Unit I Power Losses in a Wind Tunnel

Power Losses in a Wind Tunnel
The total power loss in a wind tunnel may be split into the following components.

• Losses in cylindrical parts.
• Losses in guide vanes at the corner (in closed circuit tunnels).
• Losses in diffuser.
• Losses in contraction cone.
• Losses in honey comb, screens etc.
• Losses in test-section (jet losses in case of open jet).
• Losses in exit in case of open circuit tunnel.

Unit I Special Purpose Wind Tunnels

These are tunnels with layout totally different from that of low-speed and high-speed tunnels. Some of the popular special purpose tunnels are: spinning tunnels, free-flight tunnels, stability tunnels and low-density tunnels.

Unit I Layout of Open Circuit Wind Tunnels

General Features
All modern wind tunnels have four important components;

• the Effuser
• the Working or Test-section
• the Diffuser
• the Driving unit

The Effuser

This is a converging passage located upstream of the test-section. In this passage fluid gets accelerated from rest (or from very low speed) at the upstream end of it to the required conditions at the test-section.

In general, effuser contains honey-comb and wire gauze screens to reduce the turbulence and produce an uniform air stream at the exit. Effuser is usually referred to as contraction cone.

Test-section

Model to be tested is placed here in the air-stream, leaving the downstream end of the effuser, and the required measurements and observations are made. If the test-section is bounded by rigid walls, the tunnel is called a closed throat tunnel. If it is bounded by air at different velocity (usually at rest), the tunnel is called open jet tunnel. The test-section is also referred to as working-section.

Diffuser

The diffuser is used to re-convert the kinetic energy of the air-stream leaving the working-section into pressure energy, as efficiently as possible. Essentially it is a passage in which the flow decelerates.

Driving Unit

If there were no losses, steady flow through the test-section could continue for ever, once it is established, without the supply of energy from an external agency. But in practice, losses do occur, and kinetic energy

is being dissipated as heat in vorticity, eddying motion and turbulence. Moreover, as the expansion of the diffuser cannot continue to infinity, there is rejection of some amount of kinetic energy at the diffuser exit.

This energy is also converted to heat in mixing with the surrounding air.

To compensate for these losses, energy from an external agency becomes essential for wind tunnel operation. Since power must be supplied continuously to maintain the flow, the fourth essential component

namely, some form of driving unit is essential for wind tunnel operation. In low-speed tunnels this usually takes the form of a fan or propeller.

The overall length of the wind tunnel may be shortened, and the rejection of kinetic energy at the diffuser exit eliminated, by the construction of some form of return circuit. Even then the driving unit is necessary

to overcome the losses occurring due to vorticity, eddying motion and turbulence. The skin friction at the walls and other surfaces will be large since the velocity at all points in the circuit will be large (of the same

order as the test-section velocity). Also, a construction ahead of the test–section, is necessary if the turbulence at the test-section has to be low, and particularly if the velocity distribution has to be uniform. To achieve this, usually guide vanes are placed in the corners.

Unit I Open and Closed Circuit Wind Tunnel

Figure:3 Closed Circuit Wind Tunnel

Open and Closed Circuit Wind Tunnels

Unit I Similarity Parameters

Geometric similarity
One of the most important requirements of models is that there should be geometric similarity between the model and the prototype. By geometric similarity it is meant that ratios of corresponding dimensions in the model and the prototype should be the same.

Dynamic similarity
Equally important as the geometric similarity is the requirement of dynamic similarity. In an actual flight, when the body moves through a medium, forces and moments are generated because of the viscosity of the medium and also due to its inertia, elasticity and gravity. The inertia, viscous, gravity and elastic forces generated on the body in flight can be expressed in terms of fundamental units. The important force ratios can be expressed as non dimensional numbers. For example,

 Reynolds number (Re) = Inertia force/Viscous force
 Mach number = Inertia force/Elastic force
 Froude number = Inertia force/Gravity force
 Euler's number = Inertia force / Pressure force
 Weber Number = Inertia force / Surface tension force

The principle of dynamic similarity is that a scale model under same Reynolds number and Mach number will have forces and moments on it that can be scaled directly. The flow patterns on the full scale body and the model will be exactly similar.

It is not necessary and may not be possible that all the aforesaid non dimensional numbers be simulated simultaneously in any experiment. Depending on the flow regime or the type of experiments, certain non-dimensional parameters are important. For example, in a low speed flow regime, simulation of Reynolds number in the experiments is important to depict the conditions of actual flight. In a high speed flow, simulation of Mach number is significant. It may even be necessary and significant that more than one non dimensional parameter are simulated.

Unit I Definition / Application of Wind Tunnel

Definition of a Wind Tunnel

A wind tunnel is a specially designed and protected space into which air is drawn, or blown, by mechanical means in order to achieve a specified speed and predetermined flow pattern at a given instant. The flow so achieved can be observed from outside the wind tunnel through transparent windows that enclose the test section and flow characteristics are measurable using specialized instruments. An object, such as a model, or some full-scale engineering structure, typically a vehicle, or part of it, can be immersed into the established flow, thereby disturbing it. The objectives of the immersion include being able to simulate,visualize, observe, and/or measure how the flow around the immersed object affects the immersed object.

