SciBud Wind Tunnel Questions

[deleted-71704]

Hello all,

This thread is for questions specifically relating to the Science Buddies wind tunnel design. I will answer your questions to the best of my ability, and since I designed and built it, I should be able to provide some insight.

Good luck on your wind tunnels, and feel free to post any of your questions/concerns/comments!

[amyC]

The URL for the Science Buddies Wind Tunnel resources is:

https://www.sciencebuddies.org/science- ... -toc.shtml

Amy
Science Buddies

[deleted-71704]

The following is an email conversation between myself (designer of the wind tunnel) and a man named Bill who was using the wind tunnel design. I have posted it here for the benefit of those who may find it useful. Bill's emails are in boldface to distinguish them from my own:


Hello AerospaceGuy,

My question is in regards to the wind tunnel that you built. Were you ever able to determine the actual wind speeds achieved in the test chamber. I need to build a wind tunnel to test some projects for our R&D facility that can achieve wind speeds between 5-30 MPH and I was hoping that your design would fit the bill.

Regards,

-Bill


Hi again Bill,

There is a way to mathematically determine the speeds in the test chamber. You can use the CFM (cubic feet per minute) rating of the drive section fan to determine the wind speed using this simple fluid dynamics formula:

Wind speed = (volumetric flow rate)/(flow cross section area)

In equation form: v = Q/A, where v is wind speed, Q is volumetric flow rate, and A is cross section area.

The volumetric flow rate is the CFM rating, and the flow cross section area is just 1ft^2 since the test section is 1ft by 1ft. So, you’ll end up with a number measured in feet/minute. Then just use arithmetic to convert ft/m into MPH.

The fan that I recommend using for the tunnel is between 1600CFM and 2000CFM. I did the calculations, and so my recommendation says the same thing as getting a fan between 18.18MPH and 22.73MPH. Since your range is between 5-30MPH, it would seem that the entire recommended range of fans is within your achievement domain. Also, in my design I have you use a variable voltage switch to be able to adjust the fan’s speed, if that is helpful to you. If you need to do that, I can talk you through how to use a voltage ratio to calculate the wind speed adjustment that you need to make.

As for directly measuring the wind speed in the tunnel (if you want to take data instead of calculating what the wind speed should be), then you should use a device called an anemometer, and mount that in the tunnel instead of the wind speed monitor that I have in my design. This could be more useful and more accurate than calculation, because the tunnel is never going to be perfect so your calculation will always be a little off due to equipment error. Remember, this is a high school research-level tunnel.

Did I answer your questions in a way that is helpful to you?

-AerospaceGuy


Thank you AerospaceGuy!

I would like to start by saying what a pleasure it is to deal with someone who knows what they are talking about…

I ran a guesstamation yesterday and determined that a 1600 cfm (available at the local home depot) would require approximately 6” x 6” cross sectional area in the test section to achieve 30 MPH.

I have 1600 cfm and need 2640 fpm

1600/2640 = ~.6

So I built a diffusion chamber with a 6” x 6” orifice and measured the wind speed with a small handheld anemometer that my boss had kicking around his basement. I was able to achieve 22 MPH. I then proceeded to build the test section that is 3” x 6” with a small adapter to the existing orifice. This was able to achieve 31 MPH.

(in hindsight 6” x 6” is ¼ not ½ of 12” x 12”)

Going back to try to understand the math. I know it is a simple formula but I am having trouble wrapping my head around it for some reason.

Wind speed = (volumetric flow rate)/(flow cross section area)

Solve for cross section =>

Cross section = volumetric flow rate / wind speed

Cross section = 1600 CFM / 2640 FPM

.6 ft^2 = 86.4 in^2

So it should have been about 9” x 9”? But it worked out better that I erred on the small side because I ended up having to make a restrictor.

One more question:

Does the collection chamber have any effect on the wind speeds or does it just serve to direct air from the settling chamber to the test chamber?

Thank you so much for the help and I can’t wait to poke around the website to see what else I can find to play with.

Regards,

-Bill


Hello again,

Your math looks good. Glad you could come up with a test section that works for you!

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.

I strongly encourage you to perform some research using the sources cited in my “Bibliography and Credits” section (https://www.sciencebuddies.org/science- ... aphy.shtml). This will help you better understand the effects of the collection chamber than if I were to just try to explain it over email.

Please feel free to email me with any other questions you have.

-AerospaceGuy

[epwaotl]

A few years ago I was looking at a website http://aerorocket.com/index.html for subsonic wind tunnel designs. While there I noticed that they also had supersonic wind tunnels in the form of shock tubes. From what I understand here is how they worked. A small model was put into a narrow tube and connected to electronic sensors for data gathering. The tube was connected to an air hose which was in turn hooked up to an air compressor. A thin metal membrane separated the tube (test chamber) from the hose. The metal membrane had scratches placed so that when a predetermined pressure was reached it would predictable rupture sending high pressure and high velocity air into the tube (test chamber) that would generate supersonic speeds for a very brief time. When I recently visited their web site and looked at the supersonic wind tunnel it had changed http://aerorocket.com/offer.html#anchor9 (scroll towards the bottom). The site says that it is a blow down tunnel that generates up to Mach 3 wind speeds for about 4 seconds.

I was wondering how the blow down tunnel works? Is it feasible to make either one and what materials/methods would be used?

Thanks for all your help

[mackinj]

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. Is this the threaded rod you have used?

[deleted-71704]

Hello--I will respond first to epwaotl, and then to macking in the next post.

Your question is very interesting, epwaotl. Unfortunately, supersonic miniature wind tunnels are not in my area of knowledge, so I will refer you to other sources.

