Wind Tunnel Testing and Measurement of Aerodynamic Flow Parameters

(University of Lagos Subsonic Wind Tunnel as Case Study)

Uduak O. Inyang-Udoh100404038

Department of Mechanical Engineering, University of Lagos.

This paper discusses the role of wind tunnel testing in the field of aerodynamics. It gives a basic description of

the wind tunnel, and highlights – based on configuration and application – the different kinds available.

Specifically, it shows what effects or parameters are studied by the testing. Moreover, it demonstrates how in

practice the wind tunnel is used, including how results of desired parameter may be obtained from an actual

test. Finally, worthy of note, it discusses other applications of the testing.

Keywords: airflow, blow-down tunnel, diffuser, nozzle, model, drag, instrumentation, lift, Mach number, open-

circuit, pitot tube, return-circuit , settling chamber, test section

Nomenclature

c = speed of sound, m/s

CD = Drag coefficient

CL = Lift coefficient

g = acceleration due to gravity, in m/s2

k = Specific heat ratio

Ma = Mach number

Re = Reynolds Number

µ = Viscosity

ρ = density, kg/m3

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I. Introduction

As a science, aerodynamics deals with the motion of air or gases past a moving body. It probes into the behavior of

the gas in the body’s vicinity, and what consequent effects that makes on the body. Although the field has firm

theoretical basis, it has relied heavily on experimental procedure to predict actual characteristic patterns of bodies in

airflow.

Aerodynamics, in fact, holds its advance in the past few centuries to painstaking testing or experimentation. Since it

became clear that the interplay between wings and wind was necessary for flight, enthusiasts have sought controlled

conditions for studying that interplay. Pioneering experimenters used whirling arms that bore their test shapes. The

whirling arm confirmed that objects shapes had significantly effect on air resistance (drag); and, although the arm

did help establish that propelling an object quickly in air would produce lift, it faced the drawback of turbulence.

Turbulence significantly impaired experiment results and observation. Scientist needed a better tool to provide

calmer artificial winds for continuing investigation.

In this quest, in 1871, Britain’s Francis Herbert Wenham (1824–1908) designed and operated the first enclosed wind

tunnel. The tool enabled rapid collection of the desired detailed technical data and was to set the ground for one of

the most important invention of the last century. In the late 1890’s, in Ohio, two bicycle sellers – Orville and

Wilbur Wright – were following current developments in aerodynamics and conjuring ideas for glider designs. At

the turn of the twentieth century, the flying-desirous brothers began to test gliders they had made. None had the

lifting power they had counted on. Apparently, the available published tables of air pressures on curved surfaces

were wrong. They needed a quick, controlled and reliable way of testing model wings. And the answer was to set up

their personal wind tunnel.

In 1901, they constructed a 1.8-metre wind tunnel in their shop and began experiments with model wings. They

tested more than 200 different shapes of wing shapes by attaching airfoils to balances for measuring drag and lift [8].

The balances converted airfoil performance into measurable mechanical action that the brothers used to compute

their calculations. From the results of their tests, they made the first reliable tables of air pressure on curved

surfaces. These tables made it possible for them to design a machine that could fly. In 1902, they built their most

aerodynamically-advanced glider and, moreover, in 1903, invented the first aeroplane [8].

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Since then, wind tunnel testing has

helped design engineers and

manufacturers determine aerodynamic

performances as they build vehicles –

ranging form cars to space shuttles –

that must move past air, especially at

considerable speeds. The testing usually

involves subjecting small-sized design

replica (models) to conditions in

proportion to that to be encountered in

service, or proving full sized prototypes

under identical service conditions.

Thorough designs do not eliminate the

need of testing. Testing is imperative to

ascertain that a design will perform as

desired/required under actual operation.

Testing may reveal flaws or

inadequacies in design, prompting

improvement(s) or even a complete

redesign; or augment the designer’s

confidence; making it integral to all

industrial manufacture of propulsion

vehicles.

II. Wind Tunnels: General Description and Types

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Figure 2. NASA wind tunnel with a plane model.

