Module 1 Flashcards
Wind Tunnel Familiarization
Aerodynamicists use ________s to test models of proposed aircraft.
wind tunnel
The model to be tested in the wind tunnel is placed in the _______ of the tunnel.
test section
The speed in the test section is determined by the ______of the tunnel.
design
For subsonic flow:
air density:
cross-sectional area:
velocity:
pressure:
increase or decrease?
air density: constant
cross-sectional area: decrease
velocity: increase
pressure: decrease
(and vice versa)
The choice of speed range affects the design of the wind tunnel due to __________.
compressibility effects
The ___________ is the settling chamber on a closed return
tunnel, or the open room of an open return design.
plenum
For supersonic flows, the air density changes in the tunnel because of __________
compressibility.
between density and velocity, which of them changes faster in supersonic flows?
density
why density changes faster than velocity in supersonic flows
because of a factor the square of the mach number
In a supersonic flow, what is the relationship of cross-sectional area, velocity, and pressure.
cross-sectional area: decrease
velocity: decrease
pressure: increase
vice versa
True or False:
Only compressible flows experience mass flow choking
True
How does mass flow choking happen?
As a subsonic flow is contracted, the velocity and Mach number increase. When the velocity reaches the speed of sound (M = 1), the flow chokes and the Mach
number cannot be increased beyond M = 1.
how to obtain the highest possible velocity in subsonic wind tunnel?
the test section is placed at the end of the contraction
section and upstream of the diffuser.
how to obtain the highest possible velocity in supersonic wind tunnel?
we contract the flow until it chokes in the throat of a nozzle. We then diffuse the flow which increases
the speed supersonically. The test section of the supersonic tunnel is placed at the
end of the diffuser.
In test section of a subsonic tunnel design, mach number is _________
less than 1
In a subsonic tunnel design, which velocity is higher: in plenum or test section
test section
In a subsonic tunnel design, which pressure is higher: in plenum or test section
plenum
In a subsonic tunnel design, which mach number is higher:
in diffuser or in test section
Test Section
In a subsonic tunnel design, which pressure is higher: in the diffuser or the test section
diffuser
In a subsonic tunnel design, which velocity is higher: in the diffuser or the test section
test section
in a supersonic tunnel design, the mach number in the throat is equal to ______
1
in terms of mach number in a supersonic tunnel design, arrange the following from slowest to fastest:
speed of sound
throat
test section
speed of sound
throat
test section
in terms of velocity in a supersonic tunnel design, arrange the ff. from slowest to fastest:
test section
throat
throat
test section
in terms of pressure in a supersonic tunnel design, arrange the ff. from lowest to highest :
throat
test section
test section
throat
__________are devices which provide an airstream flowing under controlled conditions so that items of interest to aeronautical engineers can be
tested.
Wind tunnels
__________ are those whose usual operating speeds require the inclusion of compressible flow effects.
High-speed tunnels
in the high- speed field we usually talk about _______________—the ratio of a given velocity to the speed of sound in the air about the body—as a more typical parameter than velocity.
“Mach number”
A lower limit of “high speed” might be considered to be where the Mach number is approximately ______—about _______ mph for standard sea level conditions.
0.5
380
The power to drive a low-speed wind tunnel varies as the _______ of the________ in the wind tunnel.
cube
velocity
Because of the power requirements, high-speed wind tunnels are often of the “__________” type,
intermittent
It is a type of wind tunnel in which energy is stored in the form of pressure or vacuum or both and is allowed to drive the tunnel only a few seconds out of each pumping
hour.
Intermittent Type
The problems of _____________ are common to all supersonic wind tunnels. So is the problem of providing the necessary ____________, and thus
the necessary power, to achieve supersonic flow.
nozzle and diffuser design
pressure ratio
Wind tunnels for intermittent operation may be of the _________or ___________type.
blow-through
suction
explain how blow-through tunnel operates
Compressed air from the reservoir passes through a valve, a drier to remove moisture, and gauzes to stabilize the flow, entering the settling chamber. The air is then accelerated through the nozzle, achieving the desired speed in the working section for testing purposes. Finally, the air is decelerated in the diffuser before exiting the tunnel.
why does blow -through tunnel requires no great power
because reservoir can be charged up relatively slowly by means of a compressor, which does not have to be especially highly powered.
Why is the running time of a blow-through wind tunnel generally short?
