Wind Tunnel Familiarization Flashcards
(87 cards)
1
Q
use wind tunnels to test models of proposed aircraft
A
Aerodynamicists
2
Q
In the tunnel, the engineer can carefully control the flow conditions which
affect _________________
A
forces on the aircraft
3
Q
The speed in the test section is
determined by the ______ of the
tunnel.
A
design
4
Q
the choice of speed range affects the design of the wind tunnel due to _________________.
A
compressibility effects
4
Q
SUBSONIC vs. SUPERSONIC
A
SUBSONIC
1. Air density remains nearly
constant
2. Decreasing the cross-sectional
area causes the flow to increase velocity and decrease pressure, vice versa.
3. Test section is placed at the end
of the contraction section and upstream of the diffuser.
SUPERSONIC
1. Air density changes in the tunnel
because of compressibility
2. Decreasing the cross-sectional
area causes the flow to decrease
in velocity and increase pressure, vice versa.
3. Test section is placed at the end
of the diffuser.
5
Q
_____________ are devices which provide an airstream flowing under
controlled conditions so that items of interest to aeronautical engineers can
be tested.
A
Wind tunnels
6
Q
are those whose usual operating speeds require the inclusion of compressible flow effects
A
High-speed tunnels
7
Q
in the high-speed field we usually talk about __________ as a more typical parameter than velocity.
A
“Mach number”
8
Q
wind tunnels are generally
classified as
A
- low-speed,
- high-speed, and
- special purpose tunnels.
9
Q
In low-speed tunnels, the predominant factors influencing the tunnel performance are
A
- inertia and
- viscosity.
10
Q
High-speed tunnels are those with test-section speed ___________ .
A
more than
650 kmph
11
Q
High-speed tunnels are generally grouped into __________ and ________ operation tunnels, based on the type of operation.
A
intermittent and
continuous
12
Q
Because of the power requirements, high-speed wind tunnels are often of the “________” type
A
intermittent
13
Q
Because of the power requirements, high-speed wind tunnels are often of the “intermittent” type, in which energy is stored in the form of pressure or vacuum or both and is allowed to _________________________________
A
drive the tunnel only a few seconds out of each pumping hour
14
Q
Common Problems in Supersonic Wind Tunnels:
A
- The problems of nozzle and diffuser design are common to all supersonic wind tunnels.
- So is the problem of providing the necessary pressure ratio, and thus the necessary power, to achieve supersonic flow.
15
Q
Solution for the Common Problems in Supersonic Wind Tunnels
A
The different designs include:
* tunnels for intermittent operation, and continuously running tunnels;
- tunnels in which the flow goes straight through and return circuit tunnels:
- tunnels in which the flow is direct, and induced flow tunnels.
16
Q
Wind tunnels for intermittent operation may be of the:
A
- blow-through or
- suction type
17
Q
air is compressed to a very high pressure in a
reservoir
A
blow-through tunnel
18
Q
blow-through tunnel
air is subsequently released through a valve into
__________and is eventually discharged into the
atmosphere.
A
the nozzle
19
Q
No great power is required, since the reservoir
can be charged up relatively slowly by means of a
compressor, which does not have to be especially
highly powered.
A
blow-through tunnel
20
Q
In a blow-through tunnel:
Disadvantages:
A
- Running time is generally short, because of the limited amount of air in the reservoir. Meaning, it is then necessary to stop the tunnel and re-charge the reservoir, and the charging time between runs may be considerable
- The stagnation temperature and pressure will vary during a run, due to the expansion of the air in the reservoir, where the flow originates
21
Q
In a typical suction type tunnel:
* Instead of the air flowing out of a compressed air reservoir,
it is _______________.
A
sucked into a previously evacuated chamber
22
Q
In a typical suction type tunnel:
Throughout a run, the __________ remain constant at the atmospheric value, since the flow through
the tunnel originates in the atmosphere outside the tunnel.
A
stagnation conditions
23
Q
In a typical suction type tunnel:
__________ are easily achieved, and this implies that high Mach numbers can be obtained, without
the use of a chamber which can withstand high
pressurization
A
Very high-pressure ratios
24
In a typical suction type tunnel:
Disadvantage:
* Running time is likely to be even shorter than that of a blow-through tunnel, especially at high Mach numbers.
25
In this type of tunnel, a high-speed jet is forced through a narrow annular slot downstream of the working section.
Induced Flow Tunnels
26
Induced Flow Tunnels
This jet induces the flow through the working section which, if the blowing pressure is __________, will be ___________.
high enough, supersonic
27
induced flow tunnels
their main advantage is that, while they have the advantage of the blow-through over the suction type tunnel in terms of running time, they do not suffer from the
_____________.
disadvantage of varying stagnation
conditions
28
_______________ may be achieved in a blow-through or induced flow tunnel if a sufficient compressed air supply is available.
