matching of components is a large portion of the science
of blown tunnel design. Often the design of blow-down
tunnel is greatly influenced by some special condition.
For example, some major component is already
available must be used for economy. In this case, the
gas dynamic facility that consists of a compressor and
an air storage tank of volume 14.14 m3 with maximum
operating pressure of 20 bar which was already
available in the laboratory was used. Under such
condition, the size, operating range and versatility of the
tunnel can be achieved easily. However, the primary
consideration in developing the wind tunnel is the
design of test section. Designers should make sure that
the test section can enclose a relatively large sub-scale
model. Therefore, an enclosed conventional solid wall
design was chosen. With respect to the available gas
dynamic facility a square test section of cross section
area 100 Sq. mm was established. The exit of the test
section is typically a free jet to the atmosphere through a
diffuser. Variable area nozzles that can produce a Mach
number range of 1.5 to 5 was designed based on the test
section dimensions using the method of characteristics
[3-5]. Next, the design for the settling chamber was
done. A settling chamber is usually a cylindrical shell,
one meter long which accepts the air from the storage
tank through a pressure regulating valve which provides
a length for settling to obtain uniform flow and then
exhausts into the inlet of the nozzle. Based on this
principle a settling chamber of 1 meter long and with 34
cm diameter was designed. After designing the
components, pipes and valves are needed to connect
them. The pipes and valves have to be selected wisely
because of the high-pressure losses. The piping should
be selected such that for the maximum mass flow, the
Mach number will be below 0.4. Also, the pressure drop
in the piping and the valves between the storage tank
and the pressure regulator valve should be calculated
because if the air storage capacity is marginal, this
pressure drop may result in a significant reduction in
available run time. Taking these into considerations, a
piping of 4 inches and a pressure regulating valve of 6
inch with 300 class steel was selected for installation.
Characterized by this design methodology, Karunya
supersonic wind tunnel was developed.
3.1 Air Supply System
The high-pressure air supply comes from two 20 HP
electric powered, high-pressure 2-stage compressor. The
air is pumped up to 150 psi, which is the maximum
pressure of the storage tanks, see in Fig. 1. Motor has
totally enclosed fan cooled, Squirrel cage induction type
with a speed of 1440 RPM. The overall dimension of
the motor is 1725*810*830 mm. Input voltage is 415V
and frequency is 50 Hz. Inter-stage coolers and a twintower dryer are used to remove moisture from the air.
After passing through an air-cooler, the air is stored in
three storage tanks, with a total volume of 14.14 m3. It
takes about 4 hours to charge all the storage tanks at the
maximum pressure.
Fig. 1. High pressure air supply system
3.2 Heat Flow Air Dryers
The dryer consists of 2 desiccants towers, a pre-filter,
and oil removing filter and after the filter. The wet
compressed air enters the pre-filter where the moisture
and dust content are removed by 5 microns. The prefilter is fitted with a double seat automatic drain valve.
Then the air passes through the oil removing filter
where the oil content in the air is removed. Heat flow air
dryer is shown in Fig. 2.
Fig. 2. Heat flow air dryers
3.3 Air Coolers
The working pressure is 30 Kg/cm2 with an operating
capacity of 35 Cubic feet per minute. Electric power
required for operation is 220 V AC. The model number
of the air coolers is AAC-004-6 and is manufactured by
GEM equipment, see in Fig. 3.
3. Configuration of Supersonic Wind Tunnel
The aerodynamic configuration that is finally selected
and developed for the wind tunnel is described below:
Fig. 3. AAC-004-6 Air coolers
International Journal of Earth Sciences and Engineering
ISSN 0974-5904, Vol. 10, No. 01 SPL, March 2017, pp. 18-23
Development and Performance Investigation of Supersonic Blow-down Wind Tunnel
3.4 Storage Tank
The size and maximum pressure of the tank, limits the
run time of the tunnel. A storage tank was available for
the supersonic wind tunnel. This tank is 1.5 m in
diameter, 8 m in length, has a volume of 14 m3 and is
rated for a pressure of 150 psi. By charging the main
storage tanks, it is estimated that the supersonic wind
tunnel is capable of achieving the required Mach
number. The storage tank shown in Fig. 4. is filled from
the high-pressure compressor by two solenoid valves at
an air distribution panel. The high-pressure air passes
through a pipe from back of the panel to air storage
tank. The pressure in the tube is closely monitored
during the air filling process.
