Re: How do I make a wind tunnel to measure car aerodynamics?

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