Continuous operation
29
continuous operation may be achieved in effectively the
same way as in a low-speed tunnel, except that instead of a fan a
compressor, usually a ____________, is incorporated in the tunnel circuit
multi-stage axial flow compressor
30
Continuous Operation
The other features of such a tunnel, i.e., liner, diffuser, etc., will be the same as in ______________, except that almost all compressor driven tunnels incorporate a __________.
intermittently operating tunnels, return circuit
31
Advantages of Intermittent:
1. They are simpler to design and less costly to build.
2. A single drive may easily run several tunnels of different capabilities.
3. 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.
4. Failure of a model will usually not result in tunnel damage.
5. Extra “power” is available to start the tunnel.
6. Loads on a model during the
establishment of high-speed flow (starting loads) are less severe because of faster starts.
32
Advantages of Continuous:
1. We are more in control of
conditions and may usually return to a given flow condition with more accuracy.
2. Since the “panic” of rapid testing
is removed, check points are more easily obtained.
3. Testing conditions can be held
constant over a long period of time.
33
A return circuit may be incorporated in a ____________, just as it may in a low-speed tunnel, and since the __________ in the return circuit is everywhere quite __________ , there is no reason for the design to be any different.
supersonic wind tunnel, speed of flow, low
34
If a return circuit is incorporated in an induced flow tunnel, then there must be a __________ somewhere to ____________ which enters the circuit through the inducing slots
break in the circuit, bleed off the excess air
35
Return Circuit Advantages:
* Some measure of power economy is achieved by using a return circuit.
* Since the same air is being re-circulated all the time, there should be no need for continuous drying
36
Return Circuit Disadvantage:
If 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
37
The phenomenon of ________ occurs when the local Mach number in a high speed tunnel reaches unity at some point in the flow other than the nozzle
throat.
choking
38
This may happen at points where the effective area of cross-section of the tunnel is reduced as a result of boundary layer thickening, shock-induced separation, or simply the presence of a model which is too thick, or at too high an incidence.
Choking
39
Advantages of the blowdown tunnel over the indraft tunnel
1. 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.
2. Cost is from slightly lower than to less than one-fourth of that of an indraft tunnel of equal Reynolds number.
3. Short-time burning tests are usually possible.
40
Some advantages of the indraft tunnel over the blowdown tunnel are:
1. 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.
41
In a low-speed tunnel, the working section consists simply of a ________.
parallel-sided duct
42
__________ is achieved by increase in blowing pressure. However, it is impossible to achieve supersonic speeds simply by ______________-.
Increase in velocity, increasing the blowing pressure
43
In a supersonic tunnel, therefore, a
____________ must be incorporated to accelerate the flow to supersonic speeds.
convergent-divergent nozzle
44
The ___________ of the wind tunnel is a convergent/divergent nozzle with a removable top part (‘liner’).
working section
45
The shape of the liner controls the
___________- at the divergent part of the working section.
maximum air velocity
46
The flow is accelerated subsonically to sonic speed at the ____________ , assuming that a sufficient pressure ratio has been established, and then accelerated
supersonically to the required Mach number in the _________, which is a parallel-sided section immediately downstream of the _________, throughout
which the Mach number should remain ____________.
throat, working section, nozzle, effectively constant
47
As the flow accelerates supersonically ______________ emanate from points on the tunnel walls. When these waves strike the opposite walls, they
may be reflected as __________, which would spoil the flow in the working section.
expansion waves, shock waves
48
Some liners are ________, i.e., one wall of the liner is plane. This makes the replacement of one liner by another much easier and cheaper, but it may
be more ___________
single-sided, difficult to achieve uniform flow
49
____________ are always rectangular in cross section, never circular or elliptical, as is the case in some low-speed tunnels.
Supersonic working sections
50
Supersonic working sections are always rectangular in cross section
There are several reasons for this, which are:
1. It is easier to construct nozzles of the required area distribution if the section is rectangular.
2. The visualization of the flow by optical methods involves the insertion of plane windows in the side walls of the working section.
3. When models are mounted in the working section, and tested, shock waves are generated by the model
51
The _______________, i.e., the slowing down of the air after it has left the working section, in the case of a supersonic tunnel has to be affected in two stages.
diffusion of the flow
52
A ____________ is required in which the flow is slowed down from supersonic to subsonic speeds, and this must be followed by a ______________ 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.
supersonic diffuser, subsonic diffuser
53
The simplest of type of supersonic diffuser consists in an arrangement
whereby a ________ is allowed to form at the ___________ of the working section.
normal shock, downstream end
54
Diffusers (Normal Shock)
This is achieved by using a ________ which is just sufficient to
maintain supersonic flow up to that station, or by ___________ which artificially generates a normal shock there.
blowing pressure, inserting some obstacle
55
Diffusers (Normal Shock)
The flow behind the shock is ________, and there is a large rise in _________. Subsonic diffusion follows
subsonic, pressure
56
Diffusers (Normal Shock)
* This form of supersonic diffuser is inefficient because of the large wastage of __________ in the shock wave. It is acceptable only if the Mach number is __________.
