PROJECT REPORT

Closed loop wind tunnel

SUBMITTED BY~:-

Prashant ambastha (108011559)

Amir subhani (10808554)

Ravi kumar (10811327)

under the supervision of “mr. anil kumar reddy”

Mechanical department

CERTIFICATE

This is to certify that AMIR SUBHANI, bearing Registration no. 10808554, has completed

capstone project titled,”CLOSED LOOP WIND TUNNEL” under my guidance and

supervision. To the best of my knowledge, the present work is the result of her original

investigation and study. No part of the dissertation has ever been submitted for any other degree

at any University.

The dissertation is fit for submission and the partial fulfillment of the conditions for the

award of.........................

Signature and Name of the Research Supervisor

Designation

School

Lovely Professional University

Phagwara, Punjab.

Date :

DECLARATION

IAMIR SUBHANI student of B.tech under Department of MECHANICAL of Lovely

Professional University, Punjab, hereby declare that all the information

furnished in this dissertation / capstone project report is based on my own intensive research and

is genuine.

This dissertation / report does not, to the best of my knowledge, contain part of my work

which has been submitted for the award of my degree either of this university or any other

university without proper citation.

Date:

Signature and Name of the student

Registration No. ...........

ACKNOWLEDGEMENT

This project involved the collection and analysis of information from a wide variety of sources

and the efforts of many people beyond us. Thus it would not have been possible to achieve the

results reported in this document without their help, support and encouragement.

I will like to express my gratitude to my mentor Mr. Anil Reddy for his help in the work leading

in this project.

I will also like to thanked to the almighty god so that we become able to do this project in a

united manner.

Abstract

Wind tunnels are tube-shaped facilities that allow engineers to move air over a vehicle as if it were

flying. They help researchers to learn more about how an aircraft will fly. NASA uses wind tunnels to test

scale models of aircraft and spacecraft. Some wind tunnels are big enough to hold full-size versions of

vehicles. By moving air around an object, the wind tunnel simulates the conditions of the object in flight.

The wind tunnels come in a variety of sizes, from those only a few inches wide to those large enough to

test a full-size airplane. Different wind tunnels move air at different speeds .Many wind tunnels use fans

to speed up the air. In a closed-loop wind tunnel, the moving air is brought back to the fan and is

continuously re-circulated through the tunnel. A close-looped tunnel efficiently produces long test

times. Other wind tunnels release pressurized air at very high speeds, for short test times. In some wind

tunnels, jet engines or rockets can be operated. In some wind tunnels, models are held in place by

magnetic fields so they don’t have to be mounted on stands that would interfere with airflow.

CONTENTS:-

1. INTRODUCTION

2. HISTORY

3. HOW IT WORKS

4. FLOW VISUALIZATION

5. QUALITATIVE METHODS

a) Smoke

b) Tuff

c) Evaporating suspensions

d) Oil

e) fog

6. CLASSIFICATION

a) Low speed wind tunnel

b) Supersonic wind tunnel

c) Flypersonic wind tunnel

7. TESTS

a) Aqua dynamic flume

b) Low speed oversize liquid testing

c) Fan testing

d) Wind engineering testing

8. CLOSED LOOP WIND TUNNEL

9. PARTS OF CLOSED LOOP WIND TUNNEL

a) Turning vans

b) Contraction section

c) Test section

d) Diffuser

e) Fan

10. OVERALL WORKING PART SUMMARY OF WIND TUNNEL

11. FORMULA’S USED IN WIND TUNNEL

a) Bernoulli’s equation

b) Drag co-efficient

c) Lift forces

12. APPLICATION OF WIND TUNNEL

13. FUTURE ASPECT OF WIND TUNNEL

14. CASE STUDY

15. CONCLUSION

16. REFRENSSES

Introduction to the topic A wind tunnel is a research tool used in aerodynamic research to study the effects of air moving past

solid objects.Wind tunnels were first proposed as a means of studying vehicles (primarily airplanes) in

free flight. The wind tunnel was envisioned as a means of reversing the usual paradigm: instead of the

air's standing still and the aircraft moving at speed through it, the same effect would be obtained if the

aircraft stood still and the air moved at speed past it. In that way a stationary observer could study the

aircraft in action, and could measure the aerodynamic forces being imposed on the aircraft.

Later on, wind tunnel study came into its own: the effects of wind on manmade structures or

objects needed to be studied when buildings became tall enough to present large surfaces to the

wind, and the resulting forces had to be resisted by the building's internal structure. Determining

such forces was required before building codes could specify the required strength of such

buildings.

Still later, wind-tunnel testing was applied to automobiles, not so much to determine

aerodynamic forces but more to determine ways to reduce the power required to move the

vehicle on roadways at a given speed. In these studies, the interaction between the road and the

vehicle plays a significant role, and this interaction must be taken into consideration when

interpreting the test results. In an actual situation the roadway is moving relative to the vehicle

but the air is stationary relative to the roadway, but in the wind tunnel the air is moving relative

to the roadway, while the roadway is stationary relative to the test vehicle. Some automotive-test

wind tunnels have incorporated moving belts under the test vehicle in an effort to approximate

the actual condition.

Measurement of aerodynamic forces

Ways that air velocity and pressures are measured in wind tunnels:

air velocity through the test section is determined by Bernoulli's principle. Measurement

of the dynamic pressure, the static pressure, and (for compressible flow only) the

temperature rise in the airflow

direction of airflow around a model can be determined by tufts of yarn attached to the

aerodynamic surfaces

direction of airflow approaching an aerodynamic surface can be visualized by mounting

threads in the airflow ahead of and aft of the test model

dye, smoke, or bubbles of liquid can be introduced into the airflow upstream of the test

model, and their path around the model can be photographed (see particle image

velocimetry)

pressures on the test model are usually measured with beam balances, connected to the

test model with beams or strings or cables

pressure distributions across the test model have historically been measured by drilling

many small holes along the airflow path, and using multi-tube manometers to measure

the pressure at each hole

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

pressure distributions can also be conveniently measured by the use of pressure-sensitive

pressure belts, a recent development in which multiple ultra-miniaturized pressure sensor

modules are integrated into a flexible strip. The strip is attached to the aerodynamic

surface with tape, and it sends signals depicting the pressure distribution along its

surface.

pressure distributions on a test model can also be determined by performing a wake

survey, in which either a single piton tube is used to obtain multiple readings

downstream of the test model, or a multiple-tube manometer is mounted downstream and

all its readings are taken

History

English military engineer and mathematician Benjamin Robins (1707–1751) invented a whirling

arm apparatus to determine drag and did some of the first experiments in aviation theory.

Sir George Clayey (1773–1857) also used a whirling arm to measure the drag and lift of various

airfoils. His whirling arm was 5 feet (1.5 m) long and attained top speeds between 10 and 20 feet

per second.

However, the whirling arm does not produce a reliable flow of air impacting the test shape at a

normal incidence. Centrifugal forces and the fact that the object is moving in its own wake mean

that detailed examination of the airflow is difficult. Francis Herbert Wenham (1824–1908), a

Council Member of the Aeronautical Society of Great Britain, addressed these issues by

inventing, designing and operating the first enclosed wind tunnel in 1871. Once this

breakthrough had been achieved, detailed technical data was rapidly extracted by the use of this

tool. Wenham and his colleague Browning are credited with many fundamental discoveries,

including the measurement of l/d ratios, and the revelation of the beneficial effects of a high

aspect ratio.