Figure 1. The Wright brothers wind tunnel

The wind tunnel is a ground based testing facility used to study the effects of winds, or airflow, on aircraft and other

vehicles and structures. It provides a controlled environment for simulating the relative motion between the vehicle

or structure and the air. When winds tunnels are used to test vehicles, they create the reverse scenario to what, in

reality, obtains: instead of the air standing still and a vehicle moving at speed through it, the air is moved at speed

past a stationary vehicle. In that way a stationary observer could study the craft in action, and could measure the

aerodynamic forces being imposed on the craft. The steady-state forces on a body held still in moving air are the

same as those when the body moves through still air given the same body shape, speed and air properties [6].

Wind tunnels may be very large and can test full-sized experimental aircrafts; but most are test scaled-down models.

Scaling laws permit the use of models rather than the full-scale crafts. Models are less costly and may be modified

more easily, and conditions may be simulated in wind tunnel that would be dangerous in real-world conditions.

Nevertheless, full-scale test wind tunnels must be used in final examination of prototypes before commercial

production commences.

Wind tunnels, summarily, work by accelerating air towards the test sample. At one end of most wind tunnels, air is

generally blown by electric fans, or other devices, such as pressurized tanks, may be used. A large nozzle then

accelerates the air to the desired speed. The air stream blows into long segment called the test section at uniform

speed. Objects to be tested are positioned in the test section. Thereafter, the wind passes into a diffuser that slows

down the airflow. The vehicle or structure being tested is secured by supports that extend form the ground or from

behind the object. The supports are fastened to measuring devices that record the force of the airflow on the vehicle

or structure. Instruments can also measure surface pressure at many places on the object. The interior of tunnel walls

must be especially smooth and preferably rounded to prevent frictional losses.

Wind tunnels are of various types depending on the air speed attained. Those in which the speed is close to that of

sound – about 1, 225 km/h – are called transonic tunnels. Subsonic wind tunnels have their air speed significantly

less than the speed of sound. Wind tunnels in which the air travels faster than the speed of sound are supersonic

tunnels. In hypersonic tunnels, air speeds are more than five times as fast as the speed of sound. Hypervelocity

tunnels yet have higher speeds than those of hypersonic tunnels.

III. Compressible Flow Background

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The airflow source, the geometry of the nozzle and location of the test section distinguish the various types.

Subsonic and transonic wind tunnels employ axial fans or blowers to pressurize air. Other wind tunnels, instead of

using a fan, maintain pressure difference to cause airflow by utilizing high pressure tanks and/or vacuum tanks. The

nozzle in a subsonic tunnel is a convergent section. Nozzles of all other kinds – transonic, supersonic and hypersonic

– initially converge and later diverge. But whereas the model in transonic nozzle is positioned in the midst of the

section (throat), the test section in supersonic and hypersonic tunnels comes at the end of the nozzle section. The

reasons for these are apparent from the study of compressible flow.

The ratio of flow to the speed of sound is generally termed the Mach number.

Ma=Vc

Hence, a flow with Ma<1 is subsonic, Ma≈1 transonic, Ma> 1 supersonic and Ma >>1 hypersonic. The sonic point

is Ma=1. If the continuity equation and the Bernoulli’s equation in differential form are combined, that is,

dρρ

+ dAA

+ dVV

=0

dPP

+VdV =0

for a steady, isentropic flow the following equation may be obtained

dAA

=−dVV

(1−Ma2)

This equation governs the shape of a nozzle or a diffuser in subsonic or supersonic isentropic flow. Noting that A

and V are positive quantities, the following are submitted:

For subsonic flow (Ma < 1), dAdV

<0

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For supersonic flow (Ma > 1), dAdV

>0

For sonic flow (Ma = 1),dAdV

=0

Thus the proper shape of a nozzle depends on the highest velocity desired relative to the sonic velocity. To

accelerate a fluid, we must use a converging nozzle at subsonic velocities and a diverging nozzle at supersonic

velocities. The highest velocity that can be achieved by a converging nozzle is the sonic velocity which occurs at the

exit of the nozzle.

