Aeronautical Laboratory Prelim

air density remains nearly constant

Describe air density in subsonic flow.

increase velocity and decrease pressure

What happens if you decrease the cross-sectional area in a subsonic flow?

end of the contraction section and upstream of the diffuser

Where is the test section placed in subsonic flow?

air density changes due to compressibility

Describe air density in supersonic flow.

decrease velocity and increase pressure

What happens if you decrease the cross-sectional area in a supersonic flow?

end of the diffuser

Where is the test section placed in supersonic flow?

Wind Tunnels

devices which provide an airstream flowing under controlled conditions so that items of interest to aeronautical engineers can be tested

Mach Number

ratio of a given velocity to the speed of sound in the air about the body

Mach Number

a more typical parameter than velocity

Intermittent Operation

may be of blow-through or suction type

Blow-Through

Suction Type

types of Intermittent Operation tunnels

Blow-through

air is compresses to a very high pressure in a reservoir

no great power is required

advantage of blow-through wind tunnel

• short running time

• stagnation temperature

• pressure varies

disadvantage of blow-through wind tunnel

Suction-type

air is sucked into a previously evacuated chamber

high Mach numbers can be obtained

advantage of suction-type wind tunnel

shorter time than blow-through

disadvantage of suction-type wind tunnel

Induced Flow Tunnels

high speed jet is forced through a narrow annular slot downstream of the working section

same time as blow-through

disadvantage of induced flow tunnel

Continuous Operation

wind tunnel that incorporates a return circuit and has revolving flow inside

return circuit

incorporated in wind tunnels which eliminates the need for driers

Choking

this phenomenon occurs when the local Mach number in a high-speed tunnel reaches unity at some point in the flow other than the nozzle throat

M = 1

What does it mean when the wind tunnel reaches unity?

• simpler to design

• less costly to build

• single drive may run several tunnels of different capabilities

• more convenient to test models since not a lot of time is spent in pumping down the whole circuit

• failure of model will not damage the tunnel

• extra power is available to start the tunnel

• loads in a model during the establishments of high-speed flow are less sevre because of faster starts

advantages of intermittent tunnels

• more control of conditions

• may return to a given flow condition with more accuracy

• the panic of rapid testing is removed; check points are more easily obtained

• testing conditions can be held constant over a long period of time

advantages of continuous tunnel

parallel-sided duct

the working sections in a low speed tunnel consists of

False. It is impossible to achieve supersonic speeds by increasing the blowing pressure.

True or False. By increasing the blowing pressure, supersonic speeds can be achieved.

increasing blowing pressure

How can increased in velocity be achieved in the working section?

Liner

can be single-sided

replacement of one liner by another is much easier and cheaper

advantage of one-sided liner

shock waves

spoils the flow in the working section

- is reflected as a results of waves striking the opposite walls

rectangular/ rectangle

shape of the working section

• easier to construct nozzles of the required area distribution

• visualization of the flow by optical methods involve the insertion of plane windows in the side walls

• shock waves are generated when models are mounted, which is reflected by tunnel walls which should not strike the model back

Why should the working section be rectangular in section?

Diffusion

slowing down of the air after it has left the working section

driers

it is needed to prevent condensation of the water vapor in the wind tunnel

drier

is not installed in the continuous tunnel

High-Speed Subsonic Tunnel

- have parallel-side liners

- speed and Mach number are increases simply by increasing the blowing pressure

special liners

required to cover the transonic range of Mach numbers from 0.85-1.15

shock tube

gun-tunnel

experimental tools for the investigation of flow at hypersonic speeds

M = 1

Mach number range of Sonic Range

0<M<1

Mach number range of Subsonic Range

0.8<M<1.2

Mach number range of Transonic Range

1.2<M<5.0

Mach number range of Supersonic Range

M>5.0

Mach number range of Hypersonic

Flow Visualization

used to make the flow patterns visible in order to get qualitative or quantitative information on them

Surface Flow Visualization

reveals the flow streamlines in the limit as a solid surfaced is approached

Surface Oil Film

Liquid Crystals and Temperature Sensitive Paints

Pressure Sensitive Paints (PSP)

examples of Surface Flow Visualization

Oil film or dots

enables obtaining a picture of the flow pattern at the surface of the model placed in the wind tunnel quickly an easily

liquid crystals

reflects only one light wavelength at each viewing angle when they are illuminated with white light under certain angle of incidence

surface-temperature distribution

can be gained by coating a test model with cholestric liquid crystals

Pressure Sensitive Paint (PSP)

spatially continuous pressure and temperature distribution on aerodynamic surfaces is important for understanding flow mechanics

conventional pressure measurements

based on pressure taps and electronically scanned transducers

pressure taps and electronically scanned transducers

conventional pressure measurements are based on

pressure taps

provide pressure information only at discrete points

True. The particle tracer can be either solid, liquid or gaseous and the fluid liquid or gaseous.

