Airflow and Compressible Flow
Visualization
Outline:
1. Flow Visualization Techniques:
a) Smoke Technique
b) Hot Wire Technique
c) Optical Methods
i.
ii.
iii.
Shadowgraph System
Schlieren System
Interferometer
2. Boundary Layer Investigation
3. Basics of Flow
4. Measuring Airflow Characteristics
a)
b)
c)
d)
e)
Measurement of Fluid Velocity
Total Head Measurement
Measurement of Static Pressure
Measurement of Mach Number
Measurement of Airspeed
i.
ii.
Hot Wire Anemometer
Yawmeters
Flow Visualization Techniques
1. Smoke Technique
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This technique simply consists in introducing into the working section
of a wind tunnel a number of fine smoke filaments, usually in the
plane of a section of the model wing under test.
Good lighting is needed if the smoke filaments are to be clearly seen,
and an open circuit tunnel is required in order to get rid of the smoke.
The main disadvantage is that usually only isolated spanwise
positions can be examined, and not the flow over the whole wing.
Flow Visualization Techniques
1. Smoke Technique
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The principal requirement of a smoke tunnel is for uniform flow with low
turbulence.
If this is not achieved, the smoke will quickly become diffused.
The speed must not be too low, or gravity will affect the paths of the
smoke particles, curving these paths downwards.
If the speed is too high, the smoke filaments may become attenuated,
even when large quantities of smoke are introduced.
Flow Visualization Techniques
1. Smoke Technique
Flow Visualization Techniques
2. Hot Wire Technique
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This technique involves using a heated wire inserted into the airstream
to produce a filament of air of different density from the mainstream.
The path of this filament can then be followed by using one of several
optical systems, in the context of their use with high-speed wind
tunnels.
Similar to the hot wire in principle is the spark technique.
 Small volumes of air are heated by the discharge of a series of
electric sparks.
 Such systems have been used to photograph velocity distributions in
boundary layers.
Flow Visualization Techniques
3. Optical Methods
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Several methods of flow visualization consist in utilizing the changes in
the refractive index of air caused by changes in density.
They may be used to investigate the flow in boundary layers, but their
main application is to the visualization of high-speed flow.
When light passes through a region of a gas in which the density varies,
the changes in the refractive index of the gas associated with these
density changes cause the direction of the rays to be altered.
The amount of the deflection is proportional to the density gradient.
Flow Visualization Techniques
3. Optical Methods
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There are three commonly used methods, the shadowgraph, schlieren
and interferometer systems.
They are all based on the same principle, but, the first two are mainly
useful simply for flow visualization, and the third may also be used for
quantitative measurement.
The shadowgraph system is the simplest, but the schlieren system is
more effective, and is the most commonly used of all three methods.
Interferometer is more difficult to interpret.
Flow Visualization Techniques
3. Optical Methods
Shadowgraph System
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Light from a point source is collimated by
a lens, so that a parallel beam passes
through the working section of the wind
tunnel on to a screen, where an image of
the working section then appears.
If there is no model in the tunnel, the
screen will be uniformly illuminated.
If a model is introduced, its shadow will
appear on the screen.
Flow Visualization Techniques
3. Optical Methods
Shadowgraph System
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If light passes through a region of
increasing density gradient, the
rays will diverge more and more,
and the corresponding region on
the screen will be lightened.
Conversely, the area on the
screen corresponding to regions of
reducing density gradient in the
working section will be darkened.
Flow Visualization Techniques
3. Optical Methods
Schlieren System
•
Light from a source is collimated by a lens, so that a parallel beam passes
through the working section of the wind tunnel, is collected by another lens
and brought to a focus, then passes through yet another lens to project an
image of the working section on to a screen.
Flow Visualization Techniques
3. Optical Methods
Schlieren System
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At the focus position, a knife edge is introduced, which can be moved into
or out of the beam to cut out some or all of the light.
When the tunnel is empty, the intensity of illumination on the screen may
be varied by moving the knife edge, but it remains uniform over the whole
image area.
Flow Visualization Techniques
3. Optical Methods
Schlieren System
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Since this system is such that the intensity of illumination is a function of
the first derivative of the gradient, it is more sensitive than the
shadowgraph.
Mirrors may be used instead of lenses. Since high quality optical
components are essential, and it is easier and cheaper to achieve good
optical performance with mirrors than with lenses, mirrors are much more
commonly used.
Color schlieren systems give attractive and easily interpreted pictures, but
the definition and contrast are usually not so good, nor so easily controlled,
as in black and white.