Application of Wind Tunnel

There are many uses of wind tunnels. They vary from ordinary to special: these include uses for Subsonic, supersonic and hypersonic studies of flight; for propulsion and icing research; for the testing of models and full-scale structures, etc. Some common uses are presented below. Wind tunnels are used for the following:

(i) To determine aerodynamic loads

Wind tunnels are used to determine aerodynamic loads on the immersed structure. The loads could be static forces and moments or dynamic forces and moments. Examples are forces and moments on airplane wings, airfoils, and tall buildings. 

In our Aerodynamics lab we use load cells to calculate the drag and lift acting on the aerofoil, cylinder, and other models ....(Do you remember in Aerodynamics Lab?)

(ii) To study how to improve energy consumption by automobiles

They can also be used on automobiles to measure drag forces with a view to reducing the power required to move the vehicle on roads and highways.

(iii) To study flow patterns

To understand and visualize flow patterns near, and around, engineering structures. For example, how the wind affects flow around tall structures such as sky scrapers, factory chimneys, bridges, fences, groups of buildings, etc. How exhaust gases ejected by factories, laboratories, and hospitals get dispersed in their environments.

(iv) Other uses include

To teach applied fluid mechanics, demonstrate how mathematical models compare to experimental results, demonstrate flow patterns, and learn and practice the use of instruments in measuring flow characteristics such as velocity, pressures, and torques.

Unit I Need of Experiments

Need of experiments

(i)Theory is incomplete and needs to be supplemented.

(ii) Information of fundamental nature needed in many areas.

Experimental information towards solving aerodynamic problems could be obtained in a number of ways. Flight tests, rocket flights, drop tests water tunnels, ballistic ranges and wind tunnels are some of the ways by which aerodynamic data can be generated. With the help of well performed experiments even information of fundamental nature could be derived.

Wind tunnel
Majority of experimental data needed in aerodynamics is generated using wind tunnels. Wind Tunnel is a device for producing airflow relative to the body under test. Wind tunnels provide uniform flow conditions in their test section.

Unit I Classification of Wind Tunnels

Wind tunnels may be classified based on any of the following:
(a) Speed, Mach no
They are classified as of low speed or high speed wind tunnels .In wind tunnel parlance, high speed wind tunnels are those operating at speeds where compressibility effects are important. They are also classified based on the Mach number of operation as subsonic, transonic, supersonic or hypersonic wind tunnels.

Range of the Mach number , M Name of flow , or conditions
M<1 Subsonic
M=1, or near 1 Transonic
1<M<3 Supersonic
3<M<5 High supersonic
M>5 Hypersonic
M>> 5 High Hypersonic

(b) Mode of operation (Pressure storage, in-draft or Pressure vacuum type.)
(c) Kind of test section (T.S) - Open, Closed or Semi enclosed

Wind Tunnel in our Aerodynamics Laboratory

Open Circuit Wind Tunnel in Aerodynamics Laboratory of 

JJ College of Engineering and Technology, Trichy, INDIA

Syllabus according to Anna University Regulations 2008

AE2353                        WIND TUNNEL TECHNIQUES         L T P C    

3  0  0  3

OBJECTIVE

To introduce the basic concepts of measurement of forces and moments on models during the wind tunnel testing.

UNIT I       WIND TUNNELS                                                                  10

Classification –non-dimensional numbers-types of similarities - Layout of open circuit and closed circuit subsonic wind tunnels – design parameters-energy ratio - HP calculations,  Calibration.

UNIT II      HIGH SPEED WIND TUNNELS                                         10

Blow down, in draft and induction tunnel layouts and their design features, Transonic, supersonic and hypersonic tunnels, their peculiarities and calibration. Helium and gun tunnels, Shock tubes

UNIT III    WIND TUNNEL MEASUREMENTS                                  12

Pressure,velocity and temperature measurements – Force measurements – types of

balances-Three component and six component balances – calibration of measuring

instruments.

UNIT IV    FLOW VISUALIZATION                                                    6

Smoke and Tuft grid techniques – Dye injection special techniques – Optical methods of flow visualization.

UNIT V      NON-INTRUSIVE FLOW DIAGNOSTICS                        7

Laser – Doppler anemometry. Particle image velocimetry. Laser induced fluorescence.

TOTAL: 45 PERIODS

TEXT BOOK

1.     Rae, W.H. and Pope, A. “Low Speed Wind Tunnel Testing”, John Wiley Publication, 1984.

REFERENCE

1.     Pope, A., and Goin, L., “High Speed wind Tunnel Testing”, John Wiley, 1985

How to be Happy in Wind Tunnel Techniques class?

• Try to attend all the classes …

• Take notes on the class, maintain a notebook…

• Just read it every evening for 30 minutes…

• Own one books on Wind Tunnel Techniques… (Low Price Editions available)

• Do the Assignments and Cycle Tests properly...

• Once again Welcome to the course 

Welcome note to the AE 2353 WIND TUNNEL TECHNIQUES

Dear all,

Welcome to the Course materials page of AE2353- WIND TUNNEL TECHNIQUES. I will try to upload or link whatever materials possible in this blog. Enjoy learning...

You are most welcome to ask questions if you have...

Have a wonderful learning this semsester....

Best Regards
PILLAI