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Author Topic: Flat bottom /tunneling  (Read 23885 times)
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Richard 2
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« Reply #15 on: March 03, 2014, 12:22:41 PM »

Gentlemen
I would like to findout about tunnel design ,and anti lift resulting from their use.
 I agree with blue about departure criteria,and effort to study it is important!
Thanks Jack Costella

Anyone have any new or updated info on tunnels or Strakes?
Richard 2
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John Burk
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« Reply #16 on: March 03, 2014, 10:47:02 PM »

Hard to argue with Speed Demon's closed on front open in back tunnel . The skirts need to hang far enough below the belly to form an actual tunnel so the whole body sees the low pressure created in the back .
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« Reply #17 on: March 04, 2014, 05:15:27 PM »

Gentlemen
I would like to findout about tunnel design ,and anti lift resulting from their use.
 I agree with blue about departure criteria,and effort to study it is important!
Thanks Jack Costella

Anyone have any new or updated info on tunnels or Strakes?
Richard 2

Richard, for what style car is it?.
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Richard 2
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« Reply #18 on: March 04, 2014, 10:01:58 PM »

mod. Roadster flat bottom 175" wb
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WOODY@DDLLC
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« Reply #19 on: March 05, 2014, 11:08:14 AM »

Here are some basic diffuser test results to ponder based on published wind tunnel tests and various anecdotal comments. Trouble is when you put any of these or other schemes on a real vehicle (with wheels & such) it all changes! Remember what was done on an F1 or Indy car was based on a different vehicle, operating conditions and rule book!

Diffusers are typically more efficient than tunnels. Strakes in the diffuser may or may not make a difference. It all depends on a lot of interacting variables.  shocked

This is THE basis for virtually all formal diffuser studies: SAE980030 The Aerodynamic Performance of Automotive Underbody Diffusers, Kevin Cooper

It's not real hard with the right tools [CFD or wind tunnel] but it is real complicated!

But if all else fails there is always monkey-see monkey-do!  grin


* Diffuser Test Bodies_e.JPG (236.63 KB, 1223x1590 - viewed 455 times.)
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« Reply #20 on: March 05, 2014, 04:34:36 PM »

Richard, we set up our Mod Roadster with skirts that pinched together under the front axle point.  There was an inlet area allowed by angling the skirts.  The skirts went straight down along the sides, followng the line of our body/chassis all the way to the back of the car.  I set it up with a gradual outward taper, as viewed in topview planform, for about 166".  That was in 1998.  There were sure a lot of folks asking questions about it, but we didnt know much yet, when we first got there with the new car.

The car had a full bellypan, with a step down under the engine/bellhousing area.  We had about an inch of front suspension, none rear.  The first time the car ran to about 175 it sucked the engine portion of the bellypan into ground contact.  I had to run tie down straps through the edge bracing to pull it up and keep it off the salt.  Note that I had done a lot of work to keep that car from sucking up dirt at Muroc and ElMirage, which may have contributed to the belly pan sucking down.

Gosh, that was a while ago, I realize! tongue

Hope this helps.

JimL
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Richard 2
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« Reply #21 on: March 06, 2014, 10:09:09 PM »

Thanks a lot for the reply's this helps reinforce my plans for the car.

JimL,  Did you ever go over 200 mph with the tunnel? And do you think the tunnel may of worked to well. By that I mean did it rob more hp than needed to go straight.

Woody, To make sure I'm correct about how to read the chart. Red is Fast and Blue is stalled air.
If there was a wind tunnel close to me I would be there quick. But as luck would have it I found a car much like mine that works well over 250 mph with some strakes in the rear. So, I guess, I fit the Monkey See Monkey Do category. grin
I also know from my real Job that what works for one may not work for another.

John Burk, I have to agree, but.

