Author Topic: "Ground Effects" Tunnels  (Read 34869 times)

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Offline Dynoroom

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"Ground Effects" Tunnels
« on: May 27, 2011, 11:49:35 AM »
Lots of aerodynamic information can be gleemed from many sources including books, magazines, internet, personal testing, ink or oil drops on your vehicle or even visual notes from others.

But my question; is there a formula to determine the L/D (lift/drag) ratio of ground effects tunnels? I have found some data from the IMSA GTP cars run in the 1980's that allow as to how much down force & drag they had with different setups but no details on the tunnels used to generate them.
My Arssumption is the tunnel cone should not exceed the 7 degree rule, and may want to be much less than that. But for a given length, height, & diameter can we somehow determine it's L/D ratio?
« Last Edit: May 27, 2011, 11:54:07 AM by Dynoroom »
Michael LeFevers
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Re: "Ground Effects" Tunnels
« Reply #1 on: May 27, 2011, 02:12:00 PM »
Mr. Dynoroom,   I have spent over 2 years researching just that subject, and have found that those people that know really don't want to tell very much of what they know.  I worked with some very sharp airplane aero people for a while, and we did figure a way to make some of their computer programs work to come up with some usefull info for tunnel design. Contact me, or see me at the lakebed in june, and we can talk.


         ED :cheers:

Offline Tman

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Re: "Ground Effects" Tunnels
« Reply #2 on: May 27, 2011, 02:32:34 PM »
Great new section to the board!

I am interested in this question as well. It came up in another thread and the JCB Diesel liner was used as an example. They use tunnels on that car.

Offline maguromic

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Re: "Ground Effects" Tunnels
« Reply #3 on: May 27, 2011, 04:06:45 PM »
Michael, I was able to catch my buddy Steve (one of the suspects on the roadster) before he left for LeMans today.  He was one of the designers for Reynard and Spice IMSA cars and has done lots of tunnels, and in his words. "There is no formula for L/D on tunnels, but 7:1 to 10:1 was often quoted for tunnels and 3:1 for (good) wings and spoilers.  That 7 degree figure was also tossed around generally, but I don't really know how much is based on fact.

The IMSA cars always ran as much tunnel as the rules allowed and the different aero setups (high downforce/medium/low downforce) were done mostly by topside stuff- which usually pulled additional air thru the  tunnels to varying degrees- and variations of inserts in the nose underside. Plus fender "S" flaps, side splitters, and wheel arch exit treatments on the car sides. The rear wing moved fore/aft and sometimes lower in various setups and the front fender louvers could vary in opening or be flush/proud, or blank. You see a lot of radiator exit wickers in photos of cars in high downforce trim, but that wasn't about downforce (usually it hurt downforce and increased drag). That was because the HDF courses were slower speed and the cars struggled to keep coolant and oil temps down".

I remember Trevor Harris mentioning something along the same lines long time ago at a IMSA race when we were struggling with our aero setup. Last year at Speed Week we went around looking at most of the tunnels on cars and did notice that many of them were not correct (mainly to steep of an exit). Tony
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Offline Tman

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Re: "Ground Effects" Tunnels
« Reply #4 on: May 27, 2011, 05:23:06 PM »
Points noted Tony, knew you would chime in here. Thanks. Trent

Offline jdincau

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Re: "Ground Effects" Tunnels
« Reply #5 on: May 27, 2011, 05:24:51 PM »
Mike,
     I have copies of two technical reports,
1. Airflow Beneath an Automobile, SAE Technical paper 741028 by Buckley and Laitone.
2. Performance and Design of Straight, Two Dimensional Diffusers, from the March 1967 Transactions of the ASME by Reneau, Johnston and Kline.
     The first is 7 pages, it is interesting to note that one of the sponsors was Collin Chapman's Group Lotus Car Co. The second is 9 pages. Both are sprinkled with formulas alas none of which calculate drag. I copied them both from the Lockheed technical library back when I was contributing to the GNP.
       Back in the 80's, with the aid of an engineer from the good old lazy L, I designed and fabricated skirts for our modified roadster which effectively converted our flat belly pan into a ground effects tunnel (diffuser). We already had rake, going from 3" ground clearance at the firewall to 9" at the rear. The short story is that it worked great, we pulled all of the weight out of the car (500+ lbs.) and traction was seemingly unaffected.
Wait for it!
However if the car pivoted even slightly because of wet salt or wind gusts the flow stalled and the car spun instantly.
We went back to ballast.
« Last Edit: May 27, 2011, 05:38:04 PM by jdincau »
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Offline Rex Schimmer

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Re: "Ground Effects" Tunnels
« Reply #6 on: May 28, 2011, 02:09:27 PM »
Mike,
I can highly recommend a book by Simon McBeath called "Competition Car Aerodynamics". Chapter 6 of this books delves into ground affects technology and provides some very good theory backed up with some very illustrative CFD prints that show what is going on in a ground affects configuration.  I am sure you can buy the book from Amazon or from the Automotive Book Store in Burbank.

