SciBud Wind Tunnel Questions

[jeffer]

Hello buddy,
I am a researcher. I have to complete thesis on it. But i am new with SciBud.
How can start it perfectly. What equipments are needed for this thesis.
Please explain it thoroughly. So that i can complete my thesis accurately.

[deleted-71704]

Hello Jeffer,

You are a researcher doing a thesis on wind tunnels? Please let me know a bit more about your thesis so I can be of better help to you.

What I can tell you right away is this:
*Science Buddies is a web site with information and resources for students in pre-college school--grades K through 12. If you are doing a thesis in college of any kind, then Science Buddies' resources are not of the caliber that you will need.
*To find great resources regarding wind tunnels and aerodynamics, visit the Bibliography and Credits page of my "How to Build a Wind Tunnel" guide: https://www.sciencebuddies.org/science- ... aphy.shtml.
*For detailed explanations of how to build and use the homemade high-school level wind tunnel, you can visit my "How to Build a Wind Tunnel" guide in its entirety here: https://www.sciencebuddies.org/science- ... -toc.shtml.

Let me know if I can be of any more help to you, Jeffer.

[r.qussous]

Hello dude =D,
My friends and I, are trying to do the same Wind tunnel project, and I would really like to know what Force sensor you used, such as model name and type, or from where we could buy such ones to be accurate for our experiment, and we also want to thank you for the time u have used to reply the posts =D

[deleted-71704]

Hello r.qussous,

The sensor I used was a dual-range force sensor produced by Vernier Scientific. This is the link to the specific product's page: http://www.vernier.com/probes/dfs-bta.html

As you can see from the page, these sensors are very expensive--over $100 each. This is why I suggest that you borrow sensors from teachers who already have them, unless of course you have no trouble affording them yourself.

Any sort of basic physics force sensor will do, as long as it is capable of sensing both pull and push forces, can be mounted on the threaded rod that you use to connect to your airfoil model in the test section, and that it can be easily and securely fastened down to your base, so that it doesn't move when the fan is on. In other words, any sensor just like the one I have linked to you will do just fine. Remember, you need 2--one for lift, one for drag.

I wish you and your friends good luck as you build my wind tunnel design for your project! Please feel free to come back here for help at any time. I'm happy to aid you in any way I can on Ask an Expert.

[Zues9]

Could anyone please give us guidance on the "settlement chamber" construction in the scibuddies wind tunnel? The project description on this is a bit imprecise and there isn't a photo that we can figure out. Appreciate your help.

[deleted-71704]

Hello,

I am the designer and builder of the wind tunnel, so I can answer any questions you may have! Please feel free to post specifics.

In general terms, the reason why there is little in the way of instructions and no photograph is because this step is simple, and up to your creativity. Basically, all that is needed is that you have two sheets of screen behind the honeycomb mesh at the entry point of the Contraction Cone. It is up to you to decide how to install those three components, since there are many different ways you can do it, depending on your materials choices, etc. In other words, I left it up to the imagination of the student, because this part of the tunnel is delicate and so the same instructions may not apply to every person's wind tunnel. Construction results vary from student to student, and so I recommend that you figure out the best way for your specific wind tunnel to have the three components attached at the Contraction Cone.

That being said, it is a bit difficult to do, so please let me know if you have any questions. I'm here to help, so if you want to run some ideas past me for advice, I'd be willing to check them out.

Also, I will search among all my photos to see if I can dig up any pictures that may be of help to you. Photos are hard to take of the Settlign Chamber because the screens and the mesh block the view of each other, but I'll see if I have anything.

[Zues9]

Thank you. I'm understanding the concept better. A photo could be helpful. We had trouble figuring out just what the honeycomb was. Would you happen to have a suggestion on what we can use for this, and where it might be available? Our initial effort on this was to simply attach a single screen to the entrance to the contraction cone, but I understand that we need to do more than that. Really appreciate your help.

[deleted-71704]

Hello again Zeus9,

The honeycomb mesh is the white grid that you see on the large end of the Contraction Cone. It is a product that you could find at a hardware store--I found mine at Lowe's. Look in the lighting department, where they have the large rectangular translucent panel sheets that go over light fixtures in the ceiling. You will find what you need there. If you don't, ask someone and bring a photo of the tunnel so they can know what the mesh looks like.

So, basically, the Settling Chamber of the Contraction Cone is the honeycomb mesh, behind which you have one or two screens stretched tightly. These serve the purpose of making the airflow smooth and straight ("laminar flow"), so that it's uniform and controlled.

I tried attaching a photo to this post, but it was rejected. However, there is still a good photo for you to check out that is already in the Wind Tunnel Guide--I recommend that you go back to the main page of the Wind Tunnel Guide and check out the first picture therel--the one of the wind tunnel as it is fully set up. Look closely at the Contraction Cone--notice that there is a dark square just behind the honeycomb mesh. This is a screen mounted inside the Cone.