I would recommend talking to the people at AeroRocket to ask about the blow down tunnel, but AeroRocket is a business and only provides information for sale. You will definitely find the information you need from that web site. However, if you are not able and/or willing to pay for it, I recommend the following sources:

To learn more about blow down tunnels, you can view NASA's page here: http://www.grc.nasa.gov/WWW/K-12/airplane/tunblow.html.

I also recommend that you re-post this question on this Ask an Expert forum. We have experts who are more familiar with supersonic tunnel testing than I am who can help you with your question. Simply choose your appropriate grade level and post this question in the Physical Sciences forum for that level.

Good luck!

[deleted-71704]

Hi mackinj,

Your post says that your threaded rod will bend with only a fraction of the airflow necessary for testing, so this leads me to believe that the threaded rod you have is made of too soft a material. You will need a sturdy rod made out of something durable and resistant to bending, like steel.

Did you purchase your rod at a hardware store? I found mine at Lowe's, and I also recommend those from The Home Depot. If you did get it from a hardware store, I suggest that you bring it back and talk to a specialist from the department where you found it. Ask him/her why your rod is so flexible, and where you can find stronger ones.

There is another possible solution to your problem: it could be that your lift sensor is so far away from where the model will be in the test chamber that it will easily bend. If you are familiar with the physics concept of torque, you know that the farther away a source of force is on a rod from its fulcrum (in other words, the farther away the model is from the point at which the rod screws into the sensor), the greater the torque exerted on the rod, and therefore the higher the likelihood that the rod will bend or break.

If you look at Figure 4C from the wind tunnel guide (https://www.sciencebuddies.org/science- ... ion4.shtml), you'll notice that my vertical rod is rather long, about a foot. This is a good maximum distance, and you shouldn't have your rod extending any longer than that. In fact, it's best if your rod is shorter, if you can mount your sensor closer to the test section. You have to make sure that the rod is firmly in place, and NEVER touches the rim of the hole through which it is inserted into the test section, because if it does, that will become the new fulcrum, throwing off the force sensor's readings.

Does this information make sense? I think that your problem lies mostly in the material of your rod--it sounds to me, from your description, that it's very flimsy and weak, and should be replaced by a stronger rod. Please let me know if you have further questions, and if my help leads you to solve your problem.

[mackinj]

Thanks. I look into the rod material and limit its length.

A second question - can you put a cup anemometer in to measure the wind speed or will this block too much air?

[deleted-71704]

Hi again mackinj,

A cup anemometer will work, but it would be much more efficient, and would disturb the airflow less (and therefore would be more accurate) if you used a fan blade anemometer, like this one: http://www.target.com/gp/detail.html/18 ... v_XSG10001.

If you can get one, I suggest using that instead of a cup anemometer, and even instead of the wind speed monitor in my own design. But, if you cannot, the cup anemometer is just fine, as long as its not too big, like this: http://www.maximum-commercial.com/produ ... meters.htm.

As a matter of fact, it would probably be cheaper to get a blade anemometer instead of a cup anemometer.

[mackinj]

Again, thanks. I did see the unit at Target but was not sure if it would work if I mounted it with the fan inside and the body outside the tunnel.

[mackinj]

Should the handheld anemometer be mounted inside the tunnel off-center like the volt meter was? I thought at first I would mount it half in the tunnel and half out so I could see the digital readout easier but then thought this might disrupt the airflow. If I need to mount it completely inside the tunnel, do you think I will be able to see the readout clearly?

[deleted-71704]

Hello again mackinj,

Very good questions. First off, you should know that the airflow behind (downwind) of the model is very important even though it's already passed over the model. This wind must be kept laminar (smoothly flowing) to ensure that no turbulence is created ahead (upwind) of the model. With this in mind, the wind speed sensor is placed offset from the center of the diffuser so that its disturbance of airflow is minimized. As I said in the writeup (https://www.sciencebuddies.org/science- ... ion2.shtml), nearly every professionally-produced high school wind tunnel includes a wind speed monitor that blocks the airflow, and they are usually mounted off to the side, as mine is. For an example, take a look at this picture of a professionally-produced high school wind tunnel, shot down the diffuser from the test section (in other words, this is what you would see if you had a camera on the back of an airplane model being tested in this tunnel): http://s495.photobucket.com/albums/rr31 ... g&newest=1.

Secondly, you can mount your sensor half in and half out of the wind tunnel--just make sure that you have enough room between the tunnel wall and the spinning part of the sensor for air to flow between. Your sensor may not be very long, but you have to make sure that it extends into the tunnel as much as possible--being too close to a wall will cause an irregular airflow and mess up your measurements. So, what you want to do is mount it so that it's not too close to the walls, but also so that it's not in the center of the airflow.

All you have to do to mount the sensor is make sure that you totally plug the hole through which you insert the sensor--use glue, construction sealant, etc. You don't want your airflow disturbed by a leak!

Does this information answer your questions?

[mackinj]

This is very helpful. I will mount the sensor as you described.

[deleted-71704]

Great! Let me know how it goes! I am interested in your experience with the wind tunnel, so please keep in touch and let me know how it's going.

[mackinj]

I am now putting the sections together and, using the small digital anemometer, am getting 17 mph wind speed. I was hoping to hit 20 mph. Any suggestions before I finish putting it all together?

The small digital anemometer works well. I am using a very strong magnet outside the tunnel and two inside the tunnel to hold it in place. This way I did not have to cut an opening in the tunnel wall and the rotor portion of the anemometer is not right next to the inside wall.

[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.