rudypoochris Posted June 18, 2007 Share Posted June 18, 2007 I am on summer break now and have decided that I kind of want to build a scale windtunnel. I would like to put 1/12th scale models in it. The problem I am having at the moment is determining the correct windspeed... Here is what I am hung up on. I was reading on wikipedia that to achieve the same results for testing, a wing 1/4 the size would need 4 times the speed. Does this mean I would need 12 times the air speed to achieve equal downforce as on the real car? Or does this mean I would need 12 times the speed to recieve equal scale downforce (as in 1/12th that of the real car). 1200mph seems ridiculously fast. I am happy with scale results since I will be calculating coefficents and such. If I do indeed need 1200mph I am thinking of simply switching over to water... thoughts? The second issue is providing the wind. I was doing some calculations and I think even the more powerful 20" "block" fans are insufficent. I am unsure. I know the taurus fan puts out something like 4000CFM which would be (4000*60) = 240000 cubic feet per hour. Assuming the opening to the tunnel is 8"x8" or .444 sq ft. The windspeed would be (240000/5280)/.444 = 102.3 mph. This would seem pretty good provided the fan can push 4000 cfm loaded and that 100 scale mph is equal to 100 real mph's. If this all checks out, how do I power it!!?!? 40 or 60 amps at 12volts... do they sell converters or something similar? 40 seems alot, let alone 60. The last issue is measuring the windspeed. How would one go about doing this acurately in a small 8" square tunnel? I haven't looked at the calcualtions yet, but is it necessary to know the exact windspeed or is an approximation based off the CFM of the fan "good enough". Thanks! Quote Link to comment Share on other sites More sharing options...
cygnusx1 Posted June 18, 2007 Share Posted June 18, 2007 There is a thread here somewhere about two years old where someone was building a scale tunnel. The roadblock was that the air speeds had to be several hundred miles an hour...something about reynolds numbers..... Quote Link to comment Share on other sites More sharing options...
OlderThanMe Posted June 18, 2007 Share Posted June 18, 2007 Yeah I am the guy that started the minature wind tunnel project... I am going to bring the project back off of the back burner just to see if I can get similar air reactions on the car as the real thing. I don't think the reynolds number has to be 100% correct to see where air is seperating from the hatch. One thing I noticed in my testing with the tiny fan was the difference in windspeeds in different areas around the car. The highest area was right behind the hatch. The next highest was under the car (on a stock height model). I tested windspeed with a fairly accurate wind speed device. Granted I'm not going to get 100% perfect results but things like the draft behind the car and pressures around the vehicle can be seen. I still want to figure out a way to do smoke. Incense was good for how light it is but it burns too slow and doesn't make enough smoke to see in the tunnel. The next thing I am going to do is get a huge fan to use, build a proper diffuser to stop the spiraling of the air in the tunnel, and then put in a thing plexiglas window. I was using toilet paper cardboard tubes stacked inside the tunnel as my diffuser before. It worked fairly well but it could still be better. The tube is from home depot and is a form for making concrete pillars. It is 8" on the inside and like 5 feet long. $8... Hmm... I don't remember what unit of measure 255 is... But with the whole Z model in there sometimes I would get close to 400 right over the hatch and like 325 under the car. Also good for curing glued parts. I was having trouble with the engine being pushed out of the tunnel from the airspeed...lol Quote Link to comment Share on other sites More sharing options...
rudypoochris Posted June 19, 2007 Author Share Posted June 19, 2007 Well to keep the same reynolds number you need 1200mph on a 1/12th scale model for a 100mph "real life" test. I am not understanding if this will make equal forces as in real life, or if it will generate scale forces. As in 1/12th the downforce. What fan are you using Olderthanme? I am thinking of going with water if 1,200mph in air is needed. Quote Link to comment Share on other sites More sharing options...
OlderThanMe Posted June 19, 2007 Share Posted June 19, 2007 What fan are you using Olderthanme? Umm... it is a small home depot fan. It was like $5. I think the wind was about 4mph or 100 scale mph for the 1/25 model. I'm going to get a box fan to use. I think it would do much better. The reynolds won't be the same but air seperation should be similar. Quote Link to comment Share on other sites More sharing options...
rudypoochris Posted June 19, 2007 Author Share Posted June 19, 2007 Wait... so you scaled down windspeed in proportion to the size of the car? So 100mph would be 8.3mph for a 1/12th scale model? Is that right...? Will that provided scale results (as in 1/12th the force being read)? Quote Link to comment Share on other sites More sharing options...
OlderThanMe Posted June 19, 2007 Share Posted June 19, 2007 Yes but the reynolds won't be right. I thought that I would be able to scale down windspeed but I don't think that will work. I'm just going to cram as much air as possible through the tube. Quote Link to comment Share on other sites More sharing options...
rudypoochris Posted June 19, 2007 Author Share Posted June 19, 2007 How important is Reynolds number to this all. It seems quite important for fine tuning things, but for the overall it doesn't seem to be a primary issue. Am I correct in understanding that Reynolds number basically decides whether or not the flow over an object is laminar or turbulent? If water is used (density is about 13 times more than air), I think a flow speed of around 92mph would equate to 100mph of air at 1/12th scale. That might be relatively doable. Then again... 92mph water is really fast... Quote Link to comment Share on other sites More sharing options...
strotter Posted June 20, 2007 Share Posted June 20, 2007 Even with bad Reynolds numbers, isn't it possible to get go/no-go data? That is, model "B" shows less lift than model "A" - though we don't know how much less lift this will translate to, we do know it's an improvement or not? Or, if model "A" shoots exhaust out of the pipe and into a 1/12-scale driver's face, and some modification on model "B" prevents it, can we suppose a similar modification of a full-size car will behave similarly? Ultimately I would suppose any change would need to be tested at full scale, but could smaller models provide a "direction" for examination? Quote Link to comment Share on other sites More sharing options...
