Wind Tunnel air flow

[CHSGreenpower]

Hi,
I'm building a small, low-speed wind tunnel to test our school's Greenpower race car designs.
Greenpower has come up on Ecomodder before I believe, it essentially involves single seat electric cars having to travel the furthest distance they can in a given time on one set of batteries. All cars use the same batteries and motor.
Anyway, I digress.
We have built the test section of a wind tunnel for 1/10th scale models of our car designs.
It measures 165 x 235 x 360mm internally, with pieces angled at 45 degrees stretching 31mm along each side from the corner to prevent pressure differences.
The largest model that it could be used for measures 120 x 120 x 280mm however typical sizes are usually about 60 x 50 x 280mm.
The wind tunnel needs to be able to produce speeds from about 25mph to 55mph or so.
I am yet to build the inlet horn, diffuser or purchase any fans yet because I didn't want to build them incorrectly so as to change the working speeds, rendering our wind tunnel almost useless to us.
What size should the inlet horn and diffuser be?
What CFM will I need to achieve these speeds?
If any of you can point me in the correct direction to find answers to either of these questions that would be great.
Thanks
Sam

[seifrob]

Hallo to Ecomodder.
May I ask you of your age, skill level and timeframe you are expected to deliver results to your team? That is important for precise answers.

Now to answers:
1) here is the link I got after minute google search:

Science Buddies: How to Build and Use a Subsonic Wind Tunnel

(but I am sure you somehow missed it during your research, and you really tried various combinations of "wind, tunnel, aerodynamics, wind tunnel" terms in google search, and this "do it for me please" forum post was your last resort).

2) More intelligent and experienced people here would tell you your effort is futile, as 1:10 scale model aerodynamics is completely different than aerodynamics of real car. To test 1:10 model you need cca 10xmore wind speed than in reality. Look at https://en.wikipedia.org/wiki/Similitude_(model). (That´s why is hour in wind tunnel so expensive and why automotive industries is still building new tunnels. You can either read Phil's aerodynamics seminars (first post in this forum, but I am sure you are now reading it) and choose one from pre-researched aerodynamic shapes, or choose one from already existing aerodynamic cars or velomobiles in existence (there are dedicated threads to interesting aerodynamic cars and velomobiles)

3) if you really mean it and want to test scale models, (and have plenty of time and money), consider using water tunnel. there should be someone who started a thread about it. use search button

4) some basic shape evaluation you can get from software tools - flow illustrator ( Flow Illustrator - free online flow simulation tool ) looks like a toy, but at least helps you to visualize the flow. There was even discussion here on ecomodder, where you can educate yourself too. use search.

But without basic understandig of aerodynamic principles are all efforts likely to fail (Halliday, Resnick, Walker - physics lessons for start, then perhaps any college physics book, than aerodynamics)

[aerohead]

Originally Posted by CHSGreenpower

Hi,
I'm building a small, low-speed wind tunnel to test our school's Greenpower race car designs.
Greenpower has come up on Ecomodder before I believe, it essentially involves single seat electric cars having to travel the furthest distance they can in a given time on one set of batteries. All cars use the same batteries and motor.
Anyway, I digress.
We have built the test section of a wind tunnel for 1/10th scale models of our car designs.
It measures 165 x 235 x 360mm internally, with pieces angled at 45 degrees stretching 31mm along each side from the corner to prevent pressure differences.
The largest model that it could be used for measures 120 x 120 x 280mm however typical sizes are usually about 60 x 50 x 280mm.
The wind tunnel needs to be able to produce speeds from about 25mph to 55mph or so.
I am yet to build the inlet horn, diffuser or purchase any fans yet because I didn't want to build them incorrectly so as to change the working speeds, rendering our wind tunnel almost useless to us.
What size should the inlet horn and diffuser be?
What CFM will I need to achieve these speeds?
If any of you can point me in the correct direction to find answers to either of these questions that would be great.
Thanks
Sam

