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Old 11-17-2015, 04:36 PM   #1 (permalink)
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I toured GM's new Reduced Scale Wind Tunnel.

copy/pasta from my blog post. To see pictures, check out my blog post. Seriously, the formatting is better and everything!



I recently had the opportunity to tour General Motors’ brand new reduced scale wind tunnel, where future car designs are tested and refined. Come take a look inside!

Disclaimer: I am currently working on contract to General Motors, but share this as an armchair aero-geek.


What makes this wind tunnel special?


GM’s full-size wind tunnel, which tests 1:1 scale vehicles as well as scale models, began operation 35 years ago and the industry has learned a lot about aerodynamics and wind tunnel design since then.

The new $30 million tunnel features a cutting-edge five belt ‘rolling road’ design that, with four rollers under the model’s carbon fiber tires and one down the middle, better simulates the effects of driving on a real road.

It is an open layout as well. This means that the testing area is not enclosed, and testers can stand very close to the model without affecting results. Since there are no walls enclosing the test area, you can actually see the return path of the wind tunnel on the far side of the car.

How fast does the air go?


The top speed of the tunnel is 155mph. As shown at the end of the video above, though, that equals out to a lot lower speed considering the model’s scale. A 1/3 scale model will only get hit with 1/3 as many air particles, so to get results consistent with full-size testing, the air has to be going 3x the speed. So, considering the size of the models being tested here, 155mph is equivalent to about 60mph for a full size car.

That’s a pretty cool car model.


You’re damn right it is. Anywhere from 30-40% the size of a real car, each 450-lb model tested here has several special features to accommodate the rolling road setup as well as return more accurate data.

The car sits on four weight-sensitive posts which are set in the tire wakes to reduce disturbance. Each wheel is custom made from solid aluminum, and each tire from carbon fiber. With the speed that these wheels turn, this was the best way to keep vibration down.

But it doesn’t stop there. Every model not only has custom machined, fully operational suspension (with springs just big enough to keep the wheels from bouncing on the rollers), but they have accurate underbodies, engine bays and grilles as well. All of this makes each scale test a bit more accurate, which means there will be less guess-work when it comes to predicting the CdA of the full size model, and even final production vehicle.

Of course, most tests do not use foam models. During the design process, studios release math data to be milled in clay onto an aero armature like this Impala below. Different grille and other aero options are rapid prototyped and swapped to find the most efficient configuration. Aero sculptors work air dams, flow trips and other tricks of the trade in clay, then the model is sent back to the design studio for aesthetic tuning.

Design changes can be tested more quickly with the new tunnel as well. The small metal holes in the side of the K2 and Cruze models are used to lift and transport each model via a large overhead crane. To minimize down time, models being tested can be swapped within a minute. While one model is testing, sculptors can try out ideas on another.

Why should I care?

With ever-tightening fuel economy regulations from the federal government as well as tougher competition among other companies’ fuel efficient offerings, aerodynamic testing actually helps us get cooler cars on the road. Lowering aero drag through styling modifications (therefore raising fuel economy regardless of power plant) allows automakers to give you more horsepower for any given platform. From a consumer’s perspective you can expect higher performance engines, higher top speeds and better fuel economy because of this fascinating and seldom-seen corner of the auto industry.

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Old 11-17-2015, 05:56 PM   #2 (permalink)
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Thanks. That answers my question about the road only rolling between the wheels. So at least the area around the tire contact patch is modeled correctly.

The weight sensitive posts don't attach to the spinning wheels? Do they attach to the sprung or unsprung weight?
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Old 11-17-2015, 07:40 PM   #3 (permalink)
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Very cool! That would have been fun. I once toured GM Powertrain Headquarters and got to see several new engines before they were released.

One little quibble...

Quote:
Originally Posted by Sven7 View Post
How fast does the air go?

