Science Buddies: "Ask an Expert"

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?

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?

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?

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.

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.

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.

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.

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.

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?

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.

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.

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.

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.

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.

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.