Flow Visualization Techniques
3. Optical Methods
Schlieren System
Flow Visualization Techniques
3. Optical Methods
Flow Visualization Techniques
3. Optical Methods
Interferometer
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Light from a source is split into two beams by a semi permeable mirror, so
that some light travels on to a totally reflecting mirror, while the rest is
reflected to another totally reflecting mirror. Both these beams are then
reflected to another semi-permeable mirror, and subsequently impinge
together on a screen.
As a visualization of the flow, the pictures obtained are much harder to
assimilate than are schlieren pictures.
It is very sensitive to even slight mechanical vibrations, and it is essential
to mount the optical components on a rigid frame independent of the
tunnel mounting.
Flow Visualization Techniques
3. Optical Methods
Interferometer
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A modification of the system is the so-called
schlieren interferometer, in which the
splitting up of the initial beam is achieved, not
by the use of semi-permeable mirrors, but by
means of a Wollaston prism, which polarizes
the beam into an ordinary and an extra
ordinary ray. This system is not so sensitive to
external vibrations, but it gives pictures which
are equally difficult to interpret.
Flow Visualization Techniques
3. Optical Methods
Mach-Zehnder Interferometer
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A light beam is first split into two parts by a beamsplitter and then
recombined by a second beamsplitter.
Flow Visualization Techniques
3. Optical Methods
Michelson Interferometer
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Produces interference fringes by splitting a beam of light so that one
beam strikes a fixed mirror and the other a movable mirror. When the
reflected beams are brought back together, an interference pattern
results.
Boundary Layer Investigation
For many studies of fluid flow, the main information required about the
boundary layer consists of a knowledge of the nature of the layer, whether
laminar or turbulent, the position of the transition point and the position of the
separation point, if any.
1. Smoke
2. Gas Filament Method
3. China Clay Method
4. Liquid Film Method
5. Wool Tufts
Boundary Layer Investigation
1. Smoke
• A very thin filament of smoke is introduced very close to the surface of
the model.
• In the laminar boundary layer, the filament remains clearly defined.
• If transition occurs, the filament is broken up suddenly, and behind the
transition point there is a region diffused smoke due to the turbulent
layer.
• The main difficulty is to ensure that there are no disturbances at the
point where the smoke is introduced into the boundary layer.
Boundary Layer Investigation
2. Gas Filament Method
• The model is coated with mercurous chloride, which stains black, if it
comes into contact with ammonia.
• A filament of ammonia is introduced into the laminar part of the boundary
layer, where it reacts with the coating to give a visible stain.
Boundary Layer Investigation
3. China Clay Method
• The surface of the model is thinly sprayed
with kaolin (China clay).
• When dry, this is a white, crystalline solid.
It is sprayed with a volatile liquid, so that
while wet it is transparent.
• The fluids commonly used for this purpose
are ethyl and methyl salicylate.
• Advantages: after one test run, the model
can be re-sprayed with fluid and used
again; the kaolin coating lasts for a
considerable time.
Boundary Layer Investigation
4. Liquid Film Method
• A simpler alternative to the China clay method
• The model is simply sprayed with a volatile oil,
and when it is placed in an airstream, the region
in which the boundary layer is turbulent dries
more quickly than the laminar region.
5. Wool Tufts
• Attached to the ends of short wires which are
fixed normally on to the surface of a wing or
model.
• When placed in an airstream, the tufts stream
outwards in the wind direction.
Basics of Flow
1. Streamline
• A line which smoothly connects velocity vectors at an instance in time. In
other words, an image of the flow characterized by streamlines is like a
snapshot of the flow at one moment in time.
2. Streakline
• A curved line formed by a string of fluid particles which have passed
through a certain point. An example of a streakline is the trail of smoke
from a chimney.
3. Pathline
• A path which a fluid particle traces. One example of a pathline is the path
defined by a balloon floating in the air.
Basics of Flow
Basics of Flow
• For a flow which does not change with time, the streamline, streakline, and
pathline are the same line.
• A flow which does not change with time is called a steady-state flow.
• On the other hand, a flow which varies with time is called a transient flow.
• For transient flows, the streamline, streakline, and pathline are all different
lines.
Measuring Airflow Characteristics
Measurement of Fluid Velocity
• It is essential, both in wind tunnel experiments and on an aircraft in flight, to
have instruments which are capable of accurate determination of the speed
of an airflow and, at high speeds, of its Mach number.
• The most common methods depend on pressure measurements.