The particle tracer can be either solid, liquid or gaseous and the fluid liquid or gaseous.

Smoke Visualization of the Flow

Visualization using Dye

Visualization by different small particles

Gas Bubble Visualization

example of Tracer Particle Methods

Tracer Particle Method

visualization technique of streamlines, filament lines or particle paths, which injects some foreign material into a flow as a tracer

where the particles are introduced

length of the exposure time

reference system from which the flow is observed or photgraphed

the curves in the tracer particle method will depend on which three factors

True. There is no difference between liquid and gaseous flows in the tracer particle methods.

True or False. There is no difference between liquid and gaseous flows in the tracer particle methods.

Smoke Visualization of the Flow

one of the oldest flow visualization technique that will continue to be an important experimental tool in the study of complex flow dynamic phenomena

low turbulence

smoke can be very useful when a wind tunnel has

Visualization using Dye

analogy of the smoke visualization technique

the mixing of smoke and air

Which is more intense? The mixing of smoke and air or the mixing of dye and water?

small ejector tube

dye can be injected in a tested flow either from a _____

visualization by different small particles

adding small particles in the flow can enable visualization and measuring of the flow velocity

0.1 to 20 microns

diameter of the particle used

Gas Bubble Visualization

visualization where tracer particles have lower (in water) or similar (in air) density to the flow

electrolysis

generates gas bubbles for gas bubble visualization

Optical Methods

method for flows that reveal their patterns by way of changes in their optical refractive index

Shadowgraph Method

Schlieren Photography

Interferometry

examples of Optical Methods

Shadowgraph Method

oldest and simplest of all optical methods for flow visualization

spherical mirror or lens

What makes the light parallel in Shadowgraph Method?

Schlieren Method

sensitive to the changes of the first derivative of density

Schlieren Method

flow visualization most frequently used in aerodynamic laboratories since it is simple and very useful

Interferometry

a phase alteration beam passes through a disturbed section of a tested field which can be compared to an undisturbed beam

greater density

In Schlieren method, light travels more slowly in _________ and is refracted towards this.

effects of interference

basis of interferometry

Interferometry

basis of this application is as old as the Schlieren Method

Hot Wire technique

a heated wire is inserted into the airstream to produce a filament of air of different density from the mainstream

Spark technique

small volumes of air are heated by the discharge of a series of electric sparks

electric sparks

In Spark Technique, small volumes of air are heated by the discharge of a series of _____________

ammonia

The model is coated with mercurous chloride which stains black when it comes into contact with _________

mercurous chloride

used to coat the model in the Gas Filament Method

black

when the model coated with mercurous chloride comes into contact with ammonia, it stains ____

Gas Filament Method

The model is coated with mercurous chloride, which stains black, if it comes into contact with ammonia.

Gas Filament Method

No special lighting arrangements are required, in this method.

China Clay Method

The surface of the model is thinly sprayed with kaolin (china clay).

kaolin

The surface of the model is thinly sprayed with __________

kaolin

another word for china clay

white

When the model dries, the kaolin appears _____

ethyl and methyl salicylate

example of volatile fluids used in China Clay method

volatile fluid

used to spray the deposit in China Clay Method

turbulent boundary layer

Which has a higher rate of evaporation in the China Clay Method?

transition point

In China Clay Method, , the kaolin again appears as a white, opaque deposit, which enables what to be located?

• after one test run, the model can be re-sprayed with fluid and used again (kaolin coating lasts for a considerable time)

• investigation covers the whole surface and not just a few isolated spanwise positions

advantages of China Clay Method

Liquid Film Method

model is simply sprayed with a volatile oil when placed in an airstream

volatile oil

used to spray the model in the Liquid Film Method

transition point

In the Liquid Film Method, the comparison of the wet and dry regions will enable what to be located?

Wool Tufts Method

visualization of boundary layer using wool tufts

wool tuft

attached to the ends of short wires which are fixed normally on the surface of a wing or model