10 bar. The settling chamber is designed to be 34 cm in
diameter and 1 m in length. The maximum flow speed is
less than the 80-100 ft/sec as recommended by Pope and
Goin [1]. The lower limit is less than the recommended
value of 10 ft/sec, which causes some heat convection
from the wall to the adjacent air.
3.7 Nozzle
The nozzle is hand actuated and has a Mach number
range of 1.5 to 4.0. The maximum pressure rating of the
nozzle is 200 psi. It is symmetric in the vertical and
horizontal direction. The area converges horizontally at
the entrance and vertically at the throat. The nozzle
consists of a rigid contoured throat section and
downstream flexible plates. The throat area can be
preset to a fixed position prior to a test for fixed Mach
number operation. This provides a uniform exit flow
over the entire Mach number range of the nozzle. The
nozzle exit has a 100 mm square cross section. Fig. 7.
illustrates the nozzle components upper plank and lower
plank.
Fig. 4. Storage tank
3.5 Gate Valve
The storage tank that is located upstream of the
supersonic wind tunnel is isolated from the rest of the
tunnel by 6 inch diameter gate valve which can regulate
pressure up to 300 psi, the gate valve is shown in Fig. 5.
Fig. 7. Nozzle upper plank and lower plank
3.8 Test Section
Fig. 5. Gate valve
3.6 Settling Chamber
A side view of the test section is shown in Fig. 8. The
test section is enclosed by a 2 cm casing, with glass
windows on the two sides of the test section to provide
substantial optical access. A model can be attached to a
strut and installed from the top opening of the test
section. It is also possible to mount models through
attachments in the diffuser.
Fig. 8. Test section
Fig. 6. Settling chamber
The configuration of the settling chamber is shown in
the Fig. 6. The settling chamber has a pressure rating of
3.9 Diffuser
The diffuser captures the flow from the test section. One
of the factors that contribute to an increase in the power
International Journal of Earth Sciences and Engineering
ISSN 0974-5904, Vol. 10, No. 01 SPL, March 2017, pp. 18-23
KALAKANDA ALFRED SUNNY, NALLAPANENI MANOJ KUMAR,
HARITHRA. M, AND RAJAKUMAR S. RAI
requirement of a supersonic wind tunnel is the
aerodynamic irreversibility in the diffuser. A typical
constant area diffuser consists of a conventional
subsonic geometry diffuser preceded by a long constant
area duct. This type of diffuser shown in Fig. 9. gives
nearly the same recompression ratio as a normal shock
even though the geometry is quite simple compared
with a variable area diffuser. The compression occurs
through a system of oblique shocks interacting with the
boundary layers. This is not the most efficient way to
recover the pressure but it is often more practical and
very stable under different Mach numbers.
due to choked flow in the throat. Equation. 2 gives the
mass flow rate for ideal conditions which is a function
of stagnation conditions and gas constant R, which is
287 KJ/kg-K for air.
1
P A * 2 1
m 0
T0 R 1
(2)
The throat area was calculated using the Mach-Area
relations to determine the throat area as a function of the
test section area At = 0.01 Sq. m, using Equation. 3.
1
At
1 2
1 2 2( 1)
M )
(
1
A* M 1
2
(3)
Throat area calculations are carried out for Mach
numbers ranging from 1.5-3.5 with different stagnation
pressures and are tabulated below in Table 1.
Table 1: Throat area for different Mach number
M
1.5
2.0
2.5
3.0
3.5
Fig. 9. Diffuser
4. Performance Test
Wind tunnel runtime is normally the amount of time the
wind tunnel operates for a specific Mach number. For
any tunnel with limited time operation, the duration of a
run is of vital importance, as it imposes limitations on
the type of tests that can be performed. A run was
considered to start when the valve opened, allowing air
to flow through the tunnel.
A preliminary runtime calculation was carried out
before the design was implemented. After developing,
wind tunnel was made to run for different Mach number
and stagnation pressure combinations and the time was
recorded and the performance is compared.
4.1 Theoretical Runtime
The runtime of the blow down supersonic wind tunnel is
represented by Equation. 1 and is the function of mass
flow rate ṁ, the initial pressure Pi, the final pressure Pf,
the initial temperature Ti, the tank volume Vt,
polytrophic coefficient n and the gas constant R.
Assuming a reasonable 50% pressure loss through the
pressure regulating valve the final pressure is obtained.
The polytrophic coefficient was chosen to be 1.