OlderThanMe Posted June 20, 2007 Share Posted June 20, 2007 Strotter: That was my main idea for my miniture tunnel. To test the bigger aspects of aerodynamics. I had an intricate system that I had come up with to measure drag. I'd tie a tiny thread to the bottom of the Z somewhere and run it forward and down through the bottom of the tunnel via a pully. This string would be tied to a 20 gram weight and that is sitting on a digital scale. Then the amount of drag would lift on the weight from the car rolling back and tell how much drag you have. I mostly wanted to test front and rear end treatments on the car. Just an idea... One thing we could do is get a model of a car and compare the results with the full size vehicle. Then we can test the small model and predict what we would see in the real tunnel. Before HybridZ goes back to the wind tunnel we should do some beta testing to see what we can design before the next test since several members are more knowledgable in aero stuff. Quote Link to comment Share on other sites More sharing options...
strotter Posted June 20, 2007 Share Posted June 20, 2007 Many years ago when I was in a fluids class, we did a series of tests using a liquid tunnel. It was strictly demonstration thing, but there were a couple of advantages to using a liquid: you didn't have to move the water as fast, meaning an aquarium pump did the job nicely; and it was possible to suspend a very fine material (some kind of soil amendment, as I recall) in the water, allowing you to make out the movement of the fluid. Now, water is a fundamentally different type of material, being incompressible and all, but as I recall it was possible to visualize turbulent flow very accurately and easily. Not the kind of thing you'll get good quantitative numbers out of, but fine for understanding the grosser flow, something I'm not clear on yet with regards to Z's. I'll dig out my Fluid Dynamics book and look it up. Quote Link to comment Share on other sites More sharing options...
rudypoochris Posted June 21, 2007 Author Share Posted June 21, 2007 Ya, liquid (water for me) is nice. Been looking into it alot. As for getting go/no-go data. That was what I was wondering. In a broad sense, ya sure. I think it would start to make a really big difference once one was to start caring about the finer points. Ex. Like at what angle will this wing be most useful, or useful at all. I would imagine that changes alot with speed and the medium. My plan was (if using air) to slot the bottom of a rectangular tunnel and pop on a platform which the car would sit on. Two springs of known constants would be hooked up and each would have a marker that pulls across a scale. The attachment points would be adjustable so you could "Zero" it out. Still thinking. The air speed issues are bothering me though. Quote Link to comment Share on other sites More sharing options...
Michael Posted June 23, 2007 Share Posted June 23, 2007 Why don't you guys contact the Mechanical or Aerospace Engineering department at your local university? They probably already have the facilties that you would like to use, and can answer your questions in detail. Quote Link to comment Share on other sites More sharing options...
rudypoochris Posted June 23, 2007 Author Share Posted June 23, 2007 Heh im an undergrad in mechanical engineering. That is a good idea though. Quote Link to comment Share on other sites More sharing options...
OlderThanMe Posted June 23, 2007 Share Posted June 23, 2007 Heh im an undergrad in mechanical engineering. That is a good idea though. I am too... though only a "fresh-more". Quote Link to comment Share on other sites More sharing options...
strotter Posted June 23, 2007 Share Posted June 23, 2007 Yeah, I got my BSME quite a long time ago. Still looking at my "Compressible Flow" book, slow going and it's mostly transonic/supersonic material. Trying to remember my differential equations... Quote Link to comment Share on other sites More sharing options...
jt1 Posted June 23, 2007 Share Posted June 23, 2007 You fellows still in school need to ask all the professors and grads about what kind of projects are going on, you might be surprised. Just be polite and ask everyone who will listen to you. Some of them might need some free help and you can get in on some interesting stuff. John Quote Link to comment Share on other sites More sharing options...
Michael Posted June 24, 2007 Share Posted June 24, 2007 For you guys currently in ME undergrad programs - this stuff should all gel by the time that you're halfway through your junior year. That is when you get the coursework with some real-world examples. To get a head start, consult the books.... An easy but useful book - intended for airplanes, but just as applicable for cars - is "Introduction to Flight", by John Anderson. Ignore the stuff on compressible flow, but concentrate on the first 4-5 chapters. Once you have read that, have a look at "Low-speed Wind Tunnel Testing", by Barlow, Rae and Pope; http://www.amazon.com/Low-Speed-Tunnel-Testing-Jewel-Barlow/dp/0471557749 Quote Link to comment Share on other sites More sharing options...
rudypoochris Posted June 24, 2007 Author Share Posted June 24, 2007 Thanks for the links! I think I will! OTM I am just a fresh-more too. Where are you located? Quote Link to comment Share on other sites More sharing options...
OlderThanMe Posted June 24, 2007 Share Posted June 24, 2007 Atlanta, GA... I went to a welding school at a local technical college during the last 2 years of my highschool. I am oficially a sophomore with my school taking freshman classes. I actually should be a junior this semester... When I broke my jaw into 4+ pieces back in October I had to withdraw from college for the semester. That put me back a year in chemistry and calculus. I went back spring semester and got some of the core classes out of the way. My college is in Pensacola, FL... Quote Link to comment Share on other sites More sharing options...
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