I don't have my work book with me,nor calculator but here are some considerations:
* a 1/10-scale passenger car tunnel would require a test section cross-sectional area of 4-meters-square minimum,to counter blockage effects.
*a minimum test section airspeed would be 200-mph to reach a critical Reynolds number/turbulent boundary layer.(dynamic similarity).
*at 200-mph,we have 17,600 feet/minute,and with 44-sq-ft of cross-sectional area,we get 774,400-Cubic Feet per Minute (CFM).
*high-speed tunnels have boundary layer thickness issues,and the ground borad must be elevated above the boundary layer,or suctioned off the flooor of the tunnel,as in the real world,the ground has no boundary layer.
*you'd need help from an air conditioning company to determine wall friction coefficients/losses,and total static pressure requirements to determine net horsepower for the air mover(s).
*Alan Pope published a great book on Low Speed Wind Tunnel Design.It's worth an inter-library loan if your school doesn't have it.
*small-scale model features cannot be successfully investigated except in near-full-scale CFD or wind tunnel.(Ferrari is spending 100-hours just to refine a single spoiler in their tunnel,at a cost of $4,000 (US)/hour.[$400,000]
*Texas Tech had a small tunnel and did 1/12th-scale work.With 50-hp (electric) they could get an 80-mph section velocity,not enough for a TBL Reynolds number,and they had to back things up in a water tow tank.

[CHSGreenpower]

Thanks to Sefirob and Aerohead for your input.
Your replies confirm my suspicions that this will only be able to be a visual educational tool for younger students - at that sort of rate it would suck itself in!
I'm a year 10 student, 15 years old, so am still learning here (as you can see). I intend to back up my results with CFD when I have the required experience to do this.
My idea was mainly to use it as an educational tool, and instead of relying on output numbers I was going to use it to answer the question 'does this offer an improvement over that?'
Some people have told me that wind tunnels are near obsolete due to CFD but I know that (like aerohead said) large names in aerodynamic design still use them.
I presume continued use of analogue methods (wind tunnel, water tow) is now mostly to back up CFD readings?
Sefirob - I'd seen that page, and used it a few times, however I came on here to see if:
1. The wind speeds I wanted would work
2. If I could make those speeds accurate by design
The answer, so it seems, to both of those questions is no.
Instead, I now intend to simply make it similarly to the Science Buddies one just as an educational tool.
The question I now ask is: Will the visual outcomes from tests in this (very inaccurate) tunnel mislead students, being a hinderance to their learning?
If so I'll quit while I'm ahead. I've not spent much money on it yet so will probably concentrate on getting my CAD skills up to scratch so I can do CFD instead.
Thanks again

[freebeard]

The question I now ask is: Will the visual outcomes from tests in this (very inaccurate) tunnel mislead students, being a hinderance to their learning?
If so I'll quit while I'm ahead. I've not spent much money on it yet so will probably concentrate on getting my CAD skills up to scratch so I can do CFD instead.

I think it was Ghandi that said it isn't important what you do, but it's very important that you do it. Time to evaluate cutting your losses on the test section. But look very closely at aerohead's bullet points. He gives you everything you need to succeed.

Bottom line you've got to stuff north of 50hp through your test section. If you reduce scale (say to 1/18th) you bump up against transonic effects above 250mph.

Have you looked at the posts from graysgarage? His techniques are better suited to your scale of operation.

The future is CFD. I can say this because of OpenVDB, which is being incorporated into current versions of Blender. Animating smoke particles is possible, and the data structure can be queried in Python so extracting useful data should be possible.

Using OpenVDB wouldn't be like a wind tunnel because the tunnel part goes away. It's complicated but [sparse, shallow inverted binary trees, bit masking, template metaprogramming, blah, blah blah] TLDR memory and computational problems go away.

When it ships in August I want to get the Beagleboard X-15. That should handle the 3D rendering handily; the problem is I suck at Blender.

[ChazInMT]

Originally Posted by seifrob

2) More intelligent and experienced people here would tell you your effort is futile, as 1:10 scale model aerodynamics is completely different than aerodynamics of real car. To test 1:10 model you need cca 10xmore wind speed than in reality.... You can either read Phils aerodynamics seminars (first post in this forum, but I am sure you are now reading it) and choose one from pre-researched aerodynamic shapes, or choose one from already existing aerodynamic cars or velomobiles in existence (there are dedicated threads to interesting aerodynamic cars and velomobiles)

The Czech man speaks the truth. Pay heed this section here and you'll rule over the competition.
Copy what's been done before.

Keep in mind 2 very often overlooked points.

1) The air is not blowing around your vehicle.....the vehicle is moving through it. The air is just sitting there stationary and you are ramming through it with your car, the distinction has huge implications when you consider that a vehicle is adding energy to the air in order to push it out of the way, it must displace the air up and to the sides.

2) Optimal aerodynamics are achieved by using the least amount of energy to move this air out of the way, AND Leave It As Undisturbed As Possible after you have passed through it.