The running time is generally short because the reservoir contains a limited amount of compressed air. Once the pressure in the reservoir drops below a certain
value, the pressure ratio across the tunnel will no longer be sufficient to give supersonic flow in the working section.
give two disadvantages of blow-through tunnel
- running time is generally short
- stagnation temperature and pressure vary during a run
How do the stagnation temperature and pressure vary during a run in a blow-through wind tunnel?
During a run in a blow-through wind tunnel, the stagnation temperature and pressure decrease due to the expansion of the air in the reservoir as it is depleted.
explain suction type tunnel
Air is drawn through the tunnel by a vacuum created in the evacuated chamber, passing through a drier, gauzes, and the settling chamber to ensure a uniform flow. The air is then accelerated through the nozzle, reaches the test speed in the working section, and is finally decelerated in the diffuser before entering the evacuated chamber.
throughout a run in a suction type tunnel, _________ remain constant at the atmospheric value.
stagnation conditions
Why do the stagnation conditions remain constant throughout a run in a suction-type wind tunnel?
because the flow originates from the atmosphere outside the tunnel, maintaining a steady atmospheric value.
advantages of suction type tunnel
- The stagnation conditions remain constant at atmospheric values since the flow originates from the atmosphere outside the tunnel.
- Very high-pressure ratios can be easily achieved.
- High Mach numbers can be obtained without requiring a chamber that can withstand high pressurization.
The running time is likely to be even shorter than that of a blow-through tunnel, especially when operating at high Mach numbers.
Induced flow tunnel
Disadvantages of suction type tunnel
The running time is likely to be even shorter than that of a blow-through tunnel, especially when operating at high Mach numbers.
in an induced flow tunnel, This____ induces the flow through the working section which, if the blowing pressure is high enough, will be supersonic.
jet
where is the jet passes in an induced flow tunnels?
slots
How is the jet through the slots provided, and what serves as its effective power supply?
The jet through the slots is provided either by a compressor or, more commonly, from a compressed air reservoir, which serves as the effective power supply, similar to a blow-through tunnel.
how to regulate the pressure ratio in an induced flow tunnel when a compressed air supply is used
a continuously variable throttle valve is required
between the reservoir and the slots.
Why is it generally impossible to achieve Mach numbers much above 1-8 in such tunnels?
because of the difficulties involved in the mixing of the two jets, to achieve Mach numbers much above 1-8.
main advantage of an induced flow tunnel
they do not suffer from the disadvantage of varying stagnation conditions since the air which flows through the working section comes from the atmosphere.
The running time depends on the ________________-it may even be a
continuously running tunnel.
magnitude of the available compressed air supply
what is common in an induced flow tunnel and a blow-through tunnel?
have an advantage in running time over suction type
_____________may be achieved in a blow-through or induced flow tunnel if a sufficient compressed air supply is available, i.e., supplied by a _____________whose capacity is such that it can supply air at the required pressure
at least at the rate air is consumed by the tunnel.
Continuous operation
compressor
Apart from this, continuous operation may be achieved in effectively the same way as in a low speed tunnel, except that instead of a ______ as compressor, usually a ________________, is incorporated in the tunnel circuit, to provide the necessary pressure
ratio for the achievement of the design Mach numbers.
fan
multi-stage axial flow compressor
The other features of a continuous operation tunnel, i.e., liner, diffuser, etc., will be the same as in intermittently operating tunnels, except that almost all compressor driven tunnels incorporate a_____________
return circuit
Advantages of incorporating a return circuit
- The return circuit helps achieve some measure of power economy
- No Need for Continuous Drying
Disadvantage of return Circuits
- the duration of the test is long, the temperature may tend to rise resulting in varying stagnation
conditions, unless some form of cooling is used.
If a __________is incorporated
in an induced flow tunnel, then there must be a break in the circuit somewhere to bleed off the excess air which enters the circuit through the inducing slots.
return circuit
How choking occurs ?
when the local Mach number in a high-speed tunnel reaches unity at some point in the flow other than the nozzle throat.
where choking naturally happen?
at points where the effective area of cross-section of the tunnel is reduced as a result of :
1.boundary layer thickening,
2.shock-induced separation, or
3. simply the presence of a model which is too thick, or at too high an incidence.
