AEROLAB 2 - WIND TUNNEL FAMILIARIZATION

Question 1

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.

Answer 1

wind tunnels ; predict the forces

Question 2

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.

Answer 2

Subsonic

Question 3

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.

Answer 3

Supersonic

Question 4

In a subsonic tunnel the area is
_________ into the test section; in
a supersonic tunnel the area is
_________.

Answer 4

contracting ; increasing

Question 5

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

Answer 5

Wind Tunnels

Question 6

These types of tunnels are those whose usual operating speeds require the inclusion of compressible flow effects.

Answer 6

High-Speed Tunnels

Question 7

High-speed wind tunnels are often of the “____________” type

Answer 7

intermittent

Question 8

Designs of Wind Tunnels

Answer 8

Intermittent, Continuous ; Return, Direct flow and Induced flow

Question 9

Wind tunnels for intermittent operation may be of the ______________________.

Answer 9

blow-through or suction type

Question 10

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.

Answer 10

blow-through tunnel

Question 11

Disadvantage of a Blow-through Wind Tunnel

Answer 11

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

Question 12

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.

Answer 12

suction type tunnel

Question 13

Disadvantage of a Suction type wind tunnel

Answer 13

Running time is likely to be even
shorter than that of a blow-through
tunnel, especially at high Mach
numbers.

Question 14

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.

Answer 14

Induced Flow Tunnels

Question 15

Advantage of Induced Flow Tunnels

Answer 15

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.

Question 16

Continuous operation may be achieved in a blow-through or induced flow tunnel if a _____________ is available.

Answer 16

sufficient compressed air supply

Question 17

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.

Answer 17

compressor, multi-stage axial flow compressor

Question 18

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 _______.

Answer 18

return circuit

Question 19

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.

Answer 19

return circuit

Question 20

Advantages of a Return Circuits

Answer 20

Power economy is achieved
no need for continuous drying

Question 21

Disadvantage of a Return Circuit

Answer 21

If the test is long, the temperature tend to rise.

Question 22

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.

Answer 22

choking

Question 23

Advantages for intermittent tunnels

Answer 23

1. Simpler design
2. A Single drive may easily run several tunnels of different capabilities
3. Model testing is more convenient
4. Failure of a model will 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 are less severe bc of faster starts

Question 24

Advantages claimed for the
continuous tunnels are

Answer 24

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.

Question 25

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.

Answer 25

Liner and Working Section

Question 26

In a supersonic tunnel, therefore, a convergent-divergent nozzle must be incorporated to _______________________.

Answer 26

accelerate the flow to supersonic speeds

Question 27

Supersonic working sections are always _________ in cross section, never circular or
elliptical, as is the case in some low speed tunnels.

Answer 27

rectangular

Question 28

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:

Answer 28

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.

Question 29

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.

Answer 29

supersonic diffuser

Question 30

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.

Answer 30

drying the air ; driers

Question 31

________ 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.

Answer 31

High-speed subsonic

Question 32

_________ are required to cover the transonic range of Mach numbers from, say, 0.85 to 1.15.

Answer 32

Special liners ; Transo Wind Tunnels

Question 33

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.

Answer 33

perforated ; Transonic Wind Tunnels

Question 34

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

Answer 34

Hyperso Wind Tunnels