MadSci Network: Engineering |
Greetings:
I suggest two types of experiments to conduct your project. The second
experiment will build on the first experiment. How far you will
progress in the
experiments will depend on how much time you have and how good you are
at
building things. You will have a visual demonstration in the first
experiment.
The two experiments are models of those actually used in wind tunnels
except
that they are a very small scale models of actual experiments.
However, by using
comparative measurements you should be able to demonstrate the
principles of
aerodynamics in automotive design. In the first experiment you can
visualize air
flow over your models. In the second and more difficult experiment,
you can
attempt to measure the relative drag forces (air resistance) of your
models.
The components that you will need to obtain or make are:
1) A wind sorce
2) A calibrated air velocity indicator
3) A wind tunnel tube with an air diffuser, a roadway and a viewing
window
4) Vehicle and reference models.
5) Very thin, black, silk or silk/rayon thread and glue
6) A balance scale or spring scale that can measure a few grams of
force
7) Perhaps a camera with a second or more exposure time.
Wind Source
The most important piece of equipment that you will need is a wind
source. This
will determine the size of the automobile models that you will be able
to test. A
tank type vacuum sweeper would make a good wind source. It would be
best to
obtain an air velocity of perhaps 60 or more miles per hour (100 km
per hour) to
demonstrate automotive aerodynamics. Some powerful shop vacuum sweepers
claim to have have an air flow greater than 60 miles per hour in a 2.5
inch (6.25
cm) diameter hose or tubing. The maximum height and width of your
model should
be less than one half of the diameter of your tunnel. If you have a
less powerful
vacuum sweeper you will have to taper the tubing down to increase the
air velocity
and this will reduce the size of your models. It may not be necessary
to get
up to 60 mph for your demonstrations.
An alternative wind source is to mount your tunnel on the side window
of an
automobile or van and do your experiments while moving at different
velocities.
This method has the advantages of simplifying the equipment and being
able to use
larger diameter wind tunnels and larger car models. The draw back is
that you will
have to conduct your experiments on the move and use photographs of
your
experiments for your presentation.
Air Velocity Indicator
You will also need to build a simple air velocity indicator and
calibrate it. I
would suggest using the simple water filled U-tube described in the
New
UNESCO Source Book for Science Teaching, United Nations Educational,
Scientific and Cultural Organization, Paris, France, 1973. The
book is out of print;
however, used copies are available from Amazon.com. for a few dollars.
You can
calibrate the U-tube velocity using an automobile.
Wind Tunnel
Building supply stores sell inexpensive cardboard tubing and plastic
drain tubing
ranging from from very small to very large diameters. At my Home Depot
store,
10 feet (3 meters) of 3 inch (7.62 cm) diameter plastic drain tubing
costs less than
$6 (USA). Use about 3 feet of tubing for your wind tunnel. Cut a large
window in the
tunnel side about 12 inches (30.5 cm) from the wind exit end and
attach a curved clear
plastic window over the window hole. To simulate a road, place a
long, thin, flat
board inside, on the bottom of the tube. You can slide this board out
to attach and
place your models in the tunnel. You need a wind diffuser in the wind
input end of
your tunnel to remove turbulance caused by the wind source. A diffuser
is usually
made in the form of horizontal and vertical fins crossing the inside
of the tube in an
egg crate fashion. The fins should be very thin so that they do not
reduce the
velocity of the air flow. If you want to make drag measurements you
will need to
place a small pulley in the bottom of the tunnel just behind the
diffuser. More about
this later.
Vehicle and Reference Models
You should make a clean aerodynamic reference model to demonstrate
laminar air flow. A elongated football shaped object with pointed ends
would be
OK. The diameter of the reference should be one half of the diameter
of the wind
tunnel. The height and width of your car models should be equal to or
less than
one half the diameter of your tunnel.
If you plan to make drag measurements the models should be as light as
possible
(balsa wood) and they should have the same crossectional area (not
shape) at their
largest crossection. Also, if you want to make drag measurements, the
models
should weigh exactly the same amount and have low friction wheels
attached
(Place some metal inside of the lighest models). You might want to
construct a
very poor aerodanamic vehicle for demonstration such as a flat fronted
van or
truck. If you want to make drag measurments it will need to have the
same
crossectional area and weight as your other models. It would help to
visualize the
air flow if you paint the models white.
Visualize Airflow
To visualize the air flow, cut many one inch (2.54 cm) long pieces of
the black
thread and place them along your model in the general direction of
airflow by
gluing the upwind ends of the threads to your models. Use only a small
dot of
glue. Cover your model with many of these threads; however, space them
far enough apart
so that they will not over lap much as they move and vibrate in the
wind. This will
take some experimenting. Be sure to attach some threads to the bottom
and along
the back end of your models, this is where most turbulance and drag
problems occur.
Laminar flow will cause the threads to be fairly still in the wind
while turbulance
will cause the ends of the strings to vibrate. The football model
should have very
little or no turbulance. The truck should produce great turbulance.
By photographing the models under test, using long exposures, the
vibrating
strings will show the degree of turbulance better than just watching
the strings.
This will also take a good deal of experimenting.
I recently flew on a commercial aircraft that had the wing above the
engine mount
instrumented in this manner. The instrumented patch was just outside
of my
window and it was interesting to visualize the air flow during
different takeoff and
landing attitudes. I assume that they were photographing this patch of
wing during
the flight.
Drag Force Measurements
This will be a difficult task and you will need much time and effort
to complete it.
For this experiment the vehicles will need low friction wheels on
them. Tie a long
thread to the front end of the models and pass the thread over a small
pulley and
out of a small hole in the bottom of the tunnel near the diffuser.
Attach the end
of the string to the top of a very sensitive ballance or spring scale
below the
tunnel. Load the scale (lower it) with a few grams of something like
beans or rice.
Tighten the string with zero air flow. As the airflow increases in the
tunnel
the drag forces will push the model backward and pull the string
upward on the
scale. The number of grams pulling upward is a measure of the drag
force. Compare the drag force of your test models to the football and
to the truck.
Your vehicle designs should fall somewhere between the drag of the two
reference models.
The solar powered electric racers, such as GM’s Sunracer, have
excellent airflow
characteristics for demonstrating low drag vehicle shapes.
A great reference for studying aerodynamics is NASA's ALLSTAR Website:
http://www.allstar.fiu.edu/
Good luck with your experiments. My spelling checker is not working so
please
excuse any typo errors in my text.
Best Regards, Your Mad Scientist
Adrian Popa
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