ani

Under guidance of
Professor S K Sane and Dr. Hemendra Arya


















Center for Aerospace System and Design Engineering
Department of Aerospace Engineering
Indian Institute of Technology, Bombay
August, 2003
2

Abstract


An experiment analyzing performance of wind tunnel and co-relating theoretical
analysis is presented. Various losses at different sections in tunnel have been calculated
and total pressure drop across the wind tunnel has been estimated using them.
Accordingly, total power required or total power invested to run a open circuit wind
tunnel is calculated at different flow velocities. An experiment is conducted to measure
the electrical power consumed by wind tunnel fans at different velocities. Calculated
power required and measured power input will give the efficiency of system.

Second task was to determine the input power required at flow velocities beyond
the maximum achievable flow velocity in tunnel. That is the present tunnel system
provides maximum velocity of 10 m/s and estimation has been conducted to calculate
power required at 25 m/s. For the same purpose trend of results of experiment was
extrapolated and again efficiencies were obtained at velocities beyond 10 m/s.

Nomenclature = Density (kg/m
3
) a
= Air density (kg/m
3
)
R = Universal gas constant (J/kg.K)
T = Temperature (K) p = Pressure drop (Pa)
Dp = Dynamic pressure head (Pa)
P = Pressure (Pa) w =
Density of water (kg/m
3
)
V = Flow Velocity (m/s)
g = Acceleration due to gravity (m/s
2
)
h = Manometeric height (mm of H
2
O)
P
t
= Total pressure (Pa)
Q = Volume flow rate (m
3
/s)

Contents

Abstract

Nomenclature
1.

Introduction
2.

Calculating theoretical power required at different flow speeds
2.1 Theory
2.2 Calculation of all K
i
s
2.3 Calculations and results
3.

Experiment to measure power consumed by fans at different flow velocities
3.1 Apparatus
3.2 Theory
3.3 Description
3.4 Precautions
3.5 Observations
3.6 Calculations and Results
4.

Extrapolation to determine the power consumption at higher velocities
5.

Co-relating theoretical and experimental results
3
6.

Conclusions
References
Appendix
A) Wattmeter Multiplier chart
B) Matlab Code
C) Charts for losses
Acknowledgement

1.

Introduction

In most basic sense, wind tunnels are ground-based experimental facilities
designed to produce flows of air (or sometimes other gases), which simulate natural flows
occurring outside the laboratory. For most aerospace engineering applications, wind
tunnels are designed to simulate flows encountered in the flight of airplanes, missiles or
space vehicles. Since last 150 years, man has always constantly endeavored in the
direction of improving performance and efficiencies of tunnel system i.e. getting
maximum velocity at minimum input power. This exercise proceeds in the same
direction.
An open wind tunnel was fabricated at IIT Bombay (Image 1) during the 2002-
summer for the purpose of designing and developing an MAV (Mini Aerial Vehicle). The
size of test section is 1m X 1m X 1.25m as shown in Fig. 1 and one can use it for the
range of velocities from 0 m/s to 10 m/s. Air inside the tunnel is circulated using six
single-phase motor-driven fans and voltage is controlled by 230 V range Auto-
Transformer. In near future the fans of this wind tunnel will be upgraded to provide
velocities in the range of 20 m/s. The sole purpose this exercise is to predict
approximately electrical power requirement and efficiency at that speed.

Image 1: Wind tunnel set up
2

4

When air flows through a channel of varying cross-section then it undergoes the
change in velocity as per the continuity principle. If there is rise in static pressure then the
dynamic pressure will drop down and vice versa. Therefore, it appears that the total
pressure, which is the sum of static pressure and dynamic pressure, remains constant.
However, in practice these velocity changes are not loss free. Some energy will be lost
(or rather turned into heat which is not significant when the fluid is incompressible). The
loss in each element of the system (bend, duct length, branches, and obstructions) is
dependent on average velocity through it. Here it was assumed that flow is uniform and
velocity is constant throughout the crossection. All these losses are measured in terms of
pressure drop ultimately which provides power required. On other hand an experiment is
conducted to measure the power supplied to fans at different flow velocities. Assuming
the fact, power is proportional to cube of flow velocities the results obtained are
extrapolated to obtain power consumption at higher velocities. Then theoretical and
experimental results at measured range and at extrapolated range are co-related to analyze
the general trend of power consumption and efficienc y of system.

