Wind Tunnel testing for the enthusiast

Shocks, Springs, Brakes, Frame, Body Work, etc

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j-c-c
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Wind Tunnel testing for the enthusiast

Post by j-c-c » Sat Oct 22, 2016 9:26 am

Anybody go this route? Any comments worth sharing or suggestions? Obviously with the title my interest and wallet are not in the F!/Nascar arena, but at some point I want to get some testing done to check off my bucket list. First question, on a non moving ground plane, is the suspension effectively locked up, or does increasing DF allowed to compress the suspension further and then additionally altering the results? Seems like allowing it to vary would be more realistic? Since engine is not running, do you run your electric fan? My thinking is the NC low speed tunnel price range, for 1 day, might even want to get a similarly interested muscle car body style mopar owner to help share the costs, and the data to see what we got, if that is efficient tunnel/time wise use, I've got a number of variations that are somewhat easy to swap out that I'm curious if they are beneficial or effective. I have no competitive class restrictions I need to adhere to.

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Re: Wind Tunnel testing for the enthusiast

Post by MadBill » Thu Nov 10, 2016 9:25 pm

Typically the ride height is fixed and the testing done at various heights and rakes, since spring rates and thus ride heights at speed are a track-specific variable.

Most competition cars do not have a fan that operates at speed but if yours does, running it would give a tiny increment of accuracy.

Given the cost of tunnel time, even though much of the content might not be applicable to your car, it would greatly benefit you to read a copy of Simon McBeath's Competition Car Aerodynamics, his CC Downforce or some similar text that gives a wealth of practical suggestions re optimizing tunnel usage.
Felix, qui potuit rerum cognscere causas.

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Re: Wind Tunnel testing for the enthusiast

Post by jacksoni » Thu Dec 01, 2016 12:07 pm

Couple months ago I spent 6+ hours in the A2 wind tunnel in NC. We made 29 pulls in that time including making changes some of which were 15 seconds, some required time to manufacture pieces to tape on. It is $495/hr. I found the equivalent of 70+ aero HP for my car which has a 2L engine that makes 290+hp NA. For my application big time worth it as there is no way I or anyone else is making 360NA HP out of a 2L engine (no power adders or fuel). A lot of little tricks helped, some didn't. Some big changes were the wrong direction. The front wheels are strapped down, the rears sit on 1/2" blocks so your ride height/rake must be adjusted to that. If not easily done with adjustable coil overs for instance, you can put blocks under the tires. The suspension moves. Lift and downforce are measured so the suspension responds to those forces. In contrast to some "common knowledge" my car did not like any rake. Setting the back down was worth a lot and improved lift both front and rear. I in fact have the capability to run my fans forward or reverse and the tunnel could measure that difference. Very small and it was felt that with the tunnel airspeed at 80mph or so, that at car speed of 170 the reverse fan deal wouldn't do much. This would take either faster tunnel airspeed (someplace else) or track testing. Spoilers and air dams and trip fences and gurney flaps etc were tested but not extensively- my budget was broken already. Bottom line, if you want to test stuff like this and can do it quickly you can find out a lot in the 2 hour minimum time. The folks there are really great. Well worth your time.

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Re: Wind Tunnel testing for the enthusiast

Post by jacksoni » Thu Dec 01, 2016 12:19 pm

In addition to the resources that MadBill suggested: the second was generated from A2 from Keith Turk's Camaro that Freiburger drove at Bonneville.

http://www.hotrod.com/articles/hdrp-060 ... icks-tips/

and

http://www.hotrod.com/articles/car-aerodynamics/

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Re: Wind Tunnel testing for the enthusiast

Post by ijames » Thu Dec 01, 2016 3:40 pm

There is a web site that focuses on improving economy at www.ecomodder.com, and they have an aerodynamics forum with some pretty knowledgeable people. Drag is drag, whether you want to apply an improvement to better fuel economy at fixed speed or to get more to speed at constant hp. One poster, screen name aerohead, has a T100 pickup that gets over 45 mpg; he's had it in a wind tunnel once and plans to go back. He highly recommends a textbook, Aerodynamics of Road Vehicles: From Fluid Mechanics to Vehicle Engineering ([Proceedings] / SAE), by Hucho, Wolf-Heinrich. Amazon has it for about $60, I've bought it but am only part way through it so far. I've learned a lot by lurking there (and here :mrgreen: ). One useful sticky thread in the aero forum is a template based on a wing (there's an infinite number of wing length to cross section heights, they chose one that seems a good compromise to wind up with a practical vehicle shape and seems to be very close to what many top fuel mileage vehicles are based on). They include instructions but basically, take a picture of your car from the side from as far away as practical to minimize lens aberrations, then put it in a photo editing program and then overlay the template, keeping the aspect ratio fixed and scaling it so that it touches the ground at the bottom and touches the highest point on the roofline at the top. The closer your vehicle comes to that shape the lower the drag will be. One thing I was surprised to learn is how important the rear of he vehicle is. As long as the front isn't totally hateful, like a vertical sheet of plywood, you can pretty much ignore it and so long as you get the rear to match the template you will be surprisingly close to what an ideal overall shape would give.

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Re: Wind Tunnel testing for the enthusiast

Post by j-c-c » Thu Dec 01, 2016 8:45 pm

I will check out the suggested reading, thank you. I'll be in Mooresville NC on Tuesday in route to PRI, to check out a known low speed full size wind tunnel. I suspect this area is still "black art" to some degree, and sharing hard fought for knowledge and aero tricks is not far enough along yet to filter down to the hobbyists level, or its just job security :D The tunnel has a list of auto aero engineers that will consult, as my knowledge is limited and tunnel time not cheap might be a helpful option to keep me focused on the big picture. Also thinking of lining up a cohort with a similar style car and goals, where he can learn from my testing vice versa. We'll see. :mrgreen:

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Re: Wind Tunnel testing for the enthusiast

Post by jacksoni » Fri Dec 02, 2016 8:56 am

The couple of Hot Rod articles and the link Carl Ijames posted are clearly stuff that has filtered to the hobbyist level. The data acquired by the various folks in the tunnels are indeed proprietary and the operators of the tunnels will not share this stuff with you. If you want to hire an aero engineer to advise you more the better but I suspect that just reading about things that work, things that don't and with generic advice from the tunnel operators will give you a lot to think about and do. Indeed if you can find some friends with similar cars to share the time and info with, more the better. But most tricks are generic and not so much specific to an individual body. The stuff Nascar comes up with you won't find published but hobbyists are more likely to do so. Depends on what you are doing. The usual "those who don't know, talk a lot, those who do aren't sayin'." :D If you talk to Geoff Eaker who runs the A2 Tunnel with his father Gary, Say hi for me.

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Re: Wind Tunnel testing for the enthusiast

Post by Olefud » Tue Dec 13, 2016 5:34 pm

Wind tunnel tests tend to be starting points mostly due to wall friction/confinement. The big boys massage the raw numbers with numerical simulations.

For your purposes you might consider a solid additive scale model of the form to be tested and a water tank "flow" tunnel. If you watch the Reynolds Number and understand flow you can get worthwhile qualitative results.

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