• At low speeds, a simple pitot-static tube is most convenient, though, as we
have seen, it may be subject to a variety of errors.
Measuring Airflow Characteristics
Total Head Measurement
• Total head is measured by means of an open-ended tube called a pitot
tube, and that such a tube gives an erroneous reading if its axis does not
point directly into the air stream (if the tube is yawed).
• The effect of yaw is to reduce the pressure recorded by the pitot tube.
• The magnitude of this effect depends on the nature of the flow into the
mouth of the tube, and, at given angles of yaw, this is governed by the ratio
of the bore, d, to the external diameter, D
• The smaller the bore, the bigger the effect of yaw.
Measuring Airflow Characteristics
Measurement of Static Pressure
• Static pressure is measured by means of a static tube, which has holes
drilled in its surface to allow equalization of pressures inside and outside
the tube.
• If the tube is yawed, there may be flow into some of the holes in some
conditions, so that the pressure recorded may be increased.
• In this case, the tube could be more sensitive to yaw than a pitot tube, and
to reduce this sensitivity it is necessary to ensure that there are several
static holes in several different radial planes.
Measuring Airflow Characteristics
Measurement of Static Pressure
• The positioning of the holes relative to the nose of the tube is also crucial.
• The flow speeds up round the nose, and if the holes are too far forward, we
may get reduced pressure readings as a result. The holes are therefore
placed some distance back from the nose.
• As the Mach number increases, the local accelerations in the flow may give
rise to regions of supersonic flow with shock waves and Consequent errors
in reading. To minimize these effects, we need to use a very thin and fairly
pointed tube.
Measuring Airflow Characteristics
Measurement of Static Pressure
For supersonic speed, possible designs for static tube includes:
(a) A Very thin, single-sided wedge
 In the design attitude, there is no flow deviation on to the lower surface,
and the pressure recorded there is the free stream pressure.
 Very sensitive to incidence changes, because they give rise to shock or
expansion waves whose intensity does vary with incidence.
Measuring Airflow Characteristics
Measurement of Static Pressure
(b) A long, thin cone with four holes located well behind the shoulder
 This device records values of the pressure which are practically the
same as the free stream values, because the expansion round the
shoulder is equal and opposite to the compression at the nose.
 It is less sensitive to incidence changes than the wedge described
above.
Measuring Airflow Characteristics
Measurement of Static Pressure
(c) A shorter, thin cone, with the four holes located forward of the shoulder.
 This instrument records an average pressure, and its reading is not
sensitive to yaw.
 However, it is less accurate than the longer cone, and it may be
necessary to apply a correction for nose angle.
Measuring Airflow Characteristics
Measurement of Mach Number
• Like the airspeed, the Mach number can be determined, either in wind
tunnel experiments or in an aircraft in flight, from pitot and static tube
readings.
• A Machmeter uses the pressures taken from a pitot and a static tube to
record the ratio of total pressure minus static pressure to static pressure,
and this ratio determines the Mach number uniquely.
𝑃𝑇 − 𝑃
𝑃
• In supersonic flow, however, the total head is necessarily measured behind
a normal shock, so that its value is less than the free stream value.
Measuring Airflow Characteristics
Measurement of Airspeed
• In the case of supersonic flow, the problems are a little more difficult. The
same airspeed indicator is used, which consists of pitot and static tubes
which now ideally record the pressure difference 𝑃𝑇 − 𝑃.
• 𝑃𝑇 is the total head behind the shock
• 𝑃 is the static pressure ahead of it.
• The airspeed indicator reading, as always, is based on this pressure
difference.
• However, a correction for Mach number will have to be applied to the
airspeed indicator reading, so that the instrument will always need to be
read in conjunction with the Machmeter.
Measuring Airflow Characteristics
Measurement of Airspeed
a. Hot Wire Anemometer
• A method of measuring airspeed which
does not depend on pressure
measurements involves the use of a hot
wire anemometer.
• A small wire is heated by an electric
current.
• If it is placed in an airflow, cooling takes
place, and the rate of cooling is a
function of the speed of the airflow.
Measuring Airflow Characteristics
Measurement of Airspeed
b. Yawmeters
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The function of a yawmeter is to determine the direction of flow, or to
determine the angle between the flow direction and some other
direction, as in the measurement of incidence.
An ordinary pitot or static tube is to some extent sensitive to yaw but
is designed to have the smallest possible sensitivity.
A yawmeter is expressly designed to have high sensitivity to yaw.
Measuring Airflow Characteristics
Measurement of Airspeed
b. Yawmeters
End of Presentation