kinetic energy, fairly low
57
Another method consists in narrowing the channel again to what is known as a
_____________ downstream of the working section, giving a gradual reduction in Mach number.
second throat
58
ideally, if the second throat had the same area as the first throat, _______ would occur there also, followed by subsonic diffusion behind it, resulting in __________.
sonic flow, no power
loss
59
Diffusers (Second Throat)
However, ___________ effects interfere with this ideal flow, and also, as we have seen, it is not possible to _________, and some __________ are inevitably sustained. In consequence, it is not possible to reduce the second throat area to this ideal value.
boundary layer, compress the flow isentropically, shock losses
60
A further factor limiting the area of the second throat is the _____________, which would prevent ________________ from ever being achieved in the working section.
possibility of choking when the tunnel is first started, supersonic flow
61
Choking implies that the local Mach number ___________ somewhere in the flow, preventing further flow acceleration upstream of this point.
becomes unity
62
___________ are almost always used in tunnels which are designed for use at higher Mach numbers.
Second throats
63
If the diffusion from supersonic to subsonic speeds can be arranged to take place through several oblique shocks, the ______________ is , in general, less than that which is sustained if the same reduction in speed is affected through a single normal shock.
power loss
64
This has led to designs for supersonic diffusers which consist of simple, wedge-shaped obstacles, introduced into the flow downstream of the
working section in order to generate such shocks.
Diffusers (Oblique Shock)
65
In effect, such a device is not much different from a rather rough and ready second throat, and it is not so efficient as one which is more carefully designed, though it may be much easier and cheaper to construct.
Diffusers (Oblique Shock)
66
An essential feature of most supersonic tunnels is some means of drying the air.
Driers
67
DRIERS
In the absence of such a device, the expansion of the air to low temperature and pressure causes ______________ contained in the air.
condensation of the water vapor
68
DRIERS
The condensed particles generate ______________, which spoil the flow in the working section
condensation shock waves
69
The ___________- is a device for
producing a flow at very high Mach
number for a very short time.
shock tube
70
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
71
a charge of air is fired along the tube, have been developed, and the end of the tube may fork into two branches so as to give increased running time', but the above account describes the basic principle of these devices.
gun-tunnel
72
High-speed subsonic tunnels have _____________, and the speed
and Mach number are increased simply by increasing the blowing
pressure, as in the case of low-speed tunnels.
parallel-sided liners
73
At some subsonic Mach number, the local flow at some point downstream becomes _________, and the tunnel is choked.
sonic
74
Choking may be delayed by using a ____________; but, if the walls are _________, sooner or later choking is bound to occur.
liner with slightly divergent walls, solid
75
t is usually impossible, then, to achieve flow at a Mach number much above 0.9 with ___________.
ordinary parallel-sided liners
76
It is also difficult to achieve supersonic flow at a Mach number below about 1.1 with a _________________, because of choking difficulties
convergent-divergent nozzle with solid walls
77
Special liners are required to cover the transonic range of Mach numbers from
0.85 to 1.15
78
Transonic Wind Tunnels
The design of such liners is based on a working section whose walls are not solid but _________. Slots or holes are made in the walls of the working section, the area of the perforations increasing with
distance along the length of the section.
perforated
79
The shock tube and gun-tunnel are experimental tools for the
____________ at hypersonic speeds.
investigation of flow
80
Hypersonic Wind Tunnels
To achieve such high Mach numbers, liners with ___________ are required, and this may involve special problems, besides necessitating very high-pressure ratios to provide the necessary expansion of the flow.
very narrow throats
81
Hypersonic Wind Tunnels
One major problem is the likelihood of _________ once the Mach number becomes very large, because of the associated
very low temperature.
liquefaction of the working fluid
82
Hypersonic Wind Tunnels (liquefaction of the working fluid)
One possible solution is to use a ___________ other than air, such as ____________, with much lower _____________, and a lower value of the adiabatic index, y, which results in reduced temperature and pressure ratios for a given Mach number
working fluid, freon gas, liquefaction temperature
83
Blow Down Wind Tunnels (Open Circuit Type) Advantages:
1. High Mach capability (up to 4)
2. Easy tunnel starting
3. Large size test section
4. Lower construction/operating
costs
5. Superior design for propulsion
experiments and smoke visualization
84
Blow Down Wind Tunnels (Open Circuit Type) Disadvantages:
1. Requirement of faster data
acquisition system
2. Noisy operation
3. Necessity of pressure regulator
valves
85
Continuous Wind Tunnels (Closed Circuit Type) Advantages:
1. Longer run times
2. Provides good flow quality in
the test section
3. The noise level is less
86
Continuous Wind Tunnels (Closed Circuit Type) Disadvantages:
1. High construction cost