The top speed of the tunnel is 155mph. As shown at the end of the video above, though, that equals out to a lot lower speed considering the modelís scale. A 1/3 scale model will only get hit with 1/3 as many air particles, so to get results consistent with full-size testing, the air has to be going 3x the speed. So, considering the size of the models being tested here, 155mph is equivalent to about 60mph for a full size car.
I get that you are trying to simplify in layman's terms that the Reynolds number needs to be constant to compare the results on the model to that of the real car. However, it doesn't really make sense to me to state that the model gets hit with 1/3rd as many air particles. Keep in mind the frontal area of the model is 1/9th (1/3rd height x 1/3rd width) of the full size car. Scaling up the speed really has more to do with the fluid in the boundary layer acting over the length of the model. Since the characteristic length is 1/3rd as long, the speed has to be 3 times as great.
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Old 11-17-2015, 08:40 PM   #4 (permalink)
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Old 11-18-2015, 08:57 AM   #5 (permalink)
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Nice blog post, wonderful of GM to allow you to publish this.

Did you have to get model releases signed?

Aero sculptors................brilliant term, one I have not read before.

Thanks for posting this.
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Old 11-18-2015, 12:04 PM   #6 (permalink)
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Quote:
Originally Posted by freebeard View Post
Thanks. That answers my question about the road only rolling between the wheels. So at least the area around the tire contact patch is modeled correctly.

The weight sensitive posts don't attach to the spinning wheels? Do they attach to the sprung or unsprung weight?
The whole model (what we would call sprung weight) rests on the pegs. The suspension does not support the model; it is just effective enough to keep the wheels from bouncing on the rollers.

Quote:
Originally Posted by darcane View Post
Very cool! That would have been fun. I once toured GM Powertrain Headquarters and got to see several new engines before they were released.

One little quibble...

I get that you are trying to simplify in layman's terms that the Reynolds number needs to be constant to compare the results on the model to that of the real car. However, it doesn't really make sense to me to state that the model gets hit with 1/3rd as many air particles. Keep in mind the frontal area of the model is 1/9th (1/3rd height x 1/3rd width) of the full size car. Scaling up the speed really has more to do with the fluid in the boundary layer acting over the length of the model. Since the characteristic length is 1/3rd as long, the speed has to be 3 times as great.
I work in "secret" every day, so it was refreshing to finally be allowed to share something.

That is how the engineers explained it. There's a lot of complicated math out there, and that explanation helped me visualize it and finally kind of understand what's happening. Whatever the actual numbers and ratios are, it gets the point across.

Quote:
Originally Posted by kach22i View Post
Nice blog post, wonderful of GM to allow you to publish this.

Did you have to get model releases signed?

Aero sculptors................brilliant term, one I have not read before.

Thanks for posting this.
Yes, fun to take some photos. I wasn't part of any select group and I'm not getting paid for the blog so I didn't get any releases. We were encouraged to take pictures and video; I'm sure GM loves the PR.

Aero sculptors... ha. When you're testing a physical model, you have to physically change it to test ideas. So you get people working in clay. I don't know if they come mostly from the aero side or sculpting side, but a lot of our sculpting staff in Design comes from a fine arts background.

My pleasure.
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Old 11-18-2015, 12:07 PM   #7 (permalink)
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Also, I unofficially asked one aero lab employee if there are any interesting aero blogs out there that I might share with you guys. This one came up:

Mulsanne's Corner, technical analysis of contemporary sports prototype racing cars

It's all about racing, but maybe there's something in there you guys will find useful.
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Old 11-18-2015, 12:48 PM   #8 (permalink)
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So, they are using air velocity to compensate for the scale of the model; not air pressure.

Thanks for the thorough report! I am looking forward to the results of their wind tunnels.
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Old 11-18-2015, 02:24 PM   #9 (permalink)
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Quote:
Originally Posted by Sven7
I work in "secret" every day, so it was refreshing to finally be allowed to share something.
So all that edgeamuckashun worked?

Quote:
Also, I unofficially asked one aero lab employee if there are any interesting aero blogs out there that I might share with you guys....
Do they now know about Ecomodder Forum > Ecomodding > Awhoadynamics?
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Old 11-18-2015, 06:33 PM   #10 (permalink)
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So all that edgeamuckashun worked?
I'm still paying on student loans, if that's what you're asking.

Quote:
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Do they now know about Ecomodder Forum > Ecomodding > Awhoadynamics?
I mentioned Ecomodder in passing but got no response. Probably because I was talking a mile a minute because OH MY GOD I'M TALKING TO A REAL AERO ENGINEER HOLY CRAP!!!! lol

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