Carl Rickard Nyberg used a wind tunnel when designing his Flagon from 1897 and onwards.

In a classic set of experiments, the Englishman Osborne Reynolds (1842–1912) of the University

of Manchester demonstrated that the airflow pattern over a scale model would be the same for

the full-scale vehicle if a certain flow parameter were the same in both cases. This factor, now

known as the Reynolds Number, is a basic parameter in the description of all fluid-flow

situations, including the shapes of flow patterns, the ease of heat transfer, and the onset of

turbulence. This comprises the central scientific justification for the use of models in wind

tunnels to simulate real-life phenomena. However, there are limitations on conditions in which

dynamic similarity is based upon the Reynolds number alone.

The Wright brothers' use of a simple wind tunnel in 1901 to study the effects of airflow over

various shapes while developing their Wright Flyer was in some ways revolutionary. It can be

seen from the above, however, that they were simply using the accepted technology of the day,

though this was not yet a common technology in America.

Subsequent use of wind tunnels proliferated as the science of aerodynamics and discipline of

aeronautical engineering were established and air travel and power were developed.

The US Navy in 1916 built one of the largest wind tunnels in the world at that time at the

Washington Navy Yard. The inlet was almost 11 feet (3.4 m) in diameter and the discharge part

was 7 feet (2.1 m) in diameter. A 500 hp electric motor drove the paddle type fan blades.

Until World War Two, the world's largest wind tunnel was built in 1929 and located in a suburb

of Paris, Chalais-Meudon, France. It was designed to test full size aircraft and had six large fans

driven by high powered electric motors.

How it works

Air is blown or sucked through a duct equipped with a viewing port and instrumentation where

models or geometrical shapes are mounted for study. Typically the air is moved through the

tunnel using a series of fans. 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. Due to the sheer volume and speed of air movement required, the fans may be

powered by stationary turbofan engines rather than electric motors.

The airflow created by the fans that is entering the tunnel is itself highly turbulent due to the fan

blade motion (when the fan is blowing air into the test section – when it is sucking air out of the

test section downstream, the fan-blade turbulence is not a factor), 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 to

smooth out the turbulent airflow before reaching the subject of the testing.

Due to the effects of viscosity, the cross-section of a wind tunnel is typically circular rather than

square, because there will be greater flow constriction in the corners of a square tunnel that can

make the flow turbulent. A circular tunnel provides a smoother flow.

The inside facing of the tunnel is typically as smooth as possible, to reduce surface drag and

turbulence that could impact the accuracy of the testing. Even smooth walls induce some drag

into the airflow, and so the object being tested is usually kept near the center of the tunnel, with

an empty buffer zone between the object and the tunnel walls. There are correction factors to

relate wind tunnel test results to open-air results.

Lighting is usually recessed into the circular walls of the tunnel and shines in through windows.

If the light were mounted on the inside surface of the tunnel in a conventional manner, the light

bulb would generate turbulence as the air blows around it. Similarly, observation is usually done

through transparent portholes into the tunnel. Rather than simply being flat discs, these lighting

and observation windows may be curved to match the cross-section of the tunnel and further

reduce turbulence around the window.

Various techniques are used to study the actual airflow around the geometry and compare it with

theoretical results, which must also take into account the Reynolds number and Mach number for

the regime of operation.

Pressure measurements

Pressure across the surfaces of the model can be measured if the model includes pressure taps. This can be useful for pressure-dominated phenomena, but this only accounts for normal forces on the body

Force and moment measurements

With the model mounted on a force balance, one can measure lift, drag, lateral forces, yaw, roll,

and pitching moments over a range of angle of attack. This allows one to produce common

curves such as lift coefficient versus angle of attack (shown).

Note that the force balance itself creates drag and potential turbulence that will affect the model

and introduce errors into the measurements. The supporting structures are therefore typically

smoothly shaped to minimize turbulence

Flow visualization

Because air is transparent it is difficult to directly observe the air movement itself. Instead,

multiple methods of both quantitative and qualitative flow visualization methods have been

developed for testing in a wind tunnel

Qualitative methods

Smoke

Tufts: - Tufts are applied to a model and remain attached during testing. Tufts can be used to

gauge air flow patterns and flow separation.

Evaporating suspensions:- Evaporating suspensions are simply a mixture of some sort or fine

powder, talc, or clay mixed into a liquid with a low latent heat of evaporation. When the wind is

turned on the liquid quickly evaporates leaving behind the clay in a pattern characteristic of the

air flow.

Oil: - When oil is applied to the model surface it can clearly show the transition from laminar to

turbulent flow as well as flow separation

Fog:-Fog (usually from water particles) is created with an ultrasonic piezoelectric nebulizer. The

fog is transported inside the wind tunnel. An electrically heated grid is inserted before the test

section which evaporates the water particles at its vicinity thus forming fog sheets. The fog

sheets function as streamlines over the test model when illuminated by a light sheet

Sublimation:- If the air movement in the tunnel is sufficiently non-turbulent, a particle stream

released into the airflow will not break up as the air moves along, but stay together as a sharp

thin line. Multiple particle streams released from a grid of many nozzles can provide a dynamic

three-dimensional shape of the airflow around a body. As with the force balance, these injection

pipes and nozzles need to be shaped in a manner that minimizes the introduction of turbulent

airflow into the airstream.

High-speed turbulence and vortices can be difficult to see directly, but strobe lights and film

cameras or high-speed digital cameras can help to capture events that are a blur to the naked eye.

Classification

1.Low-speed wind tunnel:-

Low speed wind tunnels are used for operations at very low mach number, with speeds in the test section up to 400 km/h (~ 100 m/s, M = 0.3). They are of open-return type (see figure below), or return flow (see figure below). The air is moved with a propulsion system made of a large axial fan that increases the dynamic pressure to overcome the viscous losses

Open wind tunnel

The working principle is based on the continuity and Bernoulli's equation:

The continuity equation is given by:

The Bernoulli equation states:

Putting Bernoulli into the continuity equation gives:

The contraction ratio of a wind tunnel can now be calculated by:

Closed wind tunnel

In a return-flow wind tunnel the return duct must be properly designed to reduce the pressure

losses and to ensure smooth flow in the test section. The compressible flow regime: Again with

the continuity law, but now for isentropic flow gives:

The 1-D area-velocity is known as:

The minimal area A where M=1, also known as the sonic throat area is than given for a perfect

gas:

2.Supersonic wind tunnel:-

A supersonic wind tunnel is a wind tunnel that produces supersonic speeds (1.2<M<5) The

Mach number and flow are determined by the nozzle geometry. The Reynolds number is varied

changing the density level (pressure in the settling chamber). Therefore a high pressure ratio is

required (for a supersonic regime at M=4, this ratio is of the order of 10). Apart from that,

condensation or liquefaction can occur. This means that a supersonic wind tunnel needs a drying

or a pre-heating facility. A supersonic wind tunnel has a large power demand leading to only

intermittent operation.

