Studied by 14 peopleair 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
Note
Flashcard