Figure 1. Wind Tunnel Design (Source: NASA)

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Also, based on the continuity and Bernoulli’s equations

dAdV

=dPρ

(1−Ma2)

For a subsonic flow, the term 1−Ma2 is positive; and thus dA and dP must have the same sign. That is, the

pressure of the fluid must increase as the flow area of the duct increases and must decrease as the flow area of the

duct decreases. Thus, at subsonic velocities, the pressure decreases in converging ducts (subsonic nozzles) and

increases in diverging ducts (subsonic diffusers).

In a supersonic flow, the term 1−Ma2 is negative, and thus dA and dP must have opposite signs. That is, the

pressure of the fluid must increase as the flow area decreases and vice versa. To accelerate air to supersonic

velocities, a diverging section must be added to a converging nozzle. The result is a converging-diverging nozzle.

The air first passes through a subsonic (converging) section, where the Mach number increases as the flow area of

the nozzle decreases, and then reaches the value of unity at the nozzle throat. The air will continue to accelerate as it

passes through the supersonic (diverging) section with dramatic decrease in density to keep up with the mass flow

balance(m= ρ AV ).

Whether or not the exit velocity of a given nozzle will be equal to what is desired depends on the relative pressure of

the nozzle outlet to that of the inlet. In subsonic wind tunnels, fans or blowers are sufficient to create the sufficiently

low outlet pressure (or back pressure). The back pressure determines the conditions of flow in the nozzle sections.

In supersonic and hypersonic, pressure tank or vacuum tanks are needed to maintain smaller back pressure to inlet

pressure ratio. This is necessary to attain the required tremendous speeds in these tunnels.

IV. Subsonic Wind Tunnels

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Subsonic or low- wind tunnels operate on speeds up to 480km/h. A low-speed tunnel has five fundamental features:

the settling chamber, contraction cone, test section, diffuser and drive section. Because air is generally chaotic and

swirls as it enters the tunnel, the settling chamber is necessary before the test section to settle and straighten the air,

often through the use of panels with honeycomb-shaped holes or a mesh screen. The settling chamber is also known

as plenum. The air is then immediately forced through the contraction cone, a constricted space that greatly

increases airflow velocity. The contraction cone is the nozzle of subsonic tunnels. The section decreases in area

downstream to achieve the desired velocity increase.

The model to be studied is placed in the test section, which is where sensors record data and visual observations are

made. The air subsequently flows into the diffuser, which has a conical shape that widens, and thus, smoothly slows

the air's velocity without causing turbulence in the test section.

The drive section houses the axial fan that creates high-speed airflow. In some tunnels the fan is always placed

downstream of the test section, at the end of the tunnel, rather than at the entrance. This setup allows the fan to pull

air into a smooth stream instead of pushing it, which would result in a more “choppy” airflow. For very large wind

tunnels several meters in diameter, a single large fan is not practical, and so instead an array of multiple fans are

used in parallel to provide sufficient airflow. The airflow created by the fan that is entering the tunnel section is

itself highly turbulent due to the fan blade motion, and so is not directly useful for accurate measurements. The air

moving through the tunnel needs to be relatively turbulence-free and laminar. To correct this problem, closely

spaced vertical and horizontal air vanes are used.

Low-speed wind tunnel may be open-circuit or closed-circuit. The former is open at both ends and is especially

appropriate for testing air-burning engines. Airflow through the tunnel circuit is drawn from the upstream ambient

air and discharged into the ambient air downstream. Open wind tunnel come either as blow type, where the blower is

positioned upstream; or suction type in which the fan or blower is downstream.