Restrictions for supersonic tunnel operation

Minimum required pressure ratio

Optimistic estimate: Pressure ratio the total pressure ratio over normal shock at M in test

section:

Temperature effects: condensation

Temperature in the test section:

with Tat = 330K: Tm = 70K at Mm = 4

The velocity range is limited by reservoir temperature

Power requirements

The power required to run a supersonic wind tunnel is enormous, of the order of 50 MW per

square meter of test section. For this reason most wind tunnels operate intermittently using

energy stored in high-pressure tanks. These wind tunnels are also called intermittent supersonic

blow down wind tunnels (of which a schematic preview is given below). Another way of

achieving the huge power output is with the use of a vacuum storage tank. These tunnels are

called in draft supersonic wind tunnels. Other problems operating a supersonic wind tunnel

include:

adequate supply of dry air

wall interference effects

high-quality instruments capable of rapid measurements due to short run times on

intermittent tunnels

3. Hypersonic wind tunnel

A hypersonic wind tunnel is designed to generate a hypersonic flow field in the working

section. The speed of these tunnels varies from Mach 5 to 15. As with supersonic wind tunnels,

these types of tunnels must run intermittently with very high pressure ratios when initializing.

Since the temperature drops with the expanding flow, the air inside has the chance of becoming

liquefied. For that reason, preheating is particularly critical (the nozzle may require cooling).

High pressure and temperature ratios can be produced with a shock tube.

There are several technological problems in designing and constructing a hyper-velocity wind

tunnel:

supply of high temperatures and pressures for times long enough to perform a

measurement

reproduction of equilibrium conditions

structural damage produced by overheating

fast instrumentation

power requirements to run the tunnel

Hot shot wind tunnel

One form of HWT is known as a Gun Tunnel or hot shot tunnel (up to M=27), which can be used

for analysis of flows past ballistic missiles, space vehicles in atmospheric entry, and plasma

physics or heat transfer at high temperatures. It runs intermittently, like other high speed tunnels,

but has a very low running time (less than a second). The method of operation is based on a high

temperature and pressurized gas (air or nitrogen) produced in an arc-chamber, and a near-

vacuum in the remaining part of the tunnel. The arc-chamber can reach several Map, while

pressures in the vacuum chamber can be as low as 0.1 Pa. This means that the pressure ratios of

these tunnels are in the order of 10 million. Also, the temperatures of the hot gas are up to 5000

K. The arc chamber is mounted in the gun barrel. The high pressure gas is separated by the

vacuum by a diaphragm that breaks down as its resistance is exceeded.

Prior to a test run commencing, a membrane separates the compressed air from the gun barrel

breech. A rifle (or similar) is used to rupture the membrane. Compressed air rushes into the

breach of the gun barrel, forcing a small projectile to accelerate rapidly down the barrel.

Although the projectile is prevented from leaving the barrel, the air in front of the projectile

emerges at hypersonic velocity into the working section. Naturally the duration of the test is

extremely brief, so high speed instrumentation is required to get any meaningful data.

Tests

1. Aqua dynamic Flume

The aerodynamic principles of the wind tunnel work equally on watercraft, except the water is

more viscous and so imposes greater forces on the object being tested. A looping flume is

typically used for underwater aqua dynamic testing. The interaction between 2 different types of

fluids means that pure wind tunnel testing is only partly relevant. However, a similar sort of

research is done in a towing tank

2. Low-speed Oversize Liquid Testing

Air is not always the best test medium to study small-scale aerodynamic principles, due to the

speed of the air flow and airfoil movement. A study of fruit fly wings designed to understand

how the wings produce lift was performed using a large tank of mineral oil and wings 100 times

larger than actual size, in order to slow down the wing beats and make the vortices generated by

the insect wings easier to see and understand.

3. Fan testing

Wind tunnel tests are also performed to measuring the air movement of the fans at a specific

pressure exactly. By determining the environmental circumstances during the measuring and by

revising the air-tightness afterwards, the standardization of the data is warranted. There are 2

possible ways of measurement: a complete fan or an impeller on a hydraulic installation. Two

measuring tubes enable measurements of lower air currents (< 30.000 m³/h) as well as higher air

currents (< 60.000 m³/h). The determination of the Q/h curve of the fan is one of the main

objectives. To determine this curve (and to define other parameters) air technical, mechanical as

well as electro technical data are measured:

Air technical:

Static pressure difference (Pa) Amount of moved air (m³/h) Average air speed (m/s) Specific efficiency (W/1000m³/h) Efficiency

Electro technical:

Tension (V) Current (A) Cos φ Admitted power (W) fan / impeller Rotations per minute (RPM)

4. Wind engineering testing

In Wind Engineering, Wind Tunnel Tests are often used to measure the velocity around, and

forces or pressures upon structures. Usually very tall buildings, buildings with unusual or

complicated shapes (such as a tall building with a parabolic or a hyperbolic shape), cable

suspension bridges or cable stayed bridges are analyzed in specialized atmospheric boundary

layer wind tunnels. These feature a long upwind section to accurately represent the wind speed

and turbulence profile acting on the structure. Wind tunnel tests provide the necessary design

pressure measurements for use in the dynamic analysis of the structure.

Closed loop wind tunnel In closed return wind tunnel, air is conducted from the exit of the test section back to the fan

by a series of turning vanes. Exiting the fan, the air is returned to the contraction section and

back through the test section. Air is continuously circulated through the duct work of the closed

return tunnel. The arrows on the figure denote the flow of air through the wind tunnel. In the

other major tunnel design, the open return tunnel, air that passes through the test section is

gathered from the room in which the tunnel is located.

Images for closed loop wind tunnel:

Parts of closed loop wind tunnel:-

1. Turning Vanes

The installation of turning vanes in HVAC ductwork is perhaps one of the greatest sources of

contention between mechanical contractors and HVAC engineers. Why? Because many

mechanical contractors believe that turning vanes can cause the ductwork to become less

efficient by increasing the pressure drop in the system, as well as adding time and expense to the

overall installation. This belief seems to be based in simple logic: when there is more surface

area exposed to the airflow, the amount of friction will be increased, and the harder the fan

must work to achieve the required airflows. In some cases when an HVAC system is having

particular difficulty in supplying the required amounts of airflow to all zones, many mechanical

contractors will recommend the removal of every other turning vane at each fitting in the system

in order to “reduce the friction” in the duct. This practice is a violation of SMACNA®

turning

vane spacing requirements, and has also been condemned by the Air Conditioning Contractors of

America, because it decreases the uniformity of the airflow and increases the pressure drop in the

system). The question is, does reality match up with popular belief?

THE FACTS:

When airflow changes direction in a duct that lacks turning vanes, the walls of the duct must

absorb the sudden impact of the air in order to reorient the airflow to the direction desired.

Turning vanes assist the airflow in making a smoother and more gradual change in direction,

resulting in less of an impact, and thus less force transferred (as airflow velocity increases, this

effect becomes more pronounced). While the turning vane surfaces do add a small amount of

friction, the amount of energy lost to friction from the vanes is nothing compared to the energy

lost in the impact resulting from the airflow taking an abrupt or significant change in direction.

