SciBud Wind Tunnel Questions

[WolFox]

I have a question about the wind tunnel. You say that you use a variable transformer to vary the speed of the fan. Are you not concerned with the possibility of damaging the motor by limiting the amount of voltage in the motor?

[deleted-71704]

This reply is for mackinj and WolFox:

Mackinj:
Wow, mackinj, your anemometer idea is brilliant! May I list it on the wind tunnel page as an alternative mounting method?

As for your wind speed, first I'd like to know what your fan's CFM (cubic feet per minute) rating is. Let me know, and I'll be able to help more. For now, all I can say is that it is probably due to the fact that the tunnel is not perfect, and that it will cause some losses in wind speed.

WolFox:
Thank you for your question. No, I'm not really concerned about that, and neither was my electrician consultant. What concerns do you have?

[WolFox]

In my searches online to find a suitable fan, I have found several websites that say that attempting to control the speed of an AC motor by controlling the voltage can damage the motor. One company explicitly told me that there is no way to control the speed of their fan. It is possible that they told me this for liability reasons. Where did you get your fan from?

[mackinj]

I have finished the tunnel and run some tests on it and it works well. To see pictures and data, go to http://moodle.kirkwoodschools.org, go to Aerospace Engineering, sign on as a guest (key: teach2010) and click on the first link under Unit 2 labeled Final wind tunnel pictures and data. I am now working to find a good way to screw the airfoil onto the rod and to make rods with different angles of attack. I'll let you know what I come up with.

One note on the magnet holding the anemometer. The magnet is so strong that it prevents the anemometer from spinning initially. I have started it by turning the fan on then blowing on the anemometer and then closing up the tunnel. I am going to try different magnet placements to see if I can resolve this. If not, I will be able to put the airfoil in the tunnel, zero out the sensors, start the tunnel on full power, blow on the anemometer to start it, close up the tunnel, and run several data collection runs at different airspeeds (by changing the voltage to the fan) while recording the anemoneter airspeed reading and the data probes. To determine the correction for possibly not having the probes zeroed since I moved the test section when I put the tunnel together, I will run a final data collection with the fan off. This will give me the correction factor for all the previous data.

Feel free to share the magnet idea. I'll let you know what else I find out as I work to resolve the issues above.

[mackinj]

The fan I bought was from Home Depot. It looked just like the one in the picture. I will send you the product number when I get back to school this week and look at the receipt.

[deleted-71704]

Wolfox: I'm sorry that my reply is so long in coming. My fan was purchased from Lowe's Home Improvement. I spoke to the physics teacher who helped with the electronics of the voltage adjuster as a fan speed controller, and this is what he said:

"The thing we plugged the motor into is called a variac. It's essentially an adjustable transformer to turn 120VAC into lower voltages. If it is a cheap one, the risk comes from turning the knob past 100%, which might make it a step up transformer, depending on the design. The AC motor shouldn't be exposed to extra voltage, but I don't see the harm at lower voltage.

Also I would recommend a good grounded one like the one we used.

One way around it is to use a DC motor, and then the variable voltage (supplied from a variable DC power supply) shouldn't cause any problems.

hope this helps -
do keep in mind I'm not an electrical engineer and it might be wise to get a professional's opinion on this. I'd be surprised to be wrong though."

mackinj: Thanks for sharing that link. I am encouraged to see another wind tunnel built using my design! I am curious about the anemometer's problem with the magnets: Why wouldn't it start to spin unless you blew on it? Please elaborate on that when you make your next post about your fan.

Thank you again for providing feedback and alternative ideas. Science Buddies will soon collect feedback and suggestions and post it on the wind tunnel guide. Your information, and any other info that you may come up with, will be used to develop improvement suggestions. Thanks again, I look forward to your next post.

[mackinj]

On the anemometer and magnets: The fan/shaft of the anemometer must contain metal which is affected by a magnet. If the magnet is near the anemometer, it prevents the fan from spinning using the wind force generated by the fan. Blowing on it with the fan on starts up the anemometer fan. To eliminate this problem, I got a 1/2" piece of flat iron, cut off about 6", and bent it to 90 degrees. I attached this to the back of the anemometer with rubber bands and placed 1 magnet outside the tunnel beneath the iron. This has eliminated the anemometer fan startup problem.