When choking occurs, the supersonic flow in the working section____________, and this condition sets a limit to the ___________ of the tunnel-a limit which may
vary according to the model under test, and its attitude.
breaks down
range of operation
The _____________ is used throughout the speed range.
continuous tunnel
The _________________ and _____________ are normally used for Mach numbers from
0.5 to about 5.0,
intermittent blowdown
indraft tunnels
The intermittent blowdown and indraft tunnels are normally used for Mach numbers from
_________ to about ____________
0.5
5.0
the ________________ are
normally used for higher Mach numbers. ( 5above)
intermittent pressure- vacuum tunnels
Advantages claimed for intermittent tunnels are:
- They are simpler to design and less costly to build.
- A single drive may easily run several tunnels of different capabilities.
- Model testing is more convenient, since a lot of time need not be spent in
pumping down the whole circuit and getting the drive motors up to speed. - Failure of a model will usually not result in tunnel damage.
- Extra “power” is available to start the tunnel.
- Loads on a model during the establishment of high-speed flow (starting loads) are less severe because of faster starts.
Advantages claimed for the continuous tunnels are:
- We are more in control of conditions and may usually return to a given flow
condition with more accuracy. - Since 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.
faster instrumentation is usually employed with _______________ tunnels,
intermittent
is the ratio of supply to discharge pressure
compression ratio
These almost automatically provides compression ratio ratios.
The intermittent tunnel—particularly the indraft or the pressure-vacuum
type
Some advantages of the indraft tunnel over the blowdown tunnel are:
Total air temperature at supply conditions (stagnation temperature) is
constant during a run.
2. Total air pressure at supply conditions (stagnation pressure) is constant
during a run although it may be lower than desired. There are no variations
in total pressure such as those a pressure regulator may cause.
3. The airstream is free from contaminants such as compressor oil (but may
contain dust from the desiccant of the air drier).
4. The headaches and dangers of pressure regulators are removed.
5. Loads on a model during the establishment of the high-speed flow (starting
loads) are smaller.
6. Vacuum is safer to handle than pressure.
7. The noise level is lower.
8. Obtaining low air density (corresponding to high altitude) in the tunnel is not
difficult.
9. The indraft tunnel can operate at higher Mach numbers before heating is
required to prevent the liquefaction of air during the expansion to high
speeds.
10.For a given cost, indraft tunnels are larger.
Advantages of the blowdown tunnel over the indraft tunnel are:
- It is possible to vary the Reynolds number widely at a particular Mach
number. In some cases, the value corresponding to full-scale flight may be
reached. - Cost is from slightly lower than to less than one-fourth of that of an indraft
tunnel of equal Reynolds number. - Short-time burning tests are usually possible.
In a low speed tunnel, the working section consists simply of a ____________.
parallel- sided duct
Increase in velocity is achieved by increase in blowing pressure.
However, it is impossible to achieve supersonic speeds simply by increasing the blowing pressure. In a supersonic tunnel, therefore, a ______________ must be incorporated,
convergent-divergent nozzle
it is incorporated in a tunnel to accelerate the flow to supersonic speeds.
convergent-divergent nozzle
The flow is accelerated subsonically to sonic speed at the
______________
throat
The whole channel, nozzle and working section, is called the ____________ of the tunnel.
liner
For any given liner, what are fixed properties
area distribution
mach number distribution
To change the operating Mach number, a ______________ is required.
liner of different shape
what problem can be encountered when detail shape of the nozzle is not considered?
when flow accelerates super sonically expansion waves
emanate from points on the tunnel walls. When these waves strike the opposite walls, they may be reflected as shock waves, which would spoil the flow in the
working section.
The shape of the liner walls must be calculated so as to avoid _____________,
wave reflection
advantage of using single-sided liners
This makes the replacement of one liner by another much easier and cheaper.
disadvantage of using single-sided liners
difficult to achieve uniform flow
is the slowing slowing down of the air after it has left
the working section
diffusion of the flow
two stages of diffusion in a supersonic tunnel
A supersonic diffuser is required in which the flow is slowed down from supersonic to subsonic speeds, and this must be followed by a subsonic diffuser in which the flow is reduced to the low speeds at which the air is
discharged to the atmosphere or sent on its return circuit.
There is no reason
why the subsonic diffuser should be any different from one used in a low speed
tunnel, except that it will probably need to be ________, since it must affect a bigger
_____________
longer
speed reduction.