Figures 1.1 and 1.2 below illustrate the airflow resistance that occurs in a 90° square elbow with

and without turning vanes.

Figure 1.1

Figure 2.2

From these figures, it can be seen that the elbow with the turning vanes is 800% more efficient

than the same elbow without the vanes. If the owner desires a less expensive installation, the

designer may specify radius elbows without turning vanes. A radius elbow without turning vanes

is still highly efficient, and is much easier and cheaper to fabricate and install (spatial constraints

must also be considered, as a smaller turning radius will decrease efficiency rapidly – minimum

recommended turning radius „R‟ without turning vanes is R=Width/2). Figure 1.3 below

illustrates airflow in a radius elbow.

Figure 1.3

Note also that the radius elbow without turning vanes and having a Radius/Width (R/W) ratio of

1.0 is only 28% less efficient than the elbow with turning vanes. If the radius is increased to

R/W=1.5, it will only be 12% less efficient, and if it is increased to R/W=2.0, it will have the

same efficiency as the same size elbow with turning vanes! In all cases it can be clearly seen that

as the airflow changes direction more gradually, the fitting pressure drop decreases, and with it,

the energy required by the system fan to supply the desired airflow volume.

THE CAVEATS:

There are certain instances where turning vanes can cause an increase in pressure drop, and this

article covers two such cases.

Case 1: Installation of turning vanes at the entrance to a branch duct.

The first case is when turning vanes are installed at the entrance to a duct branch. Some

contractors, in an honest effort to reduce static pressure, install turning vanes or scoops at the

entrance to a duct branch, as shown in Figure 2.1 below.

Figure 2.1

This configuration can cause large pressure losses, because the turning vanes disrupt the

uniformity of airflow in the main duct, which in turn causes a high pressure drop at the fitting.

Branches should be installed with a 45° entry or a radius branch fitting, as shown in Figure 2.3

below.

Figure 2.2

Figure 2.3

Note that the radius branch fitting is twice as efficient as the 45° entry fitting. While the radius

branch fitting is slightly more expensive to fabricate, the installation cost is the same as the 45°

entry fitting, and can greatly reduce pressure drop in systems with a high fitting count.

Case 2: Improper Turning Vane Alignment

The second example of turning vanes causing a pressure loss is where the vanes are not aligned

with the ductwork properly, increasing air turbulence and creating a drop in pressure as seen in

Figure 3.1 below.

Figure 3.1

When the turning vanes are not properly aligned to run parallel with the sides of the ductwork at

both the entrance and the exit of the vanes, the airflow will impact the sides of the duct and

create turbulence. The effects of the improperly aligned turning vanes can range from mild to

severe, and are determined by how far out of alignment the vanes are. Improper vane alignment

occurs in many cases where ductwork is installed hastily or sloppily, and can be prevented by

simply performing a final alignment check on all vanes prior to completing the installation. See

Figure 3.2 below for an example of airflow in a duct with correctly aligned turning vanes.

Figure 3.2

THE CONCLUSION:-

Turning vanes have been proven to be very valuable for reducing pressure losses and increasing

system efficiency. Designers should always specify the highest efficiency fittings possible within

the owner‟s budget, to increase system efficiency at every available opportunity. Mechanical

contractors should never take it upon themselves to add to or remove turning vanes from an

engineer‟s designs. Each system is designed to a specific total static pressure, and removing or

adding turning vanes where they have not been accounted for in the engineer‟s calculations will

make the system function differently than intended. In a worst case scenario, the changes to the

system may cause it to become incapable of supplying the required airflows to all zones

2. Contraction Section

The contraction or "nozzle" accelerates the flow from the settling chamber into the test section,

further reducing percentage variations in velocity. The old-style contraction shape with a small

radius of curvature at the wide end and a large radius at the narrow end to provide a gentle entry

to the test section is not the optimum. There is a danger of boundary-layer separation at the wide

end, or perturbation of the flow through the last screen. Good practice is to make the ratio of the

radius of curvature to the flow width about the same at each end. However, too large a radius of

curvature at the upstream end leads to slow acceleration and therefore increased rate of growth of

boundary-layer thickness, so the boundary layer - if laminar as it should be in a small tunnel -

may suffer from Taylor-Gentler "centrifugal'' instability when the radius of curvature decreases.

Bell and Mehta discuss boundary-layer predictions for contractions in "Boundary-Layer

Predictions for Small Low-Speed Contractions" AIAA J., Vol. 27, p. 372, (1989). T. Morel gives

rules for design of contractions with polynomial curves for wall shapes (J. Fluids Egg Vol. 97, p.

225, 1975 for ax symmetric contractions and Vol. 99, p. 371, 1977 for two-dimensional shapes).

Brassard in an unpublished project report "Transformation of a Polynomial for a Contraction

Wall Profile" describes a generalization of the fifth-order polynomial proposed by Bell and

Mehta, to extend the range of shapes and provide different radii of curvature at the two ends.

The contraction area ratio should be "as large as possible", to reduce the total-pressure loss

through the screens. In medium-size tunnels, the area ratio is limited by the desirability of easy

access to the test section while the operator is standing on the floor of the laboratory, implying

that the test section floor should be no more than 4-5 ft. above the floor of the room. The ceiling

height imposes another limit. There is no overwhelming reason why a contraction should be

symmetrical top and bottom - modern potential-flow codes can easily calculate the pressure

distribution on a chosen asymmetric shape - but the only practical examples are "one-sided"

contractions looking like half a conventional shape. An area ratio of 9 is acceptable for a typical

tunnel with three screens each with a pressure-drop coefficient of 1.2: the contribution to the

power factor (inversely proportional to the square of the area ratio)is then 3x1.2/81 = 0.044. The

area ratio of 31 used in the RAE 4 ft x 3 ft tunnel is unnecessarily large, and led to flow

separation in the region of the rapid change of wall angle between the wide-angle diffuser and

the contraction.

Effect of contraction section:-

The effect of a contraction on unsteady velocity variations and turbulence is more complicated: the

reduction of x-component (axial) fluctuations is greater than that of transverse fluctuations. A simple

analysis due to Brandt predicts that the ratio of root-mean-square axial velocity fluctuation to mean

velocity will be reduced by a factor 1/N2, as for mean-velocity variations, while the ratio of lateral rams

fluctuations to mean velocity is reduced only by a factor of N: that is, the lateral fluctuations (in m/s,

say) increase through the contraction, because of the stretching and spin-up of elementary longitudinal

vortex lines. Bachelor, The Theory of Homogeneous Turbulence, Cambridge 1953, gives a more refined

analysis, but Brandt’s results are good enough for tunnel design. The implication is that tunnel free-

stream turbulence is far from isotropic. The axial-component fluctuation is easiest to measure, e.g. with

a hot-wire anemometer, and is the "free-stream turbulence" value usually quoted. However it is smaller

than the others, even if it does contain a contribution from low-frequency unsteadiness of the tunnel

flow as well as true turbulence.