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Figure 2. Open Wind Tunnel (Source: NASA)

In the Closed-circuit tunnel, wind is re-circulated within the duct; hence its name. It also termed return-flow or

Prandtl tunnel. Here, the air is conducted from the exit of the test section, via a diffuser, back to the fan by series of

turning vanes. The fan increases the pressure of the air to overcome the friction losses in the tunnel circuit. Airflow

from the fan diffuses out and is made to pass through vanes to smooth out the turbulent airflow before reaching the

subject of the testing again. In comparison to the open-circuit tunnel, the closed-circuit tunnel has a low operating

cost. Once the air is circulating in the tunnel, the fan and motor only need to overcome losses along the wall and

through the turning vanes. The fan does not have to constantly accelerate air. Also, relative to the open-circuit

tunnel, it has superior flow quality and quieter operation. It is however more expensive to construct because of the

added vanes and ducting. It is usually the choice for large wind tunnels.

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Figure 3. Closed Wind Tunnel (Source: NASA)

V. Other Types

Transonic wind tunnels are capable of testing speeds from 0.7 to 1.4 Ma. Transonic wind tunnels are very common

in the aircraft industry as most aircrafts operate around this speed. In compressible fluid flow, as earlier noted sonic

speed can only occur when the cross-section is a minimum. Therefore, the test model is located at the throat where

sonic speed occurs. If additional power is supplied in the tunnel, it only causes supersonic flows downstream.

Transonic speeds are generally characterized with the presence of shock waves. Shock waves are associated with the

breaking of the sonic barrier. They occur as air moving at or above the sonic velocities meet surrounding air and

they result in dissipation of irrecoverable energy. The reflection of the shock waves from the walls of the test section

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presents a problem in the transonic wind tunnel. Therefore, perforated or slotted walls are required to reduce or

cancel the shock reflection from the walls.

Supersonic wind tunnels test speeds form Ma= 1.0 to 5. For supersonic speeds, the test section must be designed for

the particular Mach number required. As noted, when air accelerates past the speed of sound, it expands more

rapidly than it accelerates, so that the tunnel must be larger downstream than it is at the minimum section, where the

speed is sonic. The final Mach number is uniquely determined by the ratio of the final area to the throat area, A¿ .

AA¿=

1Ma [( 2

k+1 )(1+ k−12

Ma2)](k +1) /[2 ( k−1 )]

Most industrial (supersonic) tunnels operate intermittently from energy stored in high-pressure air tanks. The air is

discharged through a fast-acting regulator valve to the tunnel and exhausted to the atmosphere. This type is known

as the intermittent supersonic blow-down wind tunnel. The indraft, or vacuum-driven, tunnel type, on the other

hand, allows atmospheric-pressure air to flow through the wind tunnel into a vacuum tank. Advantages of the indraft

system include a constant (atmospheric) stagnation pressure and temperature, greater safety, and less noise and cost.

The principal problems in the design of a supersonic tunnel are to provide sufficient pressure ratio to start and

sustain the flow, and to supply adequate dry air. Drying of air to a dew point of about – 40°C at atmospheric

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Figure 4. Supersonic Wind Tunnel (Source: Wikipedia)

pressure may be required to avoid uneven flow and condensation shock [6]. No wall interference effects are felt in

supersonic flow, provided reflecting shock waves do not strike the model.

A supersonic tunnel operating in the range of Ma = 5 to 15 is regarded as hypersonic. When the Mach number is

above 4 or 5, extremely high pressure ratios are required and the temperature drop in the test section is so large that

liquefaction of the air would result unless the air is first heated. Most hypersonic tunnels operate intermittently and

use both the high-pressure stagnation air and a vacuum-discharge tank to obtain the overall pressure ratio required.

VI. Measurements and Instrumentation

Specifically designed mechanical, electrical, and optical devices are used to measure effects of airflow across a

model. The wind tunnel testing has always required instrumentation that is rugged, reliable and accurate. High costs

of operating wind tunnels have brought demands for shorter run time and higher productivity. To fulfill these needs,

modern instrumentation has been developed which can acquire accurate data quickly and provide information

directly during test. Measurements common to most tunnels are pressure, temperature, turbulence and flow

direction. In addition, flow visualization is common in wind tunnel testing.

