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?

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. :)

tasdrouille -

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

9x7 Supersonic Wind Tunnel
http://www.windtunnels.arc.nasa.gov/pics/1shuttle.jpg

Wind Tunnel 2005
http://www.mne.psu.edu/me415/spring0...indtunnel1.jpg

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 :

Ha ha, you'd need sewing string to do tuft tests, :D .

CarloSW2

A2WT HomePage

Windtunnel on a budget.

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 :-)

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.

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.

http://ecomodder.com/forum/attachmen...1&d=1222792994

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!
 

Great idea! From free-thinking ( "why not"). One tiny problem...who pays for all this? And, who insures this project? :eek:

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.

Whuups. I forgot that you are in Canada.

Diy
 

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!

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.

back door
 

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!

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.

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.

For a scale model, it should be very easy to build a wind tunnel from an old central A/C unit.

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.:)

I was thinking the same thing:

http://www.motormint.com/ProductImag...ze07/12003.jpg

But I've resolved myself to tuft testing with a camera rig mounted on the car, and A-B-A tests.

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 :D. 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.

Answer was already there. Miniature full size truck is 100mmx150mm=15000mm2=0.015m2
with that 5% rule I get cross sectional area of 300000mm2=0.3m2=3.3 square feet.

In cross sectional dimensions this means 450mmx665mm

That was good tip to make that tunnel portable. There is only one problem you should be able to put that car you are measuring on the side of your car. Otherwise your cars aerodynamics would influence to the cars you are measuring?

Can then some verify that same cross sectional rules apply to water tunnels?

Thanks for posting to this thread -- I'm building a 1/4 scale model of CarBEN and I'd like to build a wind tunnel to tuft test it. The model is 41" long at it's longest points, and 16 1/2" W x 16 1/2" H -- does this need to be 38 sq ft opening? That's a little more than 6' x 6' ... and then how long would it need to be?

From other topic aerohead has pointed out that the wind speed needs to be quite fast 480 MPH on smaller scale 1:24 models :/. That sounds like a small problem :D

There is the answer so 6x width of the tunnel?

Again from other topic: on your 1:4 tunnel you need 80 mph wind speed ;).

I have a knucklehead idea that borders on the ridiculous, but here goes :
If you want to test a scale model of a car with a true 65 -70 MPH windspeed, you could test it in the bed of a truck mounted high to avoid the turbulence from the cab.

Sorry if that has been mentioned before I didn't read everything here.

I'm not looking for a Cd number; all I want to do is tuft test and find the problem areas, so the proper velocity is not critical.

Outside the bun
 

Neil, obviously a wind tunnel is for understanding full field flow, but I wonder if you might get any insight by testing specific regions with a leaf blower...

You mean like this?

http://www.youtube.com/watch?v=HPG6Z_a_hLo

A very interesting topic for thought experiments. I suppose almost everyone
who has even a moderate interest in vehicle aerodynamics harbors a secret
desire to build a backyard wind tunnel.

Of all the possible configurations, my musings have settled on an open air
arrangement. Perhaps it's overly simplistic, but I have been thinking:
* Get a piece of level land in an area that is frequently very windy
(*See refs below.)
* Ensure that there is a clear path upwind; no trees, buildings. etc.
* Install a ground-level turntable (~ 20 ft diameter?) to point car upwind.
* Have rollers so powered wheels can drive un-powered wheels to wind
speed.
* Do smoke/tuft tests to your hearts content including wind off-center line.

It seems like all the height, width, and length issues would not apply.

With luck, if you built it ,""they would come."

Of course some accomodtions would have to be made for non-windy days/nights.

* Windy US places refs:
Wind Energy Resource Atlas of the United States, Map
Wind Energy Resource Atlas of the United States, Text Descriptions
Top 10 US Cities Weather Facts and Extremes

I have a couple of leaf blowers, and that could be useful! Tuft testing can take various forms.

tunnel
 

your reading should have revealed that your model cannot exceed 5% of the cross-sectional area of the tunnels test-section.
So 5% of your garage test-section will define your model size.
Verisimilitude is your next challenge.Based on your models frontal area, the test-section air velocity would need to be such that you achieved at least the same Reynolds number as say at 20mph at full-scale.
If say you had a 1/4-scale model,you'd need a minimum of 80 mph air in the test-section to get your Reynolds number.
For a 1/4 model of a 22 sq ft frontal area car you'd need a 110 sq ft section.
That would require 774,400 cubic-feet per minute of air.
It would get loud and expensive.
The tunnel at Texas Tech has only a nozzle of around 3X4 feet,@ 80 mph and requires a 50-hp motor ( 37,300 Watts ) which exceeds the typical electrical service for a home.
And my experience is that models cost more to build than a real car.

#hangintherebaby Blog on deviantART: MythBusters 7.16 - Hurricane Windows

I saw mythbusters last week and this was there. Best way to make the tunnel portable like they have done. Already contacted the manufacturer of that Medusa and try get more info. Costs of that machine was 500000 dollars.