Number over text as superscript denotes the number of reference given at the end of report.
Figure 1 : Wind Tunnel Details
2
5
2. Theoretically Calculating Power required at different flow speeds



Final output power produced by fans in an open circuit wind tunnel is invested in
overcoming various losses and ultimately flow possessing kinetic energy (Rate of energy
is power) is thrown into the ambient atmosphere. This total output power is power
required to run the wind tunnel. Here in this section this power will be calculated
theoretically at different speeds.

2.1 Theory

If p is total pressure drop in (Pa) and Q is Volume flow rate (m
3
/s) then power required
in (W) is given as

P
req
= p*Q







(1)
The total pressure drop between point 1 and point 2 due to losses is given as-
ct lengths
ion in du
e to frict
drop du
V
K
t
P
t
P
+
×
×
×
= 2
1
2
1
5
.
0
In other way,
2
1
p
p
p + =






(2)
Where the value of constant K is sum of all K
i
s calculated for each different type
of loss.
The different losses are -
1.

Losses at entry to the system from atmosphere.
2.

Losses at changes of duct area or shape.
3.

Losses at bend and changes of direction.
4.

Losses at division of flow into branches.
5.

Losses caused by obstructions, grills and louvers.
6.

Losses at discharge from system to atmosphere

2.2 Calculation of all K
i
s

1) Loss at inlet


Radius of inlet (curved part) r =0.125m,
Equivalent diameter d=1.1m
r/d = 0.1136 (r is the radius at inlet and d is the mean diameter)
From image 2, Appendix C, K
1
= 0.06 for r/d =0.125

2) Loss at obstructions,

Aerofoil dimension is 0.5*0.01
Area blocked due to obstruction = 0.5 * 0.01 = 0.005 m
2
Total wind tunnel cross section area = 1m
2
Blocked area / total area = 0.005
From image 3, appendix C, for streamlined strut K
2
= 0.01
Blocked area due to tube= length of tube*diameter of tube.
= 0.5*0.01
= 0.005
From image 3, appendix C, for round tube K
3
= 0.01

6
3) Pressure drop in duct diffuser,

A
2
=

1.6*2.4 =3.84 m
2
,

A
1
= 1m
2

A
2
/ A
1
= 3.84 (A
1,
A
2
are the inlet and outlet areas of the duct)
Calculating average height (a) and width (b) for tunnel
a = 1.3 m, b = 1.7
Calculating D
e
from eqn 4,
D
e
= 1.6 m
L = Length of diffuser = 3.75 L/ D
e
=2.343 K
4
= 0.45 (From Image 4, appendix C)

4) Loss at outlet,

For losses at outlet,
K
o
= 1 (From Image 5, appendix C)
But at outlet, velocity is different from that of the test section. For incompressible flow
continuity eqn is given as
A
1
.V
1
= A
2
.V
2








(3)
Where A
1
, V1 and A
2
, V
2
are area of crossection and velocity at test-section and outlet
respectively. V
2
= (A
1
/A
2
).V
1
= 0.2604.V
1
Pressure lose at outlet = ½.K
o
. .(V
2
)
2
= ½. (0.0678). . (V
1
)
2
Resultant K for losses at outlet = K
5
= 0.0678

5) Loss due to the duct friction,

This loss depends on equivalent diameter and the flow velocity. Equivalent diameter is
given as
1



(4)


Where a and b are dimensions of rectangular duct. Equivalent diameter for straight channel = 1.1 m
And Equivalent diameter for diffuser section = 1.51 m.


Using these values, the loss is calculated for different velocities with the he lp of
graph in image 6, appendix C. While calculating the duct friction loss, additional
factor of 1.25 was assumed to take into account bolts within the ducts. Loss is
calculated as shown in calculations.

Area of crossection of test section = 1 m
2 Flow velocity = Volume flow rate

2
.
0
6
.
0
6
.
0
)
(
265
.
1
b
a
b
a
D
e
+
=
7
2.3 Calculations and results:

1) Total pressure drop calculation due to all K
i
The value of resultant K is obtained by summing all values of K
i
s.
6028
.
0
0678
.
0
45
.
0
01
.
0
01
.
0
065
.
0
5
1
5
4
3
2
1
=
+
+
+
+
=
+
+
+
+
=
= =
=
i
i
i
K
K
K
K
K
K
K


(4)
Then, P
1 =
pressure drop = K * 0.5 * a
* (V
1
)
2





(4A)
Where, a
=

1.225 kg/m
3
.
This pressure loss is calculated at different test-section velocities (Table 1)