The basic "ground affects" configuration is and inlet section, a throat section and a diffuser section. The majority of the down force is generated in the throat section as it is the area , if designed properly, that the air is at maximum velocity and therefore at the lowest pressure. The diffuser is typically a flat plane that is parallel to the ground. The function of the diffuser or "tunnel" is to reduce the velocity of the air coming from the throat section to the velocity of the surrounding air which minimizes aero drag. The amount of down force generated in the diffuser is small compared to the amount generated by the throat section. The angle of the diffuser should be such that the air flowing thru it does not separate or stall. On a typical road racing car, F1, Indy or sport car the length of the diffuser is dictated by the ruling body and therefore teams try to have the highest diffuser angle that they can to maximize its effectiveness where as on a Bonneville car you can let the diffuser hang out the back of the car which would allow you to run a lower diffuser angle to insure its proper function.

Rex
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Offline maguromic

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Re: "Ground Effects" Tunnels
« Reply #7 on: May 28, 2011, 02:59:45 PM »
These are some diffusers from LMP cars. They are both very similar, but if you look closely you can see the subtle differences.Tony

Audi A10 Diffuser

Penske Porsche Spider LMP

 
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Offline SPARKY

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Re: "Ground Effects" Tunnels
« Reply #8 on: May 28, 2011, 08:07:34 PM »
 :-oTony Ride hight or transport hight  :?
Miss LIBERTY,  changing T.K.I.  to noise, dust, rust, BLUE HATS & hopefully not scrap!!

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Offline maguromic

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Re: "Ground Effects" Tunnels
« Reply #9 on: May 28, 2011, 08:33:29 PM »
Sparky, The Audi on top is in race trim, the one on the bottom is transport height.  Lets throw another log on the fire with some coupe and F1 diffusers.  This web site  http://www.gurneyflap.com/ has lots of good pictures of diffusers and aero stuff from all sorts of race cars.  Tony

F1 diffuser from the gurnyflap web site.

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Offline WOODY@DDLLC

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Re: "Ground Effects" Tunnels
« Reply #10 on: May 29, 2011, 09:45:39 PM »
Mike,
I can highly recommend a book by Simon McBeath called "Competition Car Aerodynamics". Chapter 6 of this books delves into ground affects technology and provides some very good theory backed up with some very illustrative CFD prints that show what is going on in a ground affects configuration.  I am sure you can buy the book from Amazon or from the Automotive Book Store in Burbank.

The basic "ground affects" configuration is and inlet section, a throat section and a diffuser section. The majority of the down force is generated in the throat section as it is the area , if designed properly, that the air is at maximum velocity and therefore at the lowest pressure. The diffuser is typically a flat plane that is parallel to the ground. The function of the diffuser or "tunnel" is to reduce the velocity of the air coming from the throat section to the velocity of the surrounding air which minimizes aero drag. The amount of down force generated in the diffuser is small compared to the amount generated by the throat section. The angle of the diffuser should be such that the air flowing thru it does not separate or stall. On a typical road racing car, F1, Indy or sport car the length of the diffuser is dictated by the ruling body and therefore teams try to have the highest diffuser angle that they can to maximize its effectiveness where as on a Bonneville car you can let the diffuser hang out the back of the car which would allow you to run a lower diffuser angle to insure its proper function.

Rex

Great primer Rex! Also check out Carrol Smith's "Engineer to Win" - Chapter Sixteen!

7° is the magic angle for no separation. The steeper angles can be driven efficiently by the position of the rear wing or exhaust. Aero has lots of interactions so use every tool you can to help to understand it. If it was easy all the rocket scientists would be doing it! :-D
All models are wrong, but some are useful! G.E. Box (1967) www.designdreams.biz

Offline SPARKY

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Re: "Ground Effects" Tunnels
« Reply #11 on: May 29, 2011, 10:39:55 PM »
I would love to hear about exhaust!!

ps 
Is the exhaust used to speed it up?

help it redirect?

or make it less dense

Or all of the above?
« Last Edit: May 30, 2011, 09:43:35 AM by SPARKY »
Miss LIBERTY,  changing T.K.I.  to noise, dust, rust, BLUE HATS & hopefully not scrap!!

"Security is mostly a superstition. It does not exist in nature, nor do the children of men as a whole experience it. Avoiding danger is no safer in the long run than outright exposure. Life is either a daring adventure or nothing."   Helen Keller

We are going to explore the racing N words NITROUS & NITRO!

Offline akk

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Re: "Ground Effects" Tunnels
« Reply #12 on: May 30, 2011, 06:31:05 PM »
The fact that you saylift to drag ratio should cause you to stop and think....

We have a very long straight away...any drag will slow you down!!!!!!!!!!

computer simulation shows that until you are running 350 mph...lead is better every time!!!

LEAD = "DRAGLESS DOWN FORCE"

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Offline Stan Back

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Re: "Ground Effects" Tunnels
« Reply #13 on: May 30, 2011, 06:49:13 PM »
Note the above author's credentials.
Past (Only) Member of the San Berdoo Roadsters -- "California's Most-Exclusive Roadster Club" -- 19 Years of Bonneville and/or El Mirage Street Roadster Records

Offline SPARKY

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Re: "Ground Effects" Tunnels
« Reply #14 on: May 30, 2011, 07:20:03 PM »
rolling reistance is also a form of drag  :-P
Miss LIBERTY,  changing T.K.I.  to noise, dust, rust, BLUE HATS & hopefully not scrap!!

"Security is mostly a superstition. It does not exist in nature, nor do the children of men as a whole experience it. Avoiding danger is no safer in the long run than outright exposure. Life is either a daring adventure or nothing."   Helen Keller

We are going to explore the racing N words NITROUS & NITRO!