Module 3 - Airflow and Compressible Flow Visualization Flashcards
Methods that have been developed for visualizing the nature of fluid flow, inside and outside the boundary layer, past wind tunnel models.
flow visualization techniques
Two types of flow visualization techniques
- smoke technique
- hot wire technique
The smoke technique simply consists in introducing into the ______________ of a wind tunnel a number of _________________, usually in the _____________ of the model wing under test.
working section, fine smoke filaments, plane of a section
As the particles of smoke move ______________, the filaments define the ______________ which, in steady flow, are the same as the ______________.
downstream, particle paths, streamlines
Allows the observation of streamline pattern
Smoke technique
Phenomena that may be demonstrated using the smoke technique
- trailing vortices
- breakdown of streamline flow in a region of separated flow
- unsteady nature of the flow in the wake of a wing or other obstacle
The principle requirement of a smoke tunnel is for _____________ with ________________. If this is not achieved, the smoke will quickly become ____________.
uniform flow, low turbulence, diffused
Smoke Tunnel: The speed must not be too low, or ___________________ of the smoke particles, curving these paths ____________. On the other hand, if the speed is too high, the smoke filaments may become ____________, even when ______________ of smoke are introduced.
gravity will affect the paths, downwards, attenuated, large quantities
_______________ is needed if the smoke
filaments are to be clearly seen, and an _______________ is required in order
to get rid of the smoke.
Good lighting, open circuit tunnel
Three types of light used in an experiment in order to determine the best light to be used for smoke tunnels and which of these is the best?
- fluorescent
- led
- halogen/cold light (best)
From a smoke generator, how is smoke being introduced into the smoke tunnel?
probes or rakes
Explain the basic workings of a smoke-wire tunnel
A smoke wire is suspended in the test section, using a mass to straighten/strengthen the wire with a DC power supply for the smoke generator. It also uses a cold light for better visualization.
What is the main disadvantage of using a smoke visualization technique?
usually only isolated spanwise positions can be examined, and not the flow over the whole wing, in other words, only 2D examinations can be observed
factors to consider before performing smoke visualization technique
- wind tunnel should be fixed
- check the oil
- check the pressure
- check the mounting of the model
- flow should be stabilized
- turn on the lights
- set the smoke generator
The hot wire technique involves using a _____________ inserted into the ___________ to produce a filament of air of _______________ from the mainstream.
heated wire, airstream, different density
Hot Wire: The path of this filament can then be followed by using ___________________, in the context of their use with _____________________.
one of several optical systems, high speed wind tunnels
A technique similar in theory to the hot wire technique
spark technique
Spark Technique: Small volumes of air are heated by the discharge of a __________________. An _____________ is used to trace paths of such ‘_____________.’
series of electric sparks, optical system, particles
Optical systems have been used to photograph _________________ in ________________________.
velocity distributions, boundary layers
For many studies of fluid flow, the main information required about the boundary layer consists of:
- knowledge of the nature of the layer, whether laminar or turbulent
- the position of the transition point
- the position of the separation point, if any
Which type of methods are beneficial in acquiring the main information required about the boundary layer?
visual methods
Methods used in boundary layer investigation
- use of smoke
- gas filament method
- china clay method
- liquid film method
- use of wool tufts
Use of Smoke: A __________________ is introduced ________________ of the model.
very thin filament of smoke, very close to the surface
Use of Smoke: The main difficulty is to ensure that there are _____________ at the point where the smoke is ______________________.
no disturbances, introduced into the boundary layer
Use of Smoke: ____________ is essential and the investigation is confined to ____________________.
