AEROLAB 2 - WIND TUNNEL FAMILIARIZATION Flashcards
Aerodynamicists use _______ to test models of proposed aircraft.
In the tunnel, the engineer can carefully control the flow conditions which affect forces on the aircraft.
By making careful measurements of the forces on the model, the engineer can ___________ on the full-scale aircraft.
And by using special diagnostic techniques, the engineer can better understand and improve the performance of the aircraft.
wind tunnels ; predict the forces
In this type of Wind Tunnel,
Air density remains nearly constant
Decreasing the cross-sectional area causes the
flow to increase velocity and decrease
pressure, vice versa.
Test section is placed at the end of the
contraction section and upstream of the
diffuser.
Subsonic
In this type of Wind Tunnel,
Air density changes in the tunnel because of
compressibility
Decreasing the cross-sectional area causes the
flow to decrease in velocity and increase
pressure, vice versa.
Test section is placed at the end of the
diffuser.
Supersonic
In a subsonic tunnel the area is
_________ into the test section; in
a supersonic tunnel the area is
_________.
contracting ; increasing
These are devices which provide an airstream flowing under controlled conditions so that items of interest to
aeronautical engineers can be tested.
Wind Tunnels
These types of tunnels are those whose usual operating speeds require the inclusion of compressible flow effects.
High-Speed Tunnels
High-speed wind tunnels are often of the “____________” type
intermittent
Designs of Wind Tunnels
Intermittent, Continuous ; Return, Direct flow and Induced flow
Wind tunnels for intermittent operation may be of the ______________________.
blow-through or suction type
In a _________________, air is compressed to a very high pressure in a reservoir.
It is subsequently released through a valve into the nozzle and is eventually discharged into the
atmosphere.
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.
blow-through tunnel
Disadvantage of a Blow-through Wind Tunnel
Running time is short because of the limited compressed air in the reservoir
The stagnation Temperature and Pressure will vary during a run, due to the expansion of air in the reservoir, where the flow originates
In a typical _________, instead of the air flowing out of a compressed air reservoir, it is sucked into a previously evacuated chamber.
Throughout a run, the stagnation conditions
remain constant at the atmospheric value, since
the flow through the tunnel originates in the
atmosphere outside the tunnel.
Also, very high-pressure ratios are easily achieved, and
this implies that high Mach numbers can be
obtained, without the use of a chamber which
can withstand high pressurization.
suction type tunnel
Disadvantage of a Suction type wind tunnel
Running time is likely to be even
shorter than that of a blow-through
tunnel, especially at high Mach
numbers.
In this type of tunnel, a high-speed jet
is forced through a narrow annular slot
downstream of the working section.
This jet induces the flow through the working
section which, if the blowing pressure is
high enough, will be supersonic.
Induced Flow Tunnels
Advantage of 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.
Continuous operation may be achieved in a blow-through or induced flow tunnel if a _____________ is available.
sufficient compressed air supply
continuous operation may be achieved in effectively
the same way as in a low speed tunnel, except that instead of a fan, a _________, usually a _____________, is incorporated in the tunnel circuit.
compressor, multi-stage axial flow compressor
The other features of such a 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
A ________ may be incorporated in a supersonic wind tunnel, just as it may in a low speed tunnel, and since the speed of flow in the return circuit is everywhere quite low, there is no reason for the design to be any different.
return circuit
Advantages of a Return Circuits
Power economy is achieved
no need for continuous drying
Disadvantage of a Return Circuit
If the test is long, the temperature tend to rise.
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.
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
Advantages for intermittent tunnels
- Simpler design
- A Single drive may easily run several tunnels of different capabilities
- Model testing is more convenient
- Failure of a model will not result in tunnel damage
- Extra power is available to start the tunnel
- Loads on a model during the establishment of high-speed flow are less severe bc 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.
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.
Liner and Working Section
In a supersonic tunnel, therefore, a convergent-divergent nozzle must be incorporated to _______________________.
accelerate the flow to supersonic speeds
Supersonic working sections are always _________ in cross section, never circular or
elliptical, as is the case in some low speed tunnels.
rectangular
Supersonic working sections are always rectangular in cross section, never circular or elliptical, as is the case in some low speed tunnels. There are several reasons for this:
It is easier to construct nozzles
Beneficial for visualization of the flow
When models are mounted in the working section, and tested, shock waves are generated by
the model. These shocks are inevitably reflected by the tunnel walls, and it is imperative that
the reflected shocks should not strike the model.
A ________ 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.
supersonic diffuser
An essential feature of most supersonic tunnels is some means of _______.
In the absence of such a device, the expansion of the air to low temperature and pressure causes condensation of the water vapor contained in the air. The condensed particles generate condensation shock waves, which spoil the flow in the working section.
drying the air ; driers
________ tunnels 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 tunnels.
High-speed subsonic
_________ are required to cover the transonic range of Mach numbers from, say, 0.85 to 1.15.
Special liners ; Transo 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 ; Transonic Wind Tunnels
The shock tube and gun-tunnel are experimental tools for the investigation of flow at hypersonic speeds. Since hypersonic aerodynamics lies outside the scope of our course, is not intended to deal further here with experimental facilities in hypersonic, except to say that it is possible to design wind tunnels for use in this speed range
Hyperso Wind Tunnels