Thanks Richard 2
 
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« Reply #22 on: March 06, 2014, 11:00:21 PM »

Our best time slip was 186, most runs were 175-183 area.  This was a 2.0 liter 4-wheel-drive Celica engine mildly modified.  I always thought keeping the air out from under was helping our speed, but I dont really know.

All I am convinced of, is that you dont want any bellypan inside a tunnel that is wimpy or poorly mounted!  I forgot to mention; the last year we ran front suspension on the car it ground the front area of the skirts down at speed.  They had about 1/2" ground clearance at rest.  I guess our simple tunnel did something.

JimL
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« Reply #23 on: March 07, 2014, 09:25:14 PM »

Woody, To make sure I'm correct about how to read the chart. Red is Fast and Blue is stalled air.
If there was a wind tunnel close to me I would be there quick. But as luck would have it I found a car much like mine that works well over 250 mph with some strakes in the rear. So, I guess, I fit the Monkey See Monkey Do category. grin
I also know from my real Job that what works for one may not work for another.

Thanks Richard 2

R2, yes the red is fast and blue is really slow or maybe even dead! tongue

The overall results for this series of tests showed a drag variation of 34% and a lift variation of 46%. Also a pitch variation of 25%. Pitch is nose up or down which loads or unloads the front and rear wheels depending on where the CG is located. These objects are much simpler than a real vehicle but they show what relatively minor changes can do. Keep in mind that what you get away with at 200 mph might not work at 225 mph or 250 mph. The forces go up 27% from 200~225 and 56% from 200~250. HP goes up 42% & 95% respectively. Imagine you are on a compound see-saw with wheels going really fast - because you are - so balance is even more important now than on the playground! Traction can hide a lot of sins and the lack of it can make you seek redemption! Or a new pair of shorts!  shocked

The A2 is not that far away and I think everyone who has been there will tell you it's time & money well spent! See you on the salt! cheers

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« Reply #24 on: March 07, 2014, 11:08:27 PM »

Thanks, to every one again.
Richard 2
« Last Edit: March 07, 2014, 11:35:08 PM by Richard 2 » Logged

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« Reply #25 on: March 07, 2014, 11:46:40 PM »

Our best time slip was 186, most runs were 175-183 area.  This was a 2.0 liter 4-wheel-drive Celica engine mildly modified.  I always thought keeping the air out from under was helping our speed, but I dont really know.

All I am convinced of, is that you dont want any bellypan inside a tunnel that is wimpy or poorly mounted!  I forgot to mention; the last year we ran front suspension on the car it ground the front area of the skirts down at speed.  They had about 1/2" ground clearance at rest.  I guess our simple tunnel did something.

JimL

You think??? grin grin
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« Reply #26 on: July 10, 2014, 02:55:22 PM »


The A2 is not that far away and I think everyone who has been there will tell you it's time & money well spent! See you on the salt! cheers


I've not been in A2, does it run the same boundary suction and wheel rotation systems as A1? If so I would question validity of results specifically in the areas of underfloor flow. Not saying the results are wrong, I have met Gary Romberg before and he is a very smart and cool guy, and I know the tunnels get good correlation for NASCAR, plus I know for what we are doing few people can afford Windshear.

But if you are specifically looking at small interactions under the car then I would suggest that directly sucking the airflow through the floor in the test section is not going to give a good representation of how air moves under a car. You really need a moving belt with the boundary layer taken care of before it gets to the vehicle.

This is something I have worked on, on a number of different race (and road) cars and it does not have a simple answer. CFD is a good tool for it but struggles with accurately modeling splitter separations that occur on real cars, thus you often gain too much confidence in a CAD design only to find you are short on front downforce on the real car. Or even worse the pitch sensitivity is through the roof. The famous flying Merc at LeMans was caused by a pitch change of about 1/10 of a degree!

I have actually been thinking of writing a paper for the LSR community on this very subject to aid teams in designing safer race cars, as of yet I have not found a backer to help with the testing and my boss isn't very keen on