The contraction is the last component before the test section, and should further reduce the variations

of velocity components in time and space that are created in the return circuit or the laboratory and

then attenuated by the honeycomb and screens. The effectiveness of a contraction of area ratio N in

reducing variations of mean axial velocity over the cross-section can be seen by applying Bernoulli's

equation to an incompressible flow with a small region of increased speed somewhere in the cross

section. The total pressure of the main stream is pi + (1/2) U2, where pi is the initial static pressure, and

the total pressure in the high-velocity region, which is assumed to have the same initial static pressure,

is pi + (1/2) (U+ U)2. At the exit, the main stream velocity is approximately NU (we neglect the extra

mass flow in the high-speed region) and the static pressure is therefore pi - (1/2) U2(N2-1). The velocity

in the high-speed region is now determined from Bernoulli's equation, using this value of static pressure

and the above-mentioned value of total pressure in the high-speed region, which is not altered by the

essentially in viscid flow through the contraction. Neglecting terms of order U2, we find that the

contraction reduces the fractional velocity variation U/U by a factor of about 1/N2. Variations of more

than a fraction of a percent in mean velocity (except near the test-section walls) imply either too few

screens, too small a contraction ratio, or asymmetry in the flow into the settling chamber. Asymmetry in

a closed-circuit tunnel is usually the result of separation or incipient separation in the diffuser, which

may in turn be the result of poor design of the corner vanes, leading to under-turning or over-turning of

the flow leaving the corner. In an open-circuit tunnel, asymmetry in the return flow in the laboratory can

cause trouble.

3. Test section

The test section is the part of the wind tunnel in which the model is placed. For low speed tunnel

operation, the test section has the smallest cross-sectional area and the highest velocity within the

tunnel.

4. Diffuser

The diffuser is the gradually-expanding passage following the test section (e.g. Fig. ) in which

the flow speed decreases and the pressure rises. The recovery of pressure from kinetic energy

reduces the power needed to drive the tunnel: in the case of open-circuit tunnels the diffuser also

reduces drafts in the laboratory. The pressure rise is less than that given by Bernoulli's equation,

because of losses due to skin friction and resulting growth of boundary-layer displacement

thickness.

The usual design rule for subsonic diffusers is that the total

included angle of a portion (frustum) of a circular cone with the

same length and area ratio as the diffuser should not exceed 5

deg. This is well below the angle for maximum pressure recovery, which is nearer 10 deg., but at

angles of more than about 5 deg. the boundary layer is close enough to separation for the flow to

be unsteady. The 5 deg. rule will fail if the test section is unusually long, so that the boundary-

layer thickness at entry to the diffuser is unusually large. (We expect the behavior of boundary

layers in adverse pressure gradient to depend on some dimensionless parameter such as

( /UCL)ducal/do, which must be small enough to avoid separation. Here and UCL are strictly the

local boundary layer thickness and centerline velocity - related by Bernoulli's equation to the

pressure gradient, which is what really matters - but the evolution of is determined by its value

at the beginning of the diffuser.)

The ideal diffuser shape is probably a gradually decreasing rate of expansion but this is difficult

to build, except as a series of straight-walled sections. In general-purpose wind tunnels which

may be used for tests of models big enough to disturb the flow in the diffuser, it is safest to keep

to a conservative angle from the start.

It is unusual to have an area ratio of more than three at the fan: to an (optimistic) in viscid one-

dimensional approximation, this recovers 95 percent of the kinetic energy in the flow in the form

Supersonic diffuser:-

Supersonic tunnels, in which a diverging diffuser after the test section would produce a further

increase in Mach number, are equipped with a second throat at the end of the test section (Fig.):

the first throat is the one upstream of the test section through which the flow accelerates through

the speed of sound. In the converging section leading to the second throat the flow is decelerated

to slightly above sonic speed (obeying the one-dimensional in viscid compressible flow

equations to a first approximation); in the diverging section downstream of the throat the Mach

number rises again, until a shock wave or waves produce a reduction to subsonic speed. It may

be shown that a shock wave in the converging portion of the second throat would be unstable,

and in practice the second-throat Mach number is chosen large enough for the breakdown shock

system to be located well downstream of the throat, to ensure stability under all operating

conditions.

When the tunnel is started up, the second throat must be rather larger than the first throat in order

for the latter to choke first, so that supersonic flow can be established in the test section. The in

viscid-flow equations are not accurate during startup because a strong shock wave passes down

the test section, reducing the speed below that of sound and usually causing massive boundary-

layer separation. Therefore the second throat has to be significantly larger than the first, and to

achieve the best possible supersonic diffusion in tunnels for high supersonic speeds the second

throat is closed in after starting (reducing the local Mach number), by adjusting the shape of the

walls. This is cost-effective only in large tunnels or if drive power is restricted.

A conventional subsonic diffuser is usually installed after the second throat, even in the case of

an open-circuit blow down or suck down tunnel. The boundary layers on the tunnel walls will be

driven close to separation by interaction with the shock wave(s) in the second throat, and may

even separate if the Mach number entering the shock wave is more than about 1.3. Therefore the

subsonic diffuser (or its first leg, preceding the first corner, in the case of a closed-circuit tunnel)

should have a smaller angle than the 5 deg. recommended for a low-speed tunnel, or be preceded

by a constant-area section to allow the boundary layers to recover.

5. Fan Most closed-circuit tunnels are driven by axial-flow fans, which produce a static pressure, rise

(with no appreciable change in axial velocity or dynamic pressure unless the pressure rise is

comparable with the absolute pressure) at one point in the circuit, to compensate for the total-

pressure losses in the rest of the circuit.

The design of axial fans for tunnels covers a wide range. Lightly-loaded

fans, which usually have a high ratio of tip speed to axial velocity ("fine

pitch") and a correspondingly high relative velocity at the blades, produce

the required pressure rise with a fairly small blade area and look very much

like aircraft propellers. However, fans with high tip speeds cause a great

deal of vibration if the approaching stream is not uniform over the cross-

section, and a tip speed of more than 150 or 200 m/s in air implies a relative Mach number

approaching that at which shock waves occur, again resulting in noise and vibration. In modern

low-speed tunnels, therefore, the fan tip speed is kept as low as possible, not more than two or

three times the local axial velocity, and the blade arrangements more closely resemble one stage

of an axial-flow compressor, with a stator row in front of the rotor.

Because it is necessary to return to uniform, non-swirling flow in a circular or polygonal section

downstream of the fan, the diameter of the central nacelle ("boss", "hub") is usually a smaller

fraction of the fan diameter than in multi-stage compressor practice, and rarely exceeds 0.5 to 0.6

of the fan diameter. As a result the space between adjacent blades, measured around the

circumference, varies considerably from root to tip. The space/chord ratio typifies the fan

loading, and it also determines the allowances to be made for the effect of one fan blade upon the

adjacent ones. Near the tip the interference is small and propeller design rules can be used, but

near the root the space/chord ratio is smaller and turbo machinery design rules apply.

Unfortunately there is often a region near mid-radius where neither set of rules is accurate and

some kind of interpolation is needed. It is usual to keep the cross-sectional area of the air stream

nearly constant over the length of the nacelle, so the outer casing has to bulge outwards. The

structural alternatives are a fiberglass molding or, more crudely, an expanding cone, a cylindrical

section and a contracting cone, in that order.

