Designing an experiment w/ a wind tunnel

[crystalsunset]

For a school science project I am supposed to design an experimental process that utilizes the wind tunnel my school owns. I've built four small model towers out of wood - one a triangular prism, one a rectangular prism, one a cylinder, and one a pyramid - that all have the same height and base width.

The idea is to see which shape of tower will stand up the best against wind. The problem is, once I put the towers into the wind tunnel, I have no idea how to measure which one has the best design for resisting wind force. I need to have numerical data that I can graph and analyze.

I've tried a pretty terrible experiment involving measuring how much a tower will tip when sunken into a bed of clay, but that didn't work well with the powerful wind tunnel. I could try to improve this idea, possibly, but I don't know how.

Anybody have any ideas? I'm not looking for a very specific thing, like data or complex equations or anything, just some ideas for what dependent variable I can measure and how.

 

[minger]

What capabilities does the wind tunnel have? What resources do you have access to (e.g. load cells, strain gauages, etc)?

 

[KLoux]

I've seen some simple wind tunnel experiments carried out with some small scales (like food scales) and something supporting the test object that rests on the scales. You could probably get some indication by using just one scale and measuring the moment created by the wind. Can you picture how that would work? Imagine you're looking at it from the side...

-Kerry

 

[FredGarvin]

Any time you do a test or experiment, you have to do at least two things:

1) Describe what you want to have as a result of the test, i.e. what do you want to prove?

2) Determine a way to measure or take data to quantify what you see.

You have already, in words, described what you want to get from the test:

to see which shape of tower will stand up the best against wind

You need to think about what data you can collect to help you prove your hypothesis.

Forgetting about HOW you would collect the data for a second...what measurements would you think you need to prove your theory?

 

[crystalsunset]

Okay, I've done some more research and I was thinking I could perhaps calculate the drag behind each object. The problem is, all the equations I've found seem to be very complex and require velocity to find the drag. Is there a simple way to find this?

The wind tunnel is not very complex. It can tell me the speed of the wind and the pressure inside the chamber, but not much else.

 

[FredGarvin]

You can calculate the drag based on what the wind tunnel tells you is the average velocity in the test section. However, in most tests, you want to correlate the calculated or theoretical values with a real world measured value. That is the point of testing.

If this is the way you want to go (I think it's a good project idea) you need to think of a way to measure the drag force on each building. How might you do that?

 

[crystalsunset]

This is where I'm uncertain how to proceed. I'm not very familiar with drag and aerodynamics, and the research I've done seems to indicate that to find drag, the object in question must be moving. My towers are going to be stationary.

 

[FredGarvin]

That is not true at all. Whether the wind is stationary and the object moving or the other way around, all that matters is the speed of the wind relative to the object. If you think about it, how are wind tunnel tests conducted on airplanes? Do you really think the airplane moves in the wind tunnel?

 

[crystalsunset]

Yes, I see now. :)

What if I mounted graph paper behind the towers, then attached long strings to the sides of the tower to see how they move in the wind? I could film this on my camera and slow it down to view the distance the strings moved, or I could dip the ends in ink or paint and measure the lines they draw against the paper.

 

[xxChrisxx]

I assume this is a low velocity wind tunnel (laminar flow), so you can use pressure probes to find the dynamic and static pressure and use bernoullis eq to calculate the velocity.

 

[FredGarvin]

The strings would give you a measure of the turbulence around the buildings, which is a good thing. You can also introduce smoke via a smoke wand or incense stick to see the flow (flow visualization). Both are very valid techniques. However, like Chris mentioned, pressure readings around each building as well as force readings at the base of each building is really what you need to quantify your observations.

 

[Helios]

I seem to remember that for a model plane in a wind tunnel, that if you decrease the scale of plane by 2, then the wind speed must be increased by a factor of 4 for the same effects.

 

[FredGarvin]

No one has mentioned anything about similitude yet...

 

[minger]

For proper similitude, one can use the Buckingham Pi Theory to obtain dimensionless parameters that can be used. It would be a much better approach than a rule-of-thumb estimation.

 

[xxChrisxx]

I would argue that dynamic similarity is not an issue. He isn't comparing it to a real tower, just the differences between the models. So long as the inlet conditions are the same a valid comparison can be made.

 

[crystalsunset]

Well, I've got a pretty good idea for what my experiment will be now. Thanks to everybody for all your advice. I'm going to look into pressure probes and other ways of measuring force at different points around the towers. It's just a small science project, so I'm not required to get extremely precise, complex data. Thanks again. =)

 

[minger]

I would argue that dynamic similarity is not an issue. He isn't comparing it to a real tower, just the differences between the models. So long as the inlet conditions are the same a valid comparison can be made.

True. However, to me, it would be fun to estimate the amount of lateral load some skyscrapers take though. Moot point though, the OP has what he needs.