On the fan purchase: I bought it at Lowe's. 30425 1620 CFM GABLE VENT W/HUM ($105)

Two engineers (1 from Boeing) are on our school board and were amazed at the quality and capability of the wind tunnel. Thanks for making the design available to us.

[mackinj]

On the original wind tunnel that we were supposed to buy, the test area was 5.25" x 5.25" x 16". The activity related to the tunnel limited the student designed airfoil to 4" x 4" x 4". Given that our tunnel has a test section of 12" x 12" x 24", what would be the maximum airfoil size we can test? Is this just a ratio of the original sizes? Is there a specific inner wall to airfoil model clearance that needs to be maintained for airflow? Any guidance is appreciated.

[deleted-71704]

Hi mackinj,

Regarding the airfoil wingspan which may be tested in the test section, it is a rule of thumb to have some amount of clearance between the wall of the test section and the tip of the wing. If you check the sources on which I relied, as well as others you may find, you'll see that there is some disagreement as to an exact ratio. I would suggest leaving a clearance of about .25-.5 inches on either side, which would mean that you'd want an airfoil of about 11-11.5 inches of maximum span. Again, that's a maximum, and it's a suggestion, not an experimentally-verified number. Actually, it's probably best to test various spans to find the most reliable maximum. It will vary from tunnel to tunnel, so it may be good to find which is the best maximum for your particular tunnel.

As for the two engineers, would you please give a bit more detail about their impressions?

[mackinj]

One of the engineers was from Boeing and was very impressed with the capability. He had built a wind tunnel which had limited capabilities as a science fair project when he was in school and felt your design had the capabilities needed for a student to assess their airfoil design. He also was impressed that, with the wind tunnel, students could do a complete design process like they do at Boeing starting with computer simulations then making a model airfoil then testing and refining it in the wind tunnel.

The other engineers has been involved in the design of pumps and was also impressed that the tunnel gave students the opportunity to test their own design.

[deleted-71704]

Thanks, mackinj. It is encouraging to hear their positive assessments!

Please keep your questions coming as they arise. I am happy to help you, and look forward to hearing about how you will be/are using the tunnel. We are collecting information about the first people we hear from who are making the tunnel, and will be updating the how-to guide, as we've discussed before. Let me know if you have further questions, or if not, how the tunnel is performing.

Thanks again mackinj.

[mackinj]

I am getting some pulsing of the air which is causing data fluctuations. This has happened since the beginning and the probe software has been able to average it out. In trying to find the source of the pulsing, I removed the first section. This resulted in more pulsing. So it appears my flow straightener needs some work. Do the screens have to be puller very tight to reduce pulsing? If so, how did you attach the screens. I simply stapled them to the wood and tried to get them tight but there is still some play in them. Let me know what you think I should try to reduce the pulsing.

[deleted-71704]

The screens do need to be tight, and yes it is rather difficult to get them taught, as the Collection Chamber is a small, enclosed space. Do the best that you can with those screens.

Something else you can do is lengthen the honeycomb mesh--the part in the very front of the chamber which has all the cells and helps straighten the air out. You can try setting another cut of that same mesh in front of/behind the one you already have mounted in there, making sure that they are flush with each other so you are effectively doubling the length of the cell walls, and introducing no further sources of turbulence. Or, you could find a longer mesh than the one you used, and try that.

Let me know if that helps. The trouble with this tunnel is that it's hand-made and built for a high-school budget, so it's not going to be perfect. You can always improve it though, you just have to get creative sometimes! Let me know if you are successful with the honeycomb mesh, or if you have questions about what I meant.

[raiku11]

Where were you able to find the honeycomb mesh? I have been searching across the internet and been asking some people around my school and town and nobody seems to know about where I can purchase it.

[deleted-71704]

Hi raiku11,

The place to go for that part is a hardware store--I suggest Lowe's or Home Depot, or maybe even OSH. Go to the lighting department, and look for coverings for the ceiling light fixtures which use the long fluorescent tube light bulbs. If you need help finding them, ask a lighting department worker.

[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.