The earliest method used to measure a large number of pressures in tunnel tests is with multi-tube manometer

boards. Single pitot tubes can also used to obtain multiple reading downstream the model. Pressure distributions can

more conveniently be measured by the use of pressure-sensitive paint, in which higher local pressure is indicated by

lowered fluorescence of the paint at that point. Many techniques have now been developed to provide direction

conversion of pressure to electrical signals. Individual strain gauge, capacitance, or force balance transducers may

be used. When pressure fluctuations are significant, the electronically scanned pressure (ESP) measuring system

allows for accurate measurements.

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Most temperature measurements in tunnel tests are made with devices as thermocouples, thermistors, and platinum

resistance temperature sensors. The airstream temperature measured is the total, or stagnation1, temperature. The

static, or actual temperature is computed from this measurement.

TO

T=1+( k−1

2 )Ma2

T 0=T+ V2 CP

2

The level of velocity fluctuations is important in tunnel testing because it influences the point on the model at which

the boundary layer changes from laminar to turbulent. This point of transition affects the aerodynamic drag forces

on the model. Two instruments used to measure turbulence are the thermal anemometer and the laser velocimeter.

Thermal anemometer is used to obtain the instantaneous velocity and flow angle measurement. The laser

velocimeter is an optical technique that provides remote measurement of mean velocity and turbulence in the flow

field of a tunnel model.

The flow direction and the degree to which the flow is parallel to the tunnel walls are vital in the analysis of pressure

and forces on a test model in a wind tunnel. Several devices –such as yaw sphere, wedge, cone and vane –are used,

depending on the flow velocity.

Flow visualization is most commonly carried out by injecting solid, liquid or gas particles into the stream, and

viewing them in reflected or dispersed light It is assumed that the particles have a very low inertia and acquire the

local direction of motion of the fluid, and that they are of sufficiently small weight to preclude any disturbances due

to gravity. Visualization techniques include smoke filaments, the observation of very small particles which occur

naturally in the stream and can be seen with the aid of a microscope and an intense light source, and the observation

of fixed tufts, used widely for investigations near the surface of a body.

1 A stagnation property is the value of that property the air possesses if/when it is brought to rest. The actual property the air possesses while it is still flowing air is the static property.

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VII. Similarity and Lift and Drag

The basis for model testing is that there is some identifiable way the model relates to its full-sized object

dimensional similarities. Reynolds number is a non-dimensional parameter representing the ration of the momentum

forces to the viscous forces in the flow.

ℜ= ρVLμ

w h ere L ,∈general , repesents t h e object dimension

Osborne Reynolds demonstrated in experiment s that the fluid flow over a scale model would be the same for the

full scale object if these parameters combination, or the Reynolds number, were the same in both cases. For

example, a ¼-scale model tested at the flight velocity of its full-size counterpart would only simulate the real world

system (atmosphere) only if the pressure on it is four times that of the atmosphere; for then would they be similar,

having the same Reynolds number.

The force that air exerts on an object moving past it is called drag. The drag force can be measured directly by

simply attaching the body subject to the fluid flow to a calibrated spring and measuring the displacement tin the flow

direction. Drag is undesirable in the movement of bodies through air and usually sought to be minimized. Reduction

in drag is closely associated in reduction in fuel consumption of vehicles. On the other hand, as a body moves past a

fluid, normal pressure plus a tangential shear acts to create lift. Airplane wings are shaped and positioned to create

lift at minimal drag.

The drag and lift forces depend on

the density of the fluid, the upstream

velocity, and the size, shape and

orientation of the body among other

things, and it is not practical to list

these forces for a variety of

14Figure 5. Forces on a wing advancing in air

situations. Instead, dimensionless numbers, that represent the drag and lift characteristics of the body, is convenient

to work with. They are:

Drag coefficient C D=FD

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ρV 2 A

Lift coefficient CL=F L

12

ρ V 2 A

W here FD∧FL are t hedrag∧lift components of t h e force t h e air ex erts respectively

The fluid forces may also generate moments and cause the body to rotate. The moment about the flow direction is

called the rolling moment, the moment about the flow direction is called the yawing moment, and the moment about

the side force direction is called the pitching moment.