Axial fans have a much more limited operating range (in terms of the pressure-rise coefficient, defined

as the ratio of pressure rise through the fan to the dynamic pressure of the flow) than centrifugal

blowers. If the pressure coefficient is required to be too high, the fan blades will stall, usually beginning

at the root where the blade angle to the plane of rotation is largest, so that the flow direction may be

downstream near the blade tips and upstream near the roots. If the pressure coefficient is too low, then

a negative-angle stall near the tips will occur. Axial-flow fans, usually with the electric motor mounted in

the central boss, are commercially available but high-performance tunnels normally have custom-built

fans. A good detailed account of axial-flow fan design, nominally for mine ventilation fans but applicable

to wind tunnels as well, is given by R.A. Wallis, Axial Flow Fans and Ducts, Wiley-Interscience (1983).

A "blower" tunnel is driven by an impeller at entry, usually a true (centrifugal) blower which is

almost always of the backward-airfoil or squirrel-cage type rather than the old-fashioned radial-

blade variety seen, for example, in car water pumps and domestic hair dryers. The airfoil blades

of a centrifugal blower run at nominally the same angle of attack all along the span, and the

reduction in pressure rise as the blades stall is gradual, without much deterioration in outlet flow

steadiness and uniformity. It must be said that the outlet flow frm a centrifugal blower is

disturbed at the best of times, but not much worse than the likely condition of the flow at exit

from the main diffuser of a closed-circuit tunnel.

Centrifugal blowers are almost always bought "off the shelf" as low-pressure commercial

blowers intended for ventilation systems, so the design process reduces to searching the

manufacturer's catalog for a unit that produces the required total-pressure rise at the required

volume flow rate. These blowers are usually made of sheet metal and should be distinguished

from high-pressure centrifugals with cast metal cases.

Overall working parts summary of wind tunnel

Aerodynamicists use wind tunnels to test models of proposed aircraft. In the tunnel, the engineer

can carefully control the flow conditions which affect forces on the aircraft. By making careful

measurements of the forces on the model, the engineer can predict the forces on the full scale

aircraft. And by using special diagnostic techniques, the engineer can better understand and

improve the performance of the aircraft.

Wind tunnels are designed for a specific purpose and speed range. There is a wide variety ofwind

tunnel

type and model instrumentation. The tunnel shown in the figure is a low-speed, closed tunnel

which we

are viewing from above. We can use this figure to study the various parts of a wind tunnel.

The air inside the tunnel is made to move by the fan on the far side of the tunnel. In this figure,

air moves counter-clockwise around the circuit. The fan is turned by a large, electrically-

powered drive motor. Leaving the fan, the air is turned in the corners by turning vanes. The

turning vanes are a cascade of airfoils which minimize the total pressure loss through the corners.

Leaving the corner at the upper left of the figure, the air passes through some flow straighteners

before entering the test section. The purpose of the flow straighteners is to make the flow in the

test section as uniform as possible. The test section is the part of the wind tunnel in which the

model is placed. For low speed tunnel operation, the test section has the smallest cross-sectional

area and the highest velocity within the tunnel. Leaving the test section, the air enters the

diffuser where it is expanded and slowed before returning to the fan. Again, the diffuser is

employed to minimize losses in the tunnel. For this closed circuit wind tunnel, there are two

more corners with turning vanes before the air is brought back to the fan.

Aerodynamicists use wind tunnels to test models of proposed aircraft. In the tunnel, the engineer

can carefully control the flow conditions which affect forces on the aircraft. By making careful

measurements of the forces on the model, the engineer can predict the forces on the full scale

aircraft. And by using special diagnostic techniques, the engineer can better understand and

improve the performance of the aircraft.

Wind tunnels are designed for a specific purpose and speed range. There is a wide variety of

wind tunnel types and model instrumentation. The tunnel shown in the figure is a low-speed,

closed tunnel which we are viewing from above. We can use this figure to study the various

parts of a wind tunnel.

The air inside the tunnel is made to move by the fan on the far side of the tunnel. In this figure,

air moves counter-clockwise around the circuit. The fan is turned by a large, electrically-

powered drive motor. Leaving the fan, the air is turned in the corners by turning vanes. The

turning vanes are a cascade of airfoils which minimize the total pressure loss through the corners.

Leaving the corner at the upper left of the figure, the air passes through some flow straighteners

before entering the test section. The purpose of the flow straighteners is to make the flow in the

test section as uniform as possible. The test section is the part of the wind tunnel in which the

model is placed. For low speed tunnel operation, the test section has the smallest cross-sectional

area and the highest velocity within the tunnel. Leaving the test section, the air enters the

diffuser where it is expanded and slowed before returning to the fan. Again, the diffuser is

employed to minimize losses in the tunnel. For this closed circuit wind tunnel, there are two

more corners with turning vanes before the air is brought back to the fan.

Formula used in Wind Tunnel:-

1.Bernoulli’s Equation

The continuity equation stated above is a very handy equation useful in almost any engineering

application. It shows how a person can relate geometric conditions to flow conditions at any

given point were the area and the velocity are known. But for other applications it is easier to

obtain a pressure reading at a given point rather than pulling out a ruler and measuring the area at

a given point. In fact if the area has a complicated geometry (for example a rupture in a tank) an

exact measurement of it could be a tedious process. Lets assumed for this study that instead of a

geometry reading, you have a reading of pressures to work with. The question being: Is it

possible to calculate velocities is the pressures are known or vice-versa ? And the answer to that

is: yes. The equation that would relate pressure to velocities (actually one of the most useful

equation in engineering) is called Bernoulli's equation and is given as:

Bernoulli's equation is only valid if one assumes the following: incompressible fluid (fluid

velocity less than one third the speed of sound) and inviscid flow (this just means that the point

in question along the flow is going to be away from where the flow and the object come into

contact). This equation basically tells us that, as the flow progresses from one point to another,

an increase in speed will be accompanied by a decrease in pressure. Lets look at the same

converging tube as before. It is possible to prove that pressure will decrease as velocity increase.

This was proved in the example before. But in this case all we have to prove is that the pressure

change between station 2 and station 1 is negative,

where: (from the results shown before)

So, starting with Bernoulli's equation and solving for the pressure difference:

Since V1 is less than V2 then the subtraction in the parenthesis will yield a negative result,

therefore as velocity increases pressure decreases

2.Drag coefficient

The drag coefficient is a number that aerodynamicists use to model all of the complex

dependencies of shape, inclination, and flow conditions on aircraft drag. This equation is simply

a rearrangement of the drag equation where we solve for the drag coefficient in terms of the

other variables. The drag coefficient Cd is equal to the drag D divided by the quantity: density r

times half the velocity V squared times the reference area A.

Cd = D / (A * .5 * r * V^2)

The quantity one half the density times the velocity squared is called the dynamic pressure q. So

Cd = D / (q * A)

The drag coefficient then expresses the ratio of the drag force to the force produced by the

dynamic pressure times the area.

This equation gives us a way to determine a value for the drag coefficient. In a controlled

environment (wind tunnel) we can set the velocity, density, and area and measure the drag

produced. Through division we arrive at a value for the drag coefficient. As pointed out on the

drag equation slide, the choice of reference area (wing area, frontal area, surface area, ...) will

affect the actual numerical value of the drag coefficient that is calculated. When reporting drag

coefficient values, it is important to specify the reference area that is used to determine the

coefficient. We can predict the drag that will be produced under a different set of velocity,

density (altitude), and area conditions using the drag equation.