Lastly, feel free to use us as a "secret weapon" there is a ton of knowledge here on ecomodder with people who would relish the chance to help guide your project to a successful conclusion. Think of us as an adviser committee. You will have to put up with some snarkiness sometimes, and you may stir up trouble when you turn members against themselves in an effort to give you the best advise, but we'll all really have fun and you'll gain a lot and learn a ton in the process.

So put up some more details about the rules you're bound to and what the cars that won in the past look like. If we see winners from the last few years we can pick them apart and figure out where the room for improvement is. There's nothing we like more here than playing the critic to somebodies work! We'll pluck em like a chicken!

Welcome!!

[seifrob]

Thanks for correction, apologies to Phil. (hard to concentrate while typing on the miniscule Xperia of mine.). Going to fix in the original post.

[freebeard]

Cool. Now ChazInMT can mute that RED text.

[aerohead]

Originally Posted by CHSGreenpower

Thanks to Sefirob and Aerohead for your input.
Your replies confirm my suspicions that this will only be able to be a visual educational tool for younger students - at that sort of rate it would suck itself in!
I'm a year 10 student, 15 years old, so am still learning here (as you can see). I intend to back up my results with CFD when I have the required experience to do this.
My idea was mainly to use it as an educational tool, and instead of relying on output numbers I was going to use it to answer the question 'does this offer an improvement over that?'
Some people have told me that wind tunnels are near obsolete due to CFD but I know that (like aerohead said) large names in aerodynamic design still use them.
I presume continued use of analogue methods (wind tunnel, water tow) is now mostly to back up CFD readings?
Sefirob - I'd seen that page, and used it a few times, however I came on here to see if:
1. The wind speeds I wanted would work
2. If I could make those speeds accurate by design
The answer, so it seems, to both of those questions is no.
Instead, I now intend to simply make it similarly to the Science Buddies one just as an educational tool.
The question I now ask is: Will the visual outcomes from tests in this (very inaccurate) tunnel mislead students, being a hinderance to their learning?
If so I'll quit while I'm ahead. I've not spent much money on it yet so will probably concentrate on getting my CAD skills up to scratch so I can do CFD instead.
Thanks again

Daimler-Benz has a full-scale CFD capability which runs on a supercomputer.They report a 99% accuracy compared to full-scale tunnel testing.
The rub is that a single iteration of a body design takes more than a day's run time in the computer.
A full-scale mockup,with interchangeable surfaces in clay can be evaluated within minutes,and changes can also be accomplished within minutes.
Also,clay modelers can actually (and do) modify the clay mockup right inside the wind tunnel,literally shaving a few millimeters off a contour at a time then quickly re-testing.
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I wouldn't discourage you from doing the tunnel.Run as fast an air velocity as your budget can allow.Tell the students the 'conditions' and limitations of the tunnel technology at 1/10-scale,and look forward to more robust 'tools' for the future.
Some 'big dogs' have used 1/10-scale,and they just figure that there'll be a discrepancy from a 'perfect world' condition,but you can certainly see 'trends' with smoke,lamp black and kerosene smears,and tufts.

[freebeard]

OP -- We're hoping for a third post from you. We here pick over the bones of the past. Specifically the Dieselpunk era. You are young enough that you should be looking forward.

M-B has sunk costs in their bazillion dollar facility, meanwhile Open Source and commodity hardware advance inexorably. In addition to OpenVDB (not here yet) and the X-15 (not here yet) there is stuff further out.

Knots in chaotic waves

Waves surround us all the time: sound waves in the noise around us, light waves enabling us to see... Most significantly, the whirls and eddies form lines in space called vortices. Along these lines, the wave intensity is zero, and natural wave fields - light, sound and quantum matter - are filled with a dense tangle of these null filaments.

Mark Dennis, Professor of Theoretical Physics in the School of Physics, said: "Although the computer models were framed in the language of quantum waves, these results are expected to be completely general, suggesting a new understanding of the complexity of the three-dimensional optical and acoustic landscapes that surround us every day."

More than 40 years ago, Bristol physicians Professor Sir Michael Berry and Professor John Nye discovered vortices were originally understood to be a crucial part of wave phenomena.

This work is part of the Scientific Properties of Complex Knots (SPOCK) project, a collaboration between the Universities of Bristol and Durham.

This will filter down into sparser algorithms.

What do you think about upside-down water tunnels with hydrogen bubbles?