VIII. University of Lagos Subsonic Wind Tunnel

The university of Lagos subsonic wind tunnel is a blower type open circuit wind tunnel. A blower driven by an AC

electric motor draws in air at the inlet into the rather short diffusing section. the inlet cross section measure 32 by 49

in. and is 10 in. long. The diffuser, slowing down the flow, opens into the settling chamber fitted with honeycomb

flow straighteners designed to ensure that the flow is steady in both magnitude and direction and has a flat transverse

velocity profile. The next section is the nozzle with four curved walls wherein the airflow is accelerated before it

enters the open test section that has the same cross section as the nozzle, 18 by 18 in. and is 27 in, long. The test

section has transparent walls on its sides for observation and is fitted with heavy instrumentation.

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Figure 6. UNILAG Subsonic Wind Tunnel

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Figure 7. Side view showing the four curved walls converging nozzle, settling chamber, diffuser and motor-driven blower

A wide range of measurements and demonstrations is typically possible with such a wind tunnel including: flow

visualization studies around an aerofoil; measurement of pressure distribution around an aerofoil at various angles of

attack and around a cylinder; measurement of pressure distribution measurement of lift and drag on an aerofoil with

leading edge slot and trailing edge flap; velocity and pressure distribution measurements using a Pitot static tube and

yaw probe measurement of drag for a selection of models of different shapes but common equatorial diameter;

calibration of the Wind Tunnel velocity indicator using a Pitot static tube and inclined manometer.

Pressure with Velocity Measurement. Pressure is measured using a manometer board equipped with several tubes

for simultaneously obtaining pressure distribution over the test body. From fluid statics relation:

pb=pa−ρg(hb−ha)

Where pb is the body surface pressure, pa is the atmospheric pressure. the tops of the liquid in the tube connected to

the body and open to the atmosphere are with heights hb and ha respectively. With pa , ρ and g known, and

(hb−ha) read from the tube the pressure pb can be obtained.

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Figure 8. Test Section Interior with cylinder

Pressure can also measured using the pitot tube. One of the open ends of the tube faces directly into the flow to

measure stagnation pressure, or total pressure. If the static pressure can be measured, say using a static pressure

orifice, the free stream velocity may be obtained via Bernoulli’s equation, that is,

V=√ 2( p0−p)ρ

Where pis the static pressure and p0is the stagnation pressure. It is possible to combine the measurement of both

total and static pressure in one instrument, called Pitot-static probe.

Temperature Measurement. Stagnation temperature is measured in the settling chamber. Since there is practically

not input or removal of heat between the settling chamber and the test section, the stagnation temperature remains

constant. The flow velocity in the settling chamber does not usually exceed some tens of m/sec and the temperature,

some tens of degrees Celsius. Mercury thermometers could be used in this range but resistance thermometers and

thermocouples provide faster operation and permit remote indication. As earlier observed, the static temperature

could be obtained from this measurement.

Flow Visualization. Smoke and tufts are often used to show flow direction. Smoke is widely applied at low flow

velocities (up to 40 or 50m/sec) and consists in injecting smoke filaments into a transparent gas stream through

nozzles or openings in the model. The tuft method consists in fixing light silk threads to thin wires inside the stream.

The threads remain in a definite position in steady flow, but vibrate at points where the flow is non-steady or

turbulent. It is thus possible to establish the flow direction and regime at the surface of a model; quiescence of the

tufts indicates a laminar boundary layer. Behind the point of boundary-layer separation the vibrations of the threads

become very intense. The tuft method is widely used in qualitative analyses of flow around models, since the motion

and location of the tufts can be easily observed and photographed.

Lift and Drag Force Measurement. A lift beam balance is used to measure the lift force in the test section. The lift

apparatus consists primarily of a fulcrum supported beam restrained in rotation by a strain-gauge load cell. An arm

attached to the model will transmit the aerodynamic lift force to the load cell load. For correct operation of the lift

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balance, the load cell was required to be in tension. The lifting moment compresses it. Drag balances are used to

measure drag force.

