AEROLAB 2 prelim Flashcards
uses wind tunnels to test models of proposed aircraft.
Aerodynamicists
where is the test section located in a subsonic wind tunnel
end of the contraction section and upstream diffuser
where is the test section located in a super sonic wind tunnel
end of the diffuser
are devices which provide an airstream flowing under controlled conditions
Wind tunnel
are those whose usual operating speeds require the inclusion of compressible flow effects
High speed wind tunnels
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
High-speed wind tunnels are often of the _______ type
Intermittent
wind tunnel for intermittent operation maybe of_____
blow-through or suction type.
In a _______, air is
compressed to a very high pressure in a
reservoir.
blow-through tunnel
instead of the
air flowing out of a compressed air reservoir, it is
sucked into a previously evacuated chamber.
Suction type tunnel
In this type of tunnel, a high-speed jet
is forced through a narrow annular slot
downstream of the working section.
Induced Flow tunnel
Continuous operation may be achieved in a blow-through or induced flow tunnel if____
sufficient compressed air supply is available
Apart from this, continuous operation may be achieved in effectively
the same way as in a low speed tunnel, except that instead of a fan_____
a compressor specifically an axial flow compressor, is incorporated in the tunnel circuit
almost all compressor driven tunnels incorporate a____
return circuit
advantages of a return circuit
-saves energy and reduce costs
-there is on need for continuous drying
disadvantages of a return circuit
if the duration is too long. temperature may rise, resulting in varying stagnation condition unless some form of cooling is used
phenomenon when the local Mach number in a high-speed tunnel reaches unity at some point in the
flow other than the nozzle throat.
choking
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.
Advantages claimed for intermittent tunnels
- 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.
Supersonic working sections are always ______in cross section,
rectangular
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.
The diffusion of the flow, 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.
Diffuser
super sonic diffuser is required to turn super sonic speed to subsonic one
subsonic diffuser is then need to turn subsonic speed to lower speed at which air is discharge in the atmosphere or back in to a return circuit
An essential feature of most supersonic tunnels is some means of drying the air.
Dryer
In the absence of a drier
device, the expansion of the air to low temperature and pressure causes___
water vapor to condensate that spoils the flow in working section
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 tunnel
It is usually impossible, then, to achieve flow at a Mach number much above 0.9 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. Special liners are required to cover the transonic range of Mach numbers
from, say, 0.85 to 1.15.
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
- Hypersonic Wind Tunnels
In reality, all matter is _____
Compressible
Flows where the density of the fluid is always constant.
Incompressible flow
Speeds classified under incompressible flows are
below 100 m/s
Flows in which the change in density is not a constant.
Compressible Flow
At any point in the flow field, the flow properties such as pressure, temperature and density are related
by the equation of state given as:
P=ρRT
equation of state
From the physical principle of conservation of mass which states that “mass can neither be created nor
destroyed.”
Continuity equation
rho a v = C
The momentum equation shows the relationship of pressure in the aerodynamic flow. Indeed, the
differences in pressure from one point to another in the flow creates forces that act on the fluid elements
that cause them to move. Therefore, it is important to determine the relation between pressure and
velocity.
euler’s equation:
p = -rho V dV
what is the bernoulli equation
p + 1/2 rho v^2 = C
The momentum equation for compressible flow in a stream tube or duct with no friction is given by:
P2 + rho2(v2^2) = C
– a process where no heat is added or taken away
Adiabatic process
a process where no frictional or dissipative effects occur
Reversible process
a process which is both adiabatic and reversible
Isentropic Process
Isentropic relation
t2/t1 = (rho2/rho1)^k-1 = (p2/p1)^k-1/k
For high-speed, compressible flows the physical principle goes as follows: Energy can neither be
created nor destroyed.
energy equation
cpt1 + 1/2 V1^2 = 0