P
Vt Pi
t
1 f
Pi
m RTi
1
n
(1)
The first parameter required before for the runtime
calculation was the mass flow rate, which is a constant
A/A*
1.002
1.687
2.637
4.235
6.789
A* (Sq. m)
9.98*10e-3
5.93*10e-3
3.80*10e-3
2.36*10e-3
1.47*10e-3
Table 2. Represents the mass flow rate values and they
are estimated by carrying out calculations for Mach
numbers ranging from 1.5-3.5 with different stagnation
pressures Po (Po = 4, 6, 8) at stagnation temperature T o
(To=300 K).
Table 2: Mass flow rate calculation
M
1.5
1.5
1.5
2
2
2
2.5
2.5
2.5
3
3
3
3.5
3.5
3.5
Po
(bar)
4
6
8
4
6
8
4
6
8
4
6
8
4
6
8
To
(K)
300
300
300
300
300
300
300
300
300
300
300
300
300
300
300
A*
(Sq. m)
9.98*10e-3
9.98*10e-3
9.98*10e-3
5.93*10e-3
5.93*10e-3
5.93*10e-3
3.80*10e-3
3.80*10e-3
3.80*10e-3
2.36*10e-3
2.37*10e-3
2.36*10e-3
1.47*10e-3
1.47*10e-3
1.47*10e-3
ṁ
(kg/sec)
9.32
13.98
18.64
5.54
8.31
11.08
3.55
5.32
7.09
2.20
3.31
4.41
1.37
2.06
2.74
Table 3. Represents the theoretical runtime, which was
calculated as per the Equation. 1. While calculating this
mass flow rate ṁ, the initial pressure Pi, the final
International Journal of Earth Sciences and Engineering
ISSN 0974-5904, Vol. 10, No. 01 SPL, March 2017, pp. 18-23
Development and Performance Investigation of Supersonic Blow-down Wind Tunnel
Table 3: Theoretical Runtime calculation
M
1.5
1.5
1.5
2
2
2
2.5
2.5
2.5
3
3
3
3.5
3.5
3.5
V
(m3)
14.14
14.14
14.14
14.14
14.14
14.14
14.14
14.14
14.14
14.14
14.14
14.14
14.14
14.14
14.14
ṁ
(kg/sec)
9.32
13.98
18.64
5.54
8.31
11.08
3.55
5.32
7.09
2.20
3.31
4.41
1.37
2.06
2.74
Ti
(K)
300
300
300
300
300
300
300
300
300
300
300
300
300
300
300
Pi
(bar)
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
Pf
(bar)
6
9
12
6
9
12
6
9
12
6
9
12
6
9
12
t
(sec)
25
13
7
42
22
12
65
34
18
104
55
30
168
88
48
4.2 Experimental Runtime
The wind tunnel is made to be operated at Mach
numbers ranging from 1.5-3.5 with different stagnation
pressures Po (Po = 4, 6, 8). At this time the run time of
the tunnel was obtained by conducting the runtime test
and the experimental runtime values are tabulated below
in Table 4.
Table 4: Experimental runtime at different mach
numbers and stagnation pressures
M
1.5
1.5
1.5
2
2
2
2.5
2.5
2.5
3
3
3
3.5
3.5
3.5
Po (bar)
4
6
8
4
6
8
4
6
8
4
6
8
4
6
8
t (sec)
17
10
6
36
14
9
43
23
14
84
34
23
112
65
35
4.3 Comparison of Theoretical and Experimental
Runtime
It is observed that the tunnel operates at the range as
predicted during the preliminary calculations.
In addition to this, an account of heat, pressure and
friction losses, shockwave formations the wind tunnel
show a steady operation at the Mach range of 1.5-3.5.
Table 5. Shows the comparison of theoretical and
experimental runtime for the designed supersonic blow
down wind tunnel. It has been observed that a slight
variations and at certain stagnation pressures there is
deviation.
Table 5: Comparison of Theoretical and Experimental
Runtime
Runtime in t (sec)
P0 (bar) Theoretical Experimental
Runtime
Runtime
4
25
17
13
6
10
8
7
6
4
42
36
6
22
14
8
12
9
4
65
43
6
34
23
8
18
14
4
104
84
6
55
34
8
30
23
4
168
112
6
88
65
8
48
35
M
1.5
1.5
1.5
2
2
2
2.5
2.5
2.5
3
3
3
3.5
3.5
3.5
5. Investigation and Discussion
The variation of mass flow rates and runtime with
stagnation pressure was investigated. Fig. 10. shows that
the mass flow rate increases with the increase in
stagnation pressure. This infers that for a fixed Mach
number the mass flow rate can be increased or
decreased by varying the stagnation pressure and
temperature. Thereby the required velocity can be
obtained for a particular test.