Good lighting, isolated spanwise stations
Use of Smoke: In the laminar boundary layer, the filament remains ______________. If transition occurs, the filament is ____________ and a _______________ appears behind the transition point due to the __________________.
clearly defined, broken up, diffused smoke zone, turbulent boundary layer
Use of Smoke: If the laminar boundary layer separates, the filament is suddenly _____ well clear of the surface, but remains ___________ for some distance _____________, leaving just a few wisps of smoke in the ______________.
lifted, well defined, downstream, dead air region
Use of Smoke: The _____________ and _____________ points are thus easily and accurately located and distinguished from one another.
separation, transition
Gas Filament Method: The model is coated with _______________, which stains ________, if it comes into contact with _____________.
mercurous chloride, black, ammonia
Using the gas filament method, how can you differentiate the laminar and turbulent layer?
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. On turbulent flow, the gas is diffused beyond the transition point and produces no apparent staining.
Gas Filament Method: ___________ are possible than with smoke, because even an _________________ produces some noticeable stain. ________________________ are required, and the results can be viewed at _________________ of the test.
Higher speeds, attenuated filament, No special lighting arrangements, leisure after completion
Perhaps the most widely used method for locating the transition point
China Clay Method
Process for China Clay method
- the surface is thinly sprayed with kaolin (china clay)
- when dry, china clay is a white, crystalline solid, that is then rubbed down to a high degree of smoothness
- the deposit is then sprayed with a volatile liquid, whose refractive index is the same as that of the kaolin, so that while wet, it is transparent
- the model is then placed in an airstream
- in the turbulent boundary layer, evaporation rate is higher than the laminar region
- the surface dries first behind the transition point and when it does, it returns to a white, opaque deposit
The fluids commonly used for china clay method
ethyl and methyl salicylate
two special advantages of the china clay method
- after one test run, the model can be re-sprayed with fluid and used again as the kaolin coating lasts for a considerable time
- the investigation covers the whole surface of the model, and not just a few isolated spanwise positions
a simpler alternative to the china clay method
liquid film method
process for the liquid film method
- the model is sprayed with a volatile oil
- it is then placed in an airstream, the turbulent region dries more quickly than the laminar region.
- a comparison of the wet and dry regions locates the transition point
disadvantages of the liquid film method
the visual contrast between the laminar and turbulent regions is less good than with the china clay method, and it does not last as long
are attached to the ends of short wires which
are fixed normally on to the surface of a wing or model
wool tufts
behavior of wool tufts according to region
laminar - they remain steady
turbulent - they flutter appreciably
separated flow - they tend to point in various directions other than the wind direction
Use of Wool Tufts: They may be used in _____________ and ______________ wind tunnels, and in ____________ testing.
low speed, high-speed, full scale flight
Use of Wool Tufts: They are a useful indication, for instance, of the ______ and _______ of the stall on an ____________.
start, spread, aircraft wing
Use of Wool Tufts: It is necessary, however, to ensure that the tufts themselves have __________________, such as causing __________.
no appreciable effect, transition
Optical Methods: Several methods of flow visualization consist in utilizing the changes in the _____________ of air caused by changes in _________.
refractive index, density
They may be used to investigate the flow in boundary layers, but their main application is to the visualization of high-speed flow.
optical methods
three commonly used optical methods
- shadowgraph
- schlieren
- interferometer systems
shadowgraph and schlieren are mainly useful simply for _________________, interferometer systems may also be used for ________________
flow visualization, quantitative measurement
Of the three optical methods which is much more difficult to interpret than the other two?
interferometer systems
Of the three optical methods, what is the simplest?
shadowgraph
Of the three optical methods, what is the most effective and most commonly used?
schlieren
How does shadowgraph work?
Light from a point source is collimated by a lens (or alternatively a mirror may be used), 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 alongside a shadow of the model.
Advantages of shadowgraph
it is cheap and easy to construct
disadvantage of shadowgraph
sensitivity of the system is low and the location of shockwaves are only shown as dark lines
in shadowgraph where does the intensity of illumination depend?
second derivative of the density
how does schlieren work?
Light from a source is collimated by a lens, so that a parallel beam passes through the working section of the wind tunnel, is then collected by another lens and brought to a focus at another point, then passes through yet another lens to project an image of the working section on to a screen.