The drag coefficient contains not only the complex dependencies of object shape and inclination,

but also the effects of air viscosity and compressibility. To correctly use the drag coefficient, we

must be sure that the viscosity and compressibility effects are the same between our measured

case and the predicted case. Otherwise, the prediction will be inaccurate. For very low speeds (<

200 mph) the compressibility effects are negligible. At higher speeds, it becomes important to

match Mach numbers between the two cases. Mach number is the ratio of the velocity to the

speed of sound. At supersonic speeds, shock waves will be present in the flow field and we must

be sure to account for the wave drag in the drag coefficient. So it is completely incorrect to

measure a drag coefficient at some low speed (say 200 mph) and apply that drag coefficient at

twice the speed of sound (approximately 1,400 mph, Mach = 2.0). It is even more important to

match air viscosity effects. The important matching parameter for viscosity is the Reynolds

number that expresses the ratio of inertial forces to viscous forces. In our discussions on the

sources of drag, recall that skin friction drag depends directly on the viscous interaction of the

object and the flow. If the Reynolds number of the experiment and flight are close, then we

properly model the effects of the viscous forces relative to the inertial forces. If they are very

different, we do not correctly model the physics of the real problem and will predict an incorrect

drag.

The drag coefficient equation will apply to any object if we properly match flow conditions. If

we are considering an aircraft, we can think of the drag coefficient as being composed of two

main components; a basic drag coefficient which includes the effects of skin friction and shape

(form), and an additional drag coefficient related to the lift of the aircraft. This additional source

of drag is called the induced drag and it is produced at the wing tips due to aircraft lift. Because

of pressure differences above and below the wing, the air on the bottom of the wing is drawn

onto the top near the wing tips. This creates a swirling flow which changes the effective angle of

attack along the wing and "induces" a drag on the wing. The induced drag coefficient Cdi is

equal to the square of the lift coefficient Cl divided by the quantity: pi (3.14159) times the aspect

ratio AR times an efficiency factor e.

Cdi = (Cl^2) / (pi * AR * e)

The aspect ratio is the square of the span s divided by the wing area A.

AR = s^2 / A

For a rectangular wing this reduces to the ratio of the span to the chord. Long, slender, high

aspect ratio wings have lower induced drag than short, thick, low aspect ratio wings. Lifting line

theory shows that the optimum (lowest) induced drag occurs for an elliptic distribution of lift

from tip to tip. The efficiency factor e is equal to 1.0 for an elliptic distribution and is some value

less than 1.0 for any other lift distribution. A typical value for e for a rectangular wing is .70 .

The outstanding aerodynamic performance of the British Spitfire of World War II is partially

attributable to its elliptic shaped wing which gave the aircraft a very low amount of induced

drag. The total drag coefficient Cd is equal to the drag coefficient at zero lift Cdo plus the

induced drag coefficient Cdi.

Cd = Cdo + Cdi

The drag coefficient in this equation uses the wing area for the reference area. Otherwise, we

could not add it to the square of the lift coefficient, which is also based on the wing area.

3. Lift Forces

Lift, or downforce as its known in the motor racing world, is the force generated perpendicular to

the direction of travel for an object moving through a fluid (gas or liquid). The same effect

occurs when a fluid moves over a stationary object, such as an airfoil in a wind tunnel. Airfoils

are the most efficient shapes found so far that can generate lift while at the same time

minimizing drag.

L = CL . A . qinf

D = CD . A . qinf

where:

L is the lift force

CL is the dimensionless lift coefficient

D is the drag force

CD is the dimensionless drag coefficient

A is a reference area, usually the plan area (maximum projected area onto a plane) of an airfoil or

wing

qinf is the free stream fluid dynamic pressure from Bernoulli's equation:

qinf = 1/2 rhoinf . Vinf2

where:

rhoinf is the free stream fluid density

Vinf is the free stream fluid speed

The plan area is used to calculate the drag coefficient only for airfoils. For all other shapes, such

as cars, the reference area is the frontal area of the shape projected onto a plane normal to the

flow direction.

Application Let‟s take a closer look at the common applications for benchtop wind tunnels mentioned earlier.

The first and simplest is anemometer air flow calibration. Anemometers, which are devices to

measure air velocity, are ubiquitous in many areas of science and engineering, as well as in

certain trades. The two most common styles, each employing a different technology, are the vane

type air velocity anemometer, in which a small fan spins in response to air flow, and the hot-wire

anemometer, in which a resistive

element cools as air passes over it. In industry, these devices are used for all kinds of air flow

testing and adjustment. In addition to general workspace ventilation, they are used to monitor air

velocity in spray booths, fume hoods, clean rooms and laminar flow workstations. They are also

employed to determine flow through large filters and cooling or heating coils used for industrial

processes. Within the scientific community, anemometers find widespread use in weather

measurement and analysis, in environmental studies, and for research. Probably the most

common use of all is in the installation and maintenance of HVAC systems, where an

anemometer is essential for air balancing, flow measurements,

and troubleshooting.

For more general wind tunnel applications, a model like that shown in Figure 1 offers far more

flexibility. Thisdevice can not only be used for laboratory calibration, but is also suitable for

applications similar to those of its big brothers, such as aerodynamic studies using models.

Typical uses include product design and development, R&D projects, and university laboratory

experiments. The test chamber can accommodate custom mounting fixtures and instrumentation

and includes the capability to measure temperature, humidity, and barometric

pressure. Such laboratory-grade wind tunnels typically have a much wider range of flow rates

than the much simpler calibration wind tunnels – 25 to 9000 fpm (feet per minute) vs. 500 to

3000 fpm. Additionally, the flow

rates are continuously variable and not preset as they are in a calibration wind tunnel. To achieve

the lowest flow rates, specially designed restrictive plates are used that cut down the air flow

while maintaining the required high and low turbulence.

The latest application to emerge for these handy air flow devices is in everyday engineering

design: thermal evaluation of electrical and electronic components. This includes active devices

like circuit boards and powered components, and passive devices like heat sinks and heat

exchangers.

Future aspects of wind tunnel

Fortunately, there is an exciting new technology on the horizon which may someday replace the

computer for aerodynamic design and analysis. Two workers at UNCAF (United Nations

Computational Aerodynamics Facility) have recently made a startling discovery. They found that

by building a small wooden model of an airplane and then blowing air past it in an enclosed

tunnel, reasonably accurate predictions may be made of what the flow codes would compute.

They refer to the method they have discovered as a “wind tunnel”. At present, “wind tunnel”

modeling is still in an early and relatively crude stage, and cannot be expected to precisely

reproduce numerical results. For example, the continuous surface of a wood or metal airplane

model will never exactly duplicate the discrete nature of a computational grid. Also, some

factors, such as artificial viscosity, are neglected completely in wind tunnel modeling. It may be

especially hard to accurately predict linearized potential flow in the tunnel. Nevertheless, in

many cases, the wind tunnel agrees surprisingly well with the computer.

