Wind Tunnel Design Considerations

[deleted-363644]

Hello,

I recently stumbled upon this great website while looking up wind tunnel designs on google. I had a quick question about the project posted to this site, https://www.sciencebuddies.org/science- ... -toc.shtml, particularly in regards to this thread of answers from a few years ago.

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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-f ... 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

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It seems that the wind speed in your test section depends more upon the CFM rating of your fan, and ratio of the area of your fan to the area of your test section than it does the ratio of the area of your collection chamber entrance to the area of your test section; does that seem accurate? If so, then why bother with so large of an area for the collection chamber? Even if you go up to a ratio of say 7:1 for your collection chamber, if your fan to test area ratio is only 2:1 then you can only achieve an airspeed of double that of your fan, not 7 times that right? The air flowing through your collection chamber entrance would simply be 1/7th of the speed through your test section right? So as long as you leave the collection chamber entrance area larger than the area of the fan, the area of the fan is your only limiting factor right? Bu the above answer from the projects creator makes it seem like a larger area of the collection chamber will equate to faster speeds regardless of if the fans CFM remains the same in the diffuser in the back. Please let me know if I am incorrect and you should calculate speeds much faster than that or not with the larger collection chamber.

Cheers to you all!

-SCossano