Mass flow rate (kg/sec)
pressure Pf, the initial temperature T i, the tank volume
Vt, polytrophic coefficient n and the gas constant R
were considered.
20
Mach 1.5
15
Mach 2
10
Mach 2.5
5
Mach 3
0
0
5
Stagnation pressure (bar)
10
Mach 3.5
Fig. 10. Variation of mass flow rate with stagnation
pressure
Fig. 11 and Fig. 12 shows the variation of run time with
respect to stagnation pressure for different Mach
International Journal of Earth Sciences and Engineering
ISSN 0974-5904, Vol. 10, No. 01 SPL, March 2017, pp. 18-23
KALAKANDA ALFRED SUNNY, NALLAPANENI MANOJ KUMAR,
HARITHRA. M, AND RAJAKUMAR S. RAI
Runtime (sec)
numbers. Reduction in the run time with the increase in
the stagnation pressure is seen. This shows that there is
always time limitation for higher Mach number tests. In
order to obtain high speed for a long time, the tunnel is
to operate at the lowest possible stagnation pressure
possible.
450
400
350
300
250
200
150
100
50
0
Mach 3.5
Mach 3
Mach 2.5
Mach 2
Mach 1.5
List of Symbols
t
Vt
At
A*
M
ti
Pi
Pf
To
Po
n
γ
ṁ
R
tunnel runtime, sec
storage tank volume, m3
test section area, m2
nozzle throat area, m2
Mach number
initial temperature in the tank, K
initial pressure in the tank, bar
final pressure in the tank, bar
stagnation temperature, K
stagnation pressure, bar
polytrophic index(n=1)
ratio of specific heats (γ=1.4)
mass flow rate, kg/sec
gas constant, (R=287 J/kg-K)
Acknowledgment
4
6
8
Stagnation Pressure (bar)
Fig. 11. Variation of theoretical runtime with
stagnation pressure
We would like to thank Karunya University for support
and fund offered during the establishment of this
facility. We also thank V. Kumar, Lab Technician (GRII) and Venkatesh for helping us in setting up the
facility.
Runtime (sec)
References:
120
100
80
60
40
20
0
Mach 1.5
Mach 2
Mach 2.5
Mach 3
Mach 3.5
4
6
8
Stagnation Pressure (bar)
Fig. 12. Variation of experimental runtime with
stagnation pressure
6. Conclusion
A supersonic blow-down wind tunnel, using air from
the compressed air plant and exhausting to atmosphere,
has been engineered and built based on conventional
design principles. Nozzle blocks from Mach 3.5 and
parallel duct supersonic diffuser has been installed. Run
time varies in seconds for various mach numbers and at
stagnation pressures and the Reynolds number can be
varied by changing the density level (by changing
pressure) in the settling chamber. It has been observed
that there is a slight variations in experimental runtimes
compared to theoretical runtimes and at certain
stagnation pressures there is slight deviation also.
However, the developed supersonic blow-down wind
tunnel has been operating satisfactorily.
[1] Pope A., and Goin K. “High-Speed Wind Tunnel
Testing”, John Wiley & Sons; New York, 1965.
[2] J. Njock Libii, “Wind tunnels in Engineering
Education”, Chapter in Wind Tunnels and
Experimental Fluid Dynamics Research, In Tech,
2011.
[3] Kelly Butler, David Cancel, Brian Earley, Stacey
Morin, Evan Morrison, and Michael Sangenario,
“Design and Construction of a Supersonic Wind
tunnel”, Major Project Thesis from Bachelor of
Science, Department of Aerospace and Mechanical
Engineering, Worcester Polytechnic Institute,
March 2010.
[4] Bhavin K Bharath, “Design and Fabrication of a
Supersonic Wind tunnel”, International Journal of
Engineering and Applied Sciences, 2 (5), PP. 103107, May 2015.
[5] Kenney, J.T., Webb, L.M.: A Summary of the
Techniques of Variable Mach Number Supersonic
Wind Tunnel Nozzle Design, AGAR Dograph
3,1954.
International Journal of Earth Sciences and Engineering
ISSN 0974-5904, Vol. 10, No. 01 SPL, March 2017, pp. 18-23