It is in the _____________ that most of the power losses occur
diffuser
The simplest type of supersonic diffuser of these consists in an arrangement whereby a ____________is allowed to form at the_______________
normal shock
downstream end of the working section.
normal wave in a supersonic diffuser is achieved by using a ____________________ which is just sufficient to maintain
supersonic flow up to that station.
blowing pressure
How to artificially generate a normal shock.
or by inserting some obstacle
This form of supersonic diffuser (using artificial shockwave) is inefficient because of the ___________________. It is acceptable only if the________________- is fairly low.
Large wastage of kinetic energy in the shock wave
Mach number
what are the different supersonic diffusers?
- diffuser that allows normal shock
- diffuser that incorporates second throat downstream of the working section
Second throat downstream of the working section, giving a gradual reduction in ____________–.
Mach number
Ideally, if the second throat had the same area as
the first throat, ______________ would occur there also, followed by ________________ behind it, resulting in ______________
sonic flow
subsonic diffusion
no power loss.
incorporating second throat, However, ______________ interfere with this ideal flow,
boundary layer effects
____________ implies that the local
Mach number becomes unity somewhere in the flow, preventing further flow
acceleration upstream of this point.
choking
what limits the area of second throat?
- since it is not possible to compress the flow isentropically, and some shock losses are inevitably sustained. In consequence, it is not possible to reduce the second throat area to this ideal
value. - The possibility of choking when the tunnel is first started, which would prevent supersonic flow
from ever being achieved in the working section.
which has led to designs for supersonic diffuser which consist of simple, wedge shaped obstacles?
Power loss is, in general, less than that which is sustained if the same reduction in speed is affected through a single normal shock.
It is introduced into the flow downstream of the working section in order to generate such shocks.
simple, wedge shaped obstacles
the absence of drier results to _____________
condensation shock waves
It is a device that dries or remove the moisture from the air
drier
The most common device simply consists in a chamber filled with a______________, such as ___________________, which absorbs the water vapor from
the air as it passes through.
hygroscopic substance
silica gel
An alternative means of preventing condensation, without the use of a drier, consists in_____________before it enters the tunnel.
heating the air
Heating also has the added disadvantage of altering the stagnation conditions in the tunnel. One possible
device consists in the use of a ______________ located in the ___________________ from which many tunnels are fed with air.
heat sink
high-pressure tank or reservoir
The_____________ is simply a mass of loosely packed metal in the tank.
sink
how heat sink works?
As the air is compressed in the tank, it gives
up a good deal of its water content, which may be drained off. The temperature in the tank also rises, including that of the metal. When the air is released into
the tunnel, the expansion causes rapid cooling.
_________________ have parallel-sided liners, and the speed and Mach number are increased simply by increasing the blowing pressure, as
in the case of low speed tunnel.
High-speed subsonic tunnels
In High-speed Subsonic Tunnels, the effective cross-sectional area of the channel grows less along the length of the working section because of the ________________, and in consequence the ________________.
thickening of the boundary layer
speed of the flow increases
Choking may be delayed by using a _______ with slightly _________walls; but, if the walls are solid, sooner or later choking is bound to occur.
liner
divergent
Transonic Wind Tunnels, the design of such liners is based on a working section whose walls are not solid but ___________.
perforated
Special liners are required to cover the transonic range of Mach numbers from, say, __________________.
0.85 to 1.15
it is usually impossible, then, to achieve flow at a Mach number much above _____ with ordinary parallel-sided liners. It is also difficult to achieve supersonic flow at a Mach number below about 1.1 with a convergent-divergent nozzle with
solid walls, because of choking difficulties.
0.9
what is the principle of holes in the walls of the working section?
the excess air associated with boundary
layer growth or the presence of the model can escape through these perforations, thus eliminating the effect of a narrowing channel, and preventing
choking.
The perforated walls are surrounded by other, solid walls, and the air which has escaped through the holes into the expansion chamber between, which is known as the _________________, is fed back into the tunnel downstream of the working section
plenum chamber
The ____________ is a device for producing a flow at very high Mach number for a very short time. The device
consists simply of a long tube, in which a diaphragm separates a region in which the air is compressed to a very high pressure, from an evacuated region.
shock tube
The __________________ are experimental tools for the investigation of flow at hypersonic speeds.
shock tube and gun-tunnel