IX. Other Applications

Wind tunnels are not only helpful in improving aircrafts and spacecrafts, but also in improving a variety of industrial

and consumer products. Automobile makers heavily rely on wind tunnels; modern high-speed train manufactures

also do. The tunnels have had a significant part in the reduction of automobiles drag coefficient to values

significantly lower than those of the earliest cars.

Electronics engineers use small wind tunnels to see how airflow affects heat buildup in components. Then they can

design cooler computer chips and motherboards that last longer. Also, utilities managers use wind tunnels to test

wind turbines used to generate electricity. Wind tunnels help make the turbines and their blades more efficient,

effective and durable, so they can withstand constant, powerful gusts.

The use of wind tunnels for structural testing is yet to be widespread but is one application that could be

ubiquitously exploited. In erecting light structures, the construction industry should find the tunnel helpful especially

when they are exposed repeatedly to the unfavourable elements of weather. Among other things, the tunnel testing

could help suggest the best design contours that would be suitable for prevalent wind conditions and the best plans

pattern for maximal ventilation.

X. Conclusion

Procuring and running wind tunnels, with their test models cost much. Hence there is a general shift to computer

modeling (also called computational fluid dynamics), which is now often used in place of physical models and

tunnels. Computers enable engineers to adjust infinite variables of the model and the test section without time-

consuming (and expensive) manual labour. Physical tunnels are sometimes used only to retest the results of

computer modeling.

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Wind tunnels, however, are still in use, especially in testing final prototypes before commencing commercial

production. “And,” in the words of C. Nathan [4], “even if newer virtual technologies do eventually replace physical

wind tunnels, these marvels of engineering will always have a place in the history of humankind's development.”

References

1Cengel, Y. A., and Cimbala, J. M., Fluid Mechanics: Fundamentals and Applications, 2nd ed., McGraw-Hill, New

York, 2010, Chap. 12.

2Gorlin, S.M., and Slezinger I.I., Wind Tunnels and Their Instrumentation, Israel Program for Scientific Translation

Ltd., Jerusalem, 1966, Chaps 1, 2.

3Josué N. L., “Wind Tunnels in Engineering Education,” Wind Tunnels and Experimental Fluid Dynamics Research,

edited by Prof. J. C. Lerner, Indiana University-Purdue University Fort Wayne, USA., [online scholarly work],

2011, URL: http://www.intechopen.com/books/wind-tunnels-and-experimental-fluid-dynamics-research/wind-

tunnels-inengineering-education [cited 29 May, 2014].

4Nathan, C., How Wind Tunnels Work, HowStuffWorks, Inc [Online article], URL:

science.howstuffwork.com/wind-tunnel.htm [cited 23 April, 2014].

5Selig, M. S., Robert W. Deters, R. W., and Williamson, G. A., “Wind Tunnel Testing Airfoils at Low Reynolds

Numbers,” 49th AIAA Aerospace Sciences Meeting, AIAA 2011-875, Orlando, FL, 2011.

6Shramo, D. J., “Wind Tunnel,” McGraw-Hill Encyclopedia of Science and Technology, Vol. 4, McGraw-Hill, New

York, 1977, pp. 651-659.

7“Subsonic and Transonic Wind Tunnel,” Wikipedia [Online article], URL:

http://en.wikipedia.org/wiki/Sudsonic_and_transonic_wind_tunnel [cited 10 May, 2014]

8“Wind Tunnel,” The World Book Encyclopedia, Vol. 21, World Book Inc., London, 1995, p. 269.

9“Wind Tunnel,” Wikipedia [Online article] URL: http://en.wikipedia.org/wiki/Wind_tunnel [cited 23 April, 2014].

10“Wright brothers,” The World Book Encyclopedia, Vol. 21, World Book Inc., London, 1995, pp. 419, 420.

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