Schlieren system is ______________ than shadowgraph since intensity of illumination is a function of the _______________.
more sensitive, first derivative of gradient
The Schlieren technique may be used to show _______________ normal to the flow direction by turning the ____________ through a __________ which is beneficial for boundary layer investigations.
much smaller density variations, knife edge, right angle
most commonly used to visualize shockwaves and other gradient flows
Schlieren system
how does the schlieren interferometer work?
The splitting up of the initial beam is achieved using a Woolaston prism instead of semi-permeable mirrors, which polarizes or splits the beam and combines again to project in a screen.
Interferometer: This system is not so sensitive to _______________, but it gives pictures which are ___________________.
external vibrations, equally difficult to interpret
Visualizing flow using _____________, ________________, or _______________ makes it more intuitively understandable.
streamlines, streaklines, pathlines
The three lines are similar in ____________ but different in ____________.
appearance, definition
a line which smoothly connects velocity vectors at an instance in time
streamlines
an image of the flow characterized by ___________________ is like a snapshot of the flow at one moment in time
streamlines
a curved line formed by a string of fluid particles which have passed through a certain point
streaklines
example of a streakline
trail of smoke from a chimney
a path which a fluid particle traces
pathlines
example of a pathline
path defined by a balloon floating in the air
characteristics of streamlines
- Stream Lines are Imaginary, can be mathematically expressed
- The tangent to any point of stream line gives the direction of flow of fluid in that direction.
- Two stream lines can never intersect each other
- when the fluid particles exactly follows the stream lines then the flow is called as streamline flow
characteristics of streaklines
- Streak lines are real lines executed by the stream or chain of fluid particles
emanating from their source - Two streak lines can intersect with each other
- In stream line flow the stream lines and streak lines do coincide with each other
characteristics of path lines
- The path lines are the actual path traced by every single particle of the flow
- Two path lines can also intersect each other
- In streamline flow the path lines and stream lines do coincide with each other
A flow which does not change with time
steady-state flow
For a steady-state flow, the streamline, streakline, and pathline are _____________.
the same line
A flow which varies with time
transient flow
For transient flows, the streamline, streakline, and pathline are _________________.
all different lines
uses of flow visualization
- Understanding flow phenomenon
- Verifying model or theory results
- Easier measurements for designing
- To get a prior knowledge of solution
The most common methods of measuring fluid velocity depend on ________________.
pressure measurements
Example of pressure measurement on low speed
pitot tube
total head is measured by means of an open-ended tube called a _____________, and that such a tube gives an erroneous reading if its axis _______________ into the air stream.
pitot tube, does not point directly
Total Head Measurement: The smaller the _______, the bigger the _____________.
bore, effect of yaw
Static pressure is measured by means of a ___________, which has holes drilled in its surface to allow ______________________ inside and outside the tube.
static tube, equalization of pressures
There are various possible designs for a static tube for use at supersonic speeds. These include:
- A very thin, single-sided wedge
- A long, thin cone, with four holes located well behind the shoulder
- A shorter, thin cone, with the four holes located forward of the shoulder
uses the pressures taken from a pitot and a static tube to record the ratio of total pressure minus static pressure to static pressure
Machmeter
this ratio determines the Mach number uniquely
the ratio of total pressure minus static pressure to static pressure
devices used to measure airspeed
- pitot static tube
- hot wire anemometer
how is the pitot static used to measure airspeed
using the difference of total and static pressure, we can get the dynamic pressure and the airspeed
how is the hot wire anemometer used to measure airspeed
a small wire is heated and placed in the airflow which then cools the wire. the rate of cooling is a function of the speed of airflow and the amount of current required to maintain constant temperature is the measure of the airspeed.
device used to measure the angle variation of the flow
yawmeter
two experimental techniques which make use of the analogies between fluid flow and other physical phenomena to determine solutions to some special aerodynamic problems
electrical analogy & hydraulic analogy
how to test much larger models in flight?
- drop the model from an aircraft and observe their free flow
- mounting them outside of an aircraft in flight