Make sure that you re-read the small section of instructions regarding the Settling Chamber in the Contraction Cone portion of the Wind Tunnel Guide. Having seen the image, and read my answers, the instructions should make much more sense to you now.

I hope that those two images have clarified things for you! If not, please feel free to ask what questions you have left. Good luck!

[Zues9]

This is very helpful, and just what we needed. Thank you.

[epwaotl]

I was reading this thread and I saw this:

Your question is a very good one, by the way. The collection chamber does in fact serve to increase wind speed in the test section. This is part of what I meant about the differences between the math and actual data you would collect: the collection chamber is much more voluminous than the test section so that a large volume of air is collected and compressed to fit into the test section volume. By the continuity and Bernoulli equations, the effect produced is that the wind speed in the test section increases without the need of power. In other words, the fan draws air into the tunnel, which speeds up as it is compressed into a smaller volume.

This got me wondering if the same principle would work for a push through wind tunnel rather than a draw through wind tunnel. My wind tunnel is a pusher. The way it is set up there is a fan which lows air into a contraction zone, then a settling chamber, and then the test chamber. I was wondering if adding a large collection chamber upstream of the fan would increase the airspeed in the test chamber. If I made this modification air would flow through the large collection chamber before entering the fan and the rest of the wind tunnel.

Thanks for the help.

[deleted-71704]

Hi epwaotl,

The principle works for both types of tunnels: draw-through fans and push-through fans. The reason that the Science Buddies tunnel is a draw-through fan is because push-through fans tend to create more turbulence because they are blowing air in. Drawing air into the settling chamber makes it easier to straighten the flow into a laminar flow.

Adding a cone upstream of the fan will not really do anything for you--the fan is open at the back and is drawing air from the surroundings to blow downstream of itself. Adding a cone will only restrict the volume of air from which the fan may draw. It won't really produce any helpful effect for you.

If you want to learn more about this, I recommend that you do a science project about the best wind tunnel configuration. That would be a great project!

If you are simply looking for advice, I suggest that you build a draw-through tunnel like the Science Buddies wind tunnel design. It's simple and will produce the least amount of turbulence.

[linhely]

I have started building the wind tunnel and have a question concerning the threaded rod that goes from the Vernier sensor to the airfoil in the test section. I purchased a #6-32 threaded rod which screws into the sensor. This rod seems very flexible. The project recommends a rod that does not respond to just the airflow in the tunnel but this rods seems like it will bend with little air flow.


[Edited by Science Buddies administrators.]

[deleted-71704]

Hello linhely,

Your concern addresses a very important component of the design--the rod has to be of a metal material strong enough to resist bending caused by the airflow. The air will push back on the model and cause the rod to bend unless it is made of a stronger material. What sort of rod are you using? You will want to use something like steel if you can get it.

[acoggan]

Hello,

Using your plans as a general guide, I am in the process of building a similar wind tunnel. Based on theoretical calculations and your students’ data, I was expecting/hoping to achieve wind speeds of ~8 m/s. However, a calibrated vane anemometer installed in the diffuser reads only ~5 m/s without the test chamber in place, and only 3-4 m/s with it attached (I haven’t finished the contraction yet to be able test the tunnel as a whole).

I assume that this is because the gable fan I purchased off Amazon to save a few dollars can’t/doesn’t deliver the claimed CFM of 1620 when used in such a manner. In looking at your write-up, though, I cannot determine the exact make/model/size of fan that you used. The plastic shroud makes me think it is a Vent Air as they sell at, e.g., Lowes, but they all measure >17” in diameter (including the shroud), which isn’t consistent with your Figure 2C.

If you could provide a few more details re. the fan, I would greatly appreciate it!

[deleted-71704]

Hello acoggan,

At the moment, I cannot find a record of the exact make and model of the fan I used in the example wind tunnel, as I was trying to be general at the time and didn't make specific product recommendations. I will see if I can pay a visit to the high school which now uses that wind tunnel and take a look at the fan to determine the information you need.

In the meantime, I recommend that you construct the contraction cone and use the anemometer to test the entire wind tunnel as a whole. The point of the cone is to contract incoming air, and thus accelerate it, and so perhaps that is the missing element you need to achieve the calculated theoretical airflow you desire. The test chamber brings your number down because it limits the open quantity of available air to draw through the diffuser. Once you have attached the cone, however, that quantity will increase markedly because the cross-sectional inlet of the cone is 7X greater than that of either the test chamber or the diffuser.

If you are still having trouble with the wind speed once you have connected the cone, then we can conclude that either the fan or the wind tunnel itself are causing a loss of flow rate. I'll get the fan info if I can, and you test the full wind tunnel design, so we can go after two possibilities at once.

[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

[deleted-71704]

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?

[deleted-71704]

Hello again Chinwe,

Email Courtney Corda from Science Buddies to talk about funding details. You can contact her at [email protected].

[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.

[deleted-71704]

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?

[deleted-71704]

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?

[deleted-71704]

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_i ... ation1.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/ ... duster.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).

[deleted-71704]

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

[deleted-71704]

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!