Constructing a wind tunnel model is much quicker and less labor-intensive than running all but

the simplest computer programs. Shops such as Minicraft or Static Engineering complete even a

highly detailed titanium model in a mere matter of months. Thus, many design iterations and

trade-off studies can be conducted in a fraction of the time required via the computer. Advances

in wind tunnel technology and model fabrication are expected to proceed at a rapid pace. Many

promising techniques, such as the chiseling of facets in Plaster-of-Paris models to more closely

resemble computational panelings and grids, are already being suggested by researches around

the world. The future prospects of this amazing new wind tunnel technology are bounded only by

the imagination.

But what, you may ask, will be the fate of the millions of computational aerodynamicists

presently employed in the aerospace industry? Is the wind tunnel a threat to their job security?

While it is true that some may lose their jobs, a brand-new demand will be created for those

well-versed in the state-of-the-art wind tunnel technology. Engineering graduate schools are

already replacing courses in Finite Volume Methods and Grid Generations with curricula in

woodworking and whittling. Clearly, the engineer will be freed from the tyranny and drudgery of

computational methods, giving him more time to concentrate on creative tasks. It is doubtful,

however, that the computer will ever be completely eliminated: the thought of an airplane

designed solely from wind tunnel data without the aid of the computer seems too incredible to

believe. While the wind tunnel may never fully replace the computer, it is almost certain to

become the most useful engineering tool of the future.

Case study

Success or failure in Formula One today is governed by the car‟s aerodynamic performance.

With less than a second covering the first five places on the grid, the efficiency of a front wing

endplate is just as important as the driver‟s technique or the horsepower of the engine. Tiny

revisions can have huge consequences, which is why the Honda Racing F1 Team has invested

around £30m in a new, full-scale wind tunnel at the team‟s headquarters in Brackley, England.

The three main components of an F1 car‟s performance are the tyres, the engine and the

aerodynamics. In recent years, the Formula One regulations have become more complex and

restrictive, placing a greater emphasis on aerodynamic efficiency and attention to detail.

The cars are now more sophisticated than ever before and to be successful, the designers need

the latest tools. The new, state-of-the-art wind tunnel will play a critical role in the Honda team‟s

future.

The use of wind tunnels for aerodynamic testing in Formula One can be traced back to the BRM

team of the 1960‟s but it wasn‟t until the Lotus „ground effect‟ cars of the late 1970s that teams

really started to understand the importance of aerodynamics. By today‟s standards, even the

turbocharged cars of the mid-1980s were relatively crude.

“Now we have a very competitive Formula One environment with some very clever people, who

are well resourced and organised,” says Chief Aerodynamicist, Mariano Alperin-Bruvera. “In

the past 10-15 years, the rate of progress of the cars has been largely constant, but the amount of

effort that‟s gone into achieving that has probably increased ten-fold because we‟ve become

much better at designing race cars.”

Anyone taking a peek at the specification of the three-storey building that houses Honda‟s new

wind tunnel will be left in no doubt about the team‟s commitment. Approximately 5.5 km of

concrete piling was installed, weighing 6000 tonnes. More than 160 tonnes of reinforcement was

used in the slab and bases, with half of the slab designed to withstand the load of a 250-tonne .

The wind is generated by a huge fan, consisting of sixteen rotating, composite blades and 27

stator (static) blades, which are made of steel. Each blade is 5.3m long with a hub diameter of

2.4m. This fan is powered by a 3035hp electric three-phase induction motor generating an

astonishing 32,125lb ft of torque at 495rpm. During a test session, this giant construction will

move 895m3/sec of air to generate a wind speed of 80m/sec. That this highly complex, technical

structure was completed in just 18 months is a testament to the skill, dedication and teamwork of

the construction engineers.

The scale of the enterprise reflects Honda‟s decision to construct a full-scale wind tunnel,

capable of accommodating a real grand prix car. Until now, the team had relied on testing quarter

or half-scale models of the chassis. These tunnels had relied on a useful bit of physics, which

dictates that the air flow over a car is described by the speed multiplied by the length. If you

halve the length of the car and double the wind speed, you can achieve reasonably accurate

results.

But in today‟s highly competitive environment, „reasonably

accurate‟ is no longer good enough. The team had achieved as

much as it could with a half-scale model and now needed to be

able to resolve more subtle differences.

If the Formula One rule-makers continue to restrict the number of

days that a team may test at a circuit, wind tunnels and computer

simulations will become even more important. “We‟ll need to do

our testing in what you might call laboratory conditions,” says

Alperin-Bruvera. “If you test at a track and there‟s wind or rain, you could easily lose a day‟s

work.”

The wind tunnel is an important component in a testing chain. Parts are designed using CAD

(Computer Aided Design) technology and then tested using CFD (Computational Fluid

Dynamics) software. The latter uses sophisticated computer processing to analyse the likely

impact of a new part before it is manufactured. Parts that show a benefit on the computer will be

built and tested in the wind tunnel.

If the tunnel analysis proves successful, they will be tested „for real‟ on the car. “The tunnel

won‟t stop us taking aerodynamic measurements at a circuit,” explains Alperin-Bruvera, “but it

will help us understand the subtleties of the kits before they‟re given to our drivers.”

A wind tunnel is a stylised version of what happens in the real world. The car is held still, while

the road and the air are moved. The car can be tested at an angle to road and at an angle to the

wind. This allows the aerodynamicists to measure the affect of cross winds and the yaw rate -

what happens when the car corners.

Wind tunnel technology has improved dramatically in recent years, but it does have its

limitations. It‟s impossible, for example, to simulate gusting wind or the affect of the air flowing

across rough surfaces, such as grandstands or grass. The human element is also critical. The

technicians must understand the tunnel’s limitations and use it in the right way. This is something that

some teams do better than others.

It is impossible to underestimate the complexity of the work. On a modern F1 car, the aerodynamics

increase the mechanical grip - provided by the tyres and suspension - by a factor of two or three. Put

simply, the aerodynamics force the car into the ground, allowing it to corner at a higher speed. A

modern F1 car can corner at 4.5G and brake at 5G. At 100mph, it could drive upside down.

Conclusion

Wind tunnels are certainly not a common tool for most engineers; however, the little-known category

of benchtop wind tunnels can offer great gains in test, measurement, and design effectiveness in

the right applications. For those routinely using anemometers, a benchtop calibration wind tunnel is

a practical way to bring calibration capability in-house. For those involved in research and advanced

product development, a laboratory-grade unit can be a decisive data resource. For designers of

circuit boards, heat-generating components, heat sinks, or other cooling devices, a thermal

evaluation wind tunnel offers a brand new way to create safer, more reliable, and higher quality

products. If you fit into any of these categories and had never heard of benchtop wind tunnels,

welcome to a new path to productivity.

References

http://www.nasa.gov/audience/forstudents/5-8/features/what-are-wind-tunnels-58.html

closed+loop+wind+tunnel&aq=0&aqi=g1&aql=&gs_sm=&gs_upl=&bav=on.2,or.r_gc.r_pw.,cf

.osb&fp=7aef64bad2581979&biw=1366&bih=518

http://www.google.co.in/#pq=closed+loop+wind+tunnel&hl=en&cp=26&gs_id=6q&xhr=t&q=w

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