SciBud Wind Tunnel Questions

[Chinwe]

Hello I'm just starting my project and was wondering where and what type of fan did you buy for the windtunnel
Also where do you find egg crates

[AerospaceGuy]

To acoggan:

Unfortunately, I have not been able to get in touch with the teacher who has the tunnel, so I do not have the exact info on the specific fan I used. Have you tried checking out the variety offered by nearby hardware stores?

To Chinwe:
Hi there,

I'm excited to hear that you are building a tunnel of your own!

The fan can be bought at a hardware store such as Lowe's or Home Depot. It doesn't really matter what brand or mounting style you get, just make sure it's a fan like the one in the photos on the Science Buddies wind tunnel guide page, and that it has the proper CFM rating. Try to pick a fan that you think would easily be mounted to the end of your diffuser.

The "egg crate" is usually found in the lighting department of a hardware store. If you bring a print of a photo showing the egg crate on the wind tunnel, I'm sure that a helpful associate who works there will help you find where they keep it.

Good luck, let me know if you have any more questions.

[Chinwe]

Hello
it' me again I would first like to thank you for answering my first question because i was really at a loss as to where to find those materials.
Though another problem has arisen.
I talked to my science teacher and Dean of students about financial aid and how science buddies would contribute the same amount of money, but they said they couldn't do that and could only give me the supplies they could find in their lab. Unfortunately they don't have the dual range force probes or the Variable AC transformer. The only way you guys can help pay for my wind tunnel is if the school sends money as well right?

[AerospaceGuy]

Hello again Chinwe,

Email Courtney Corda from Science Buddies to talk about funding details. You can contact her at courtney@sciencebuddies.org.

[acoggan]

To acoggan:

Unfortunately, I have not been able to get in touch with the teacher who has the tunnel, so I do not have the exact info on the specific fan I used.

Thank you for at least trying, and mea culpa for not revisiting this thread sooner. To cut to the chase: after building a contraction (out of mat board) and correcting the anemometer readings based on Bernoulli's principle, I'm getting ~8 m/s even with a honeycomb and two anti-turbulence screens in place. So, all is well.

[AerospaceGuy]

Great, glad your tunnel is working!

[Chinwe]

Hello it's me again
sor of cutting it close here
but anyway I have a question concerning the force probes
You see I have very light airfoils and they won't be able to hold 100g more like 5g
and once I take the weight off I don't think the force probes will be able to read that my airfoil is on it
also What sort of rod are you using for the drag force probe?

[AerospaceGuy]

The vertical force probe does not read the weight of your airfoil. When your airfoil is sitting on the rod without any airflow, you define that as the "Force = 0" condition. This is part of calibration. What that rod reads is the force of the airfoil once the airflow is on: lift and/or negative lift, if applicable.

The drag force probe uses the same sort of rod as the lift force probe.

[Shaddow]

My idea is that if air can be forced opposite of the direction of the air creating the wingtip vortices at the same pressure, the induced drag can be reduced. What could be used to visually show the wingtip vortices? Also, if i took the information on the amount of drag before adding in the variable of the air blowing opposite and then with the air blowing opposite would the difference accurately show if the drag was reduced?

[AerospaceGuy]

Hello Shaddow,

Your project sounds quite interesting, and it is good that you are familiar with induced drag! Judges will be impressed by your understanding of the different types of drag experienced by an aircraft. If you haven't already, make sure you study the other types, as I'm sure you'll run into more than just induced drag fluctuation in your experiment.

The best way to show tip vortices is to send a stream or series of streams of smoke at the aircraft from upwind in the wind tunnel, like you can see here in these examples:

http://www.tritrack.net/gallery/main.php?g2_itemId=240
http://www.usfamily.net/web/stauffer/debate.html
http://www.oocities.org/ieee_tpc/ieee_images/AeolianVibration1.jpg

The effect you want to illustrate needs a good amount of visible airflow (smoke) so that it is clear, like in this full-scale example:
http://www.efluids.com/efluids/gallery/gallery_pages/cropduster.jsp

You can find readily available wind tunnel smoke source equipment online, or you can try to make something yourself. Be sure you seek supervision and/or an expert if you attempt to fabricate your own, though!

Also, make sure that you have a model small enough for this to work properly in your relatively small wind tunnel. You need to make sure that your experiment incorporates what are known as "3-D effects." Basically, this is the concept: in a wind tunnel, you only have an approximation of real-flight conditions, because the model is in a box rather than the atmosphere. The wings do not have an expanse on either side of themselves, due to the wind tunnel walls, and so the effects of airflow from the bottom over the wingtips to the top tends to be reduced or not even present in wind tunnel tests. However, this effect occurs invariably in actual flight conditions, and so "3-D effect corrections" are applied to data retrieved from wind tunnels. I suggest that you look into 3D effects, and other wind tunnel-related aerodynamics in order to properly design your experiment.

As for your second question, I do not believe you can simply measure the effect of the standard airflow, then that of your drag-reduction airflow, and then take the difference. The proper methodology would be to measure the model under the effects of both airflows, since that would mimic the in-flight conditions. Does that make sense?

You're off to a really good start. I highly recommend the following book for further study to enhance your understanding of aerodynamics: The Illustrated Guide to Aerodynamics 2nd edition, by H.C. "Skip" Smith. It really makes technical concepts, such as those you are studying, accessible to the student.

Good luck with your project! Feel free to return to Ask an Expert with your future questions.

[somebody]

Hello everyone,

I am building a wind tunnel and have some questions.
When it says screen for the settling chamber, do you mean the thin black screen in your window to keep the fies out?
How important is it? I am asking this because the egg crate mesh board available to me is large and rectangular, exactly twice the size of my contraction cone. I was thinking of usinf two egg crate boards. If I go this route, should I connect them so as to make deeper cells or separate them?

Another option that I was considering is to use the rest of the egg crate instead of the wire sheet as a safety screen at the back of the drive section.

Thanks in advance.

[somebody]

Hey everyone it is me again.

So I was reading the above Student's post and that reminded me of something.

I also plan to use smoke to increase my observation of the airflow.

My idea was to use dry ice. By submerging a good sized chunk of it into warm water right at the front of the contraction cone, the smoke could hopefully go through the tunnel.

A friend of mine who is also doing an aerodynamics project is burning incense. Will that also work? I chose dry ice because it is more readily available and safer as you aren't actually burning something (next to a big wooden thing).

[AerospaceGuy]

Hello somebody,

The point of the screens (which, yes, are like what you use in a common window) is to aid in making the flow laminar (smooth and streamlined). However, if you have an egg crate mesh that is half as deep as the contraction cone itself, then you really don't need the screen. As long as you have something deep and cell-shaped, you should be fine (although it might be interesting to do it both ways and test it, to demonstrate to science fair judges that you tried both ways and chose the better).

If you double-up, you'll want to make the cells deeper, rather than separated. Separating them would only disrupt the airflow, rather than streamline it.

Your idea of using the egg crating to be a safety screen for the fan in the diffuser sounds good to me, although if the cells are large enough for a hand to still fit through, you'll want to use something else as the screen.

And yes, I agree with you regarding the dry ice being safer than the incense. You may want to recommend to your friend to switch to dry ice from the incense. Using dry ice in your flow is going to be difficult to control, but with good practice and experimentation you'll find a way to make it work the way you want to. That's another impressive detail to be able to explain to judges!

Good luck as you build and use your wind tunnel. Please feel free to post any more questions you have here.

[somebody]

Thank you aerospaceguy for answering my questions.

I think I will use some egg crate as a safety screen in the drive section, as I am having trouble finding a wire sheet. I was thinking of using a sheet designed for cooling/ oven racks, but the holes were too large.

I have just come up with some minor questions/comments. I see that your airfoil was made up of wood. Did you use some kind of a disc sander to sand a piece of wood down to that shape?

I am also using a balance to measure lift rather than a force sensor, as it is just too expensive. As the airfoil lifts up, there will be less force pushing down on the balance, so it wil generate a lower reading. I will subtract the that from the original weight to find the force of lift. This is more of a comment and an idea for others who cannot afford or justify spending well over $100 on a force sensor, but AerospaceGuy, if you believe this will not work, let me know.

Thanks again, the information I got here was very helpful

[AerospaceGuy]

Hello again, somebody,

If you still would like to look for wire screens, they can be found in hardware stores in the outdoors sections, usually wrapped in cylinders rather than available as flat sheets. They would probably be classified as chicken wire or some other kind of outdoor animal wire fencing. However, as long as your meshing has small enough cells to keep hands or fingers from reaching the fan blades, your tunnel will be safe.

As for my airfoils, I originally tried to make them by sanding them down using a Dremel tool, but that proved impractical. It took too long, and I was never able to make an airfoil that was smooth or properly shaped. Mine were made with the assistance of someone with mechanical fabrication equipment. He had much more stable sanding machines and was able to make them for me relatively quickly and simply. If you have a wood shop class at your school, you may want to talk to the wood shop teacher about how to properly sculpt your airfoils out of blocks of soft wood.

As for your balance idea, it's perfectly fine. I know that force sensors are expensive, so if you don't have a physics teacher who will lend you some, you have to go with the next best thing. A balance is great, but it has a drawback: it will only tell you the lifting force (using the subtraction method you described). It cannot tell you what the drag force is. Proper aerodynamic analysis calls for data on lift AND drag. If you have a balance sophisticated enough to determine force in two perpendicular directions, that's great, but it sounds like yours will just tell you lift.

However, I don't think you should worry too much about that, if you are only able to use the balance. As long as you explain to the judges how your balance determines lift and that you know it neglects drag, you can make drag approximations based on the geometry of your airfoil. If you try to make airfoils look like existing ones, you can approximate that the drag response is the same. You'd have to do a bit of research on airfoil shapes (such as the NACA series, which has symmetric airfoils used in airplane tail wings, and the NLF series, which is designed to create natural laminar flow for high efficiency, used in main wings). You'd also have to use the drag force equation to estimate the drag of your small model airfoil (using its span, chord, planform area, etc.), but it would be do-able, and impressive to judges. It would also be very educational for you!

Let me know if you run into any more questions. Have fun with your aerodynamics testing!