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Old 09-30-2008, 05:14 PM   #11 (permalink)
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A2WT HomePage

Windtunnel on a budget.

Whuups. I forgot that you are in Canada.

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Old 09-30-2008, 06:22 PM   #12 (permalink)
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Originally Posted by tasdrouille View Post
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?
The big boys say do not exceed 5% blockage or buoyancy will destroy any chance of meaningful data.That would require a test section of 400 square feet for a car with 20-square feet frontal area.To get a turbulent boundary layer,at a minimum 20-mph would require 704,000 cubic feet per minute fan capacity.Typically,you need 6-pipe diameters length of tunnel to kill vorticity and get nice even laminar flow into the test section.If yo do circulation,you can save on power but you have to do alot of full area turning-vanes at every corner and then maybe flow-straighteners before the test section.If you do smoke,you have to do purge and makeup air.Just operating the tunnel heats the air because of friction,so unless you have a climatic tunnel,you have to do continuous "weather" monitoring within the tunnel to properly reduce your data at the end of the run.I want to say that GM's tunnel has a 60,000-hp electric motor to turn the 80-ft dia. fan.Fixed stator blades are typically used at the fan to help the air on and off the fan blades with minimum vorticity.----------------- NASA Ames uses a "wall" of fans,each with bell-mouth openings,and boattail stingers after the motors feeding a common duct as a draw-through configuration.Nice San Francisco Bay Area weather to go with it!------------------------ I'll have to brush up on physics to calculate the fan HP.-------------------------- You might want to play with tufts while you ponder the tunnel.I get a friend and daughter to drive chase car and photograph the tufts.Its more valid than a tunnel.Not as sexy though!-------------- If you stare at a teardrop long enough you'll develop an "eye" for the air.If what you do ,is as occurs on the teardrop,you're there!
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Old 09-30-2008, 08:21 PM   #13 (permalink)
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Phil, thanks for your input.

I'm surprised by the 5% max blockage as the A2 wind tunnel clearly is smaller than 200 square feet, yet they do testing for cars.

Anyway, even with the smallish 120 square feet tunnel I had in mind, 100kw ac engines apparengly go for more than $5k each. So it would turn out a bit expensive for a toy.

Cd, even if I was in the states I would certainly not pay for wind tunnel time. Their cousin tunnel, the AeroDyn wind tunnel, with pretty much the same dimensions charges $1590/hour.

I guess I'd be better off designing a camera rig that could be easily adapted between cars to film tufts testing.

Tufts are nice, but they don't tell you what the drag is with or without the aeromod you're testing.
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Old 10-03-2008, 01:05 PM   #14 (permalink)
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Originally Posted by tasdrouille View Post
Phil, thanks for your input.

I'm surprised by the 5% max blockage as the A2 wind tunnel clearly is smaller than 200 square feet, yet they do testing for cars.

Anyway, even with the smallish 120 square feet tunnel I had in mind, 100kw ac engines apparengly go for more than $5k each. So it would turn out a bit expensive for a toy.

Cd, even if I was in the states I would certainly not pay for wind tunnel time. Their cousin tunnel, the AeroDyn wind tunnel, with pretty much the same dimensions charges $1590/hour.

I guess I'd be better off designing a camera rig that could be easily adapted between cars to film tufts testing.

Tufts are nice, but they don't tell you what the drag is with or without the aeromod you're testing.
tasdrouille,some of the universities use a small plastic burrette as an underhood fuel tank when testing.If you can get reliable mpg figures from a prototype aeromod,you can use the delta-mpg as a back door to calculate your new Cd.If you test at a constant 55-MPH,any 2% drag reduction will yield a 1% increase in mpg.---------------------So say,you cobble up a mod out of cardboard and duct tape,test it at 55,and you get a 5% improvement at the gas pump,then you've cut your Cd by 10%,all else being equal.------------------------------ This is from SAE, and Glen Scharf at GM Aero Labs told me that it's a very accurate gauge in aero development.Something to think about and way cheaper than tunnel time!
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Old 10-03-2008, 01:54 PM   #15 (permalink)
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So, I've been reading a bit about wind tunnels lately, secretly dreaming of someday building my own.
Keep dreaming, good ones are huge and expensive.

For a full scale wind tunnel your only real option is called "the road". With two cars (test and chase) you can do tuft testing. With three (add a smoke generator on a lead car) you can do a bit more. Plus, and unlike in any but the most expensive wind tunnels, the wheels are turning, which does affect the aerodynamics. Measuring forces is difficult. I think though, that if you towed the test car on a long enough cable (to get it out of the tow car's wake) it should be possible to deduce drag by measuring the tension on that cable at a fixed test speed. Proximity to an abandoned landing strip or dry lake bed would also be a big plus.

Another option, if you can afford it, is to rent one of those car carrier trucks which has a flat bed that extends over the entire length of the truck. You know, when end is over the cab, and the back tilts down to load and unload the car. With a big stable platform like that it would be easy to put a smoke generator up at the front, and to mount cameras at fixed positions.

If you want to work at 1/24th scale the road is again your best bet. Build an open tube and mount it on the roof of a car. Direct measurement of forces becomes possible, although to get good accuracy you are going to need accelerometers on all 3 axes on both the test model and the car carrying it - and a good flat road.
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Old 10-03-2008, 03:02 PM   #16 (permalink)
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Those are good ideas. If you can't move the air, move the object. An old flatbed truck sounds like a good idea. The flatbed could be modified to integrate a balance and other measuring equipment.
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Old 10-03-2008, 03:12 PM   #17 (permalink)
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For a scale model, it should be very easy to build a wind tunnel from an old central A/C unit.
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Old 10-04-2008, 12:43 AM   #18 (permalink)
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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.
That's a really cool idea. There are portable dynos that go to events and they do very well. Maybe a railer that does only one side for efficiency of space and fans. One side of the trailer could be clear plastic and those smoke wand thingies (google this term unless you're from MIT) could be inserted through holes in the side of the wall.

In between events you could blow leaves off of people's yards.
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Old 10-04-2008, 10:00 PM   #19 (permalink)
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Another option, if you can afford it, is to rent one of those car carrier trucks which has a flat bed that extends over the entire length of the truck.
I was thinking the same thing:



But I've resolved myself to tuft testing with a camera rig mounted on the car, and A-B-A tests.
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Old 03-19-2011, 10:40 AM   #20 (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.
Can aerodhead or anyone else verify this 5x part width is tunnel width/height?. I am also starting to study and manufacture my own wind tunnel or water tunnel or even both . I dont have much problems with space needed. Plans are to built 1:24 tunnel which could fit a "full sice truck in it.

I already have the acrylic sheet for clear windows and wood material needed. Still missing the fans and honeycomps, scales etc.

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