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Old 09-29-2008, 08:32 PM   #1 (permalink)
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DIY full size wind tunnel

So, I've been reading a bit about wind tunnels lately, secretly dreaming of someday building my own.

So I'm reading about all this stuff, closed or open jet, corner vanes, contractions ratios, screens, honeycombs and what not to get the cleanest flow possible, and for a 200 square feet test area I'm ending up with something over 150 feet long and so tall that even if I had the money I would never get the permit to build it.

But what if someone was to make a garage sized tunnel? Something 24 feet long by 12 feet wide and 10 feet tall. Make one end with a bellmouth lip and the other end with what would be needed to move 600000 CFM. How dirty would the flow be? How meaningful would the measurements made in such a tunnel be for a common midsize car? What would be needed to move 600000 CFM?

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Old 09-29-2008, 09:14 PM   #2 (permalink)
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The problem as I see it from my fluid dynamics days, is getting something like laminar flow in such a short distance. First, you would have to have the same size opening as chamber size (or larger) Remember that fans have to have frames and motors and that can account for up to 50% of their area. Once you have the air intake, you need to clean it up. This can be done with a honeycomb lattice work but once again you have the area trouble.

It is still hurricane season, just park somewhere with a hurricane and video the tufts from a safe location.
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Old 09-29-2008, 09:35 PM   #3 (permalink)
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tasdrouille -

Sounds like a question for aerohead. I Googled "wind tunnel dimension" and found these :

9x7 Supersonic Wind Tunnel


Wind Tunnel 2005


Question: What would the dimensions of a wind tunnel be a 1/24 scale? The reason I ask is that the most common scale for plastic model cars is 1/24 scale. Therefore, you would have a large choice of "accurate" shapes to work from. Would the air speed have to be scaled to match? If yes, then it would have to be like this :

Code:
1/1 MPH   1/24 MPH
    24         1.000
    35         1.458
    45         1.875
    48         2.000
    55         2.292
    65         2.708
    72         3.000
    75         3.125
Ha ha, you'd need sewing string to do tuft tests, .

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Old 09-29-2008, 09:35 PM   #4 (permalink)
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Windtunnel on a budget.
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Old 09-29-2008, 10:20 PM   #5 (permalink)
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Seems to me the real problem wouldn't be building the wind tunnel itself, but getting the equipment to precisely measure the forces involved.

Why not instead put some effort into an open-source aerodynamics program? It's something that should parallelize well, and could be adapted to run on e.g. an NVidia GPU, instead of using it for video games :-)
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Old 09-30-2008, 10:52 AM   #6 (permalink)
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The problem with CFD is the time and/or cost involved to build accurate models that would account for engine bay, underside and wheel wells airflow. It's much easier to just roll the car in, and needless to say much more fun and exiting.

So, looking at the A2 wind tunnel it seems something like three 72 inches 100kw fans would be needed (shooting for 50 mph). I don't know what price 100kw motors go for but it sounds expensive.

The last 8 rear feet of the garage could form the diffuser. So that would leave 16 feet for the test section. The honeycomb could flip up like a garage door. The nozzle could be made to be removable and assembled only when the tunnel is used.

As far as balances are concerned, there was a chapter dealing with that in Pope's low speed wind tunnel testing book. It should not be too hard to build something low tech with pivots, springs and cheap digital weight scales. I'll have to go look back at the book.
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Old 09-30-2008, 01:44 PM   #7 (permalink)
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I did a quick model of what I had in mind in google sketchup. Everything is scaled on a 20 wide by 24 long and 14 high garage.

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Old 09-30-2008, 01:48 PM   #8 (permalink)
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Here's my vision. Have a couple semi's haul a portable wind tunnel around to various cities on a 'tour'. Set up the tunnel in a rented corner of the local Wal-Mart parking lot. Charge admission*. For the fee, your Cd would be calculated and areas of the vehicle identified that need the most improvement. You could attract ricers and local drag racers, as well as mom and pop for the 'gee whiz' factor. It would help educate the public on the importance of aerodynamics in relation to FE.

*Deeply discounted for active EcoModder members, of course.
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Old 09-30-2008, 01:54 PM   #9 (permalink)
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Great idea!

Quote:
Originally Posted by Tango Charlie View Post
Here's my vision. Have a couple semi's haul a portable wind tunnel around to various cities on a 'tour'. Set up the tunnel in a rented corner of the local Wal-Mart parking lot. Charge admission*. For the fee, your Cd would be calculated and areas of the vehicle identified that need the most improvement. You could attract ricers and local drag racers, as well as mom and pop for the 'gee whiz' factor. It would help educate the public on the importance of aerodynamics in relation to FE.

*Deeply discounted for active EcoModder members, of course.
Great idea! From free-thinking ( "why not"). One tiny problem...who pays for all this? And, who insures this project?
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Old 09-30-2008, 02:01 PM   #10 (permalink)
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I don't remember specifics off the top of my head but in my Fluids class we discussed wind tunnels and dimensional analysis. I do remember that we had calculated that the cross sectional area of the tunnel would have to be at least 5 times that of the object (car). If it was not large enough you would run into effects of the air compressing in the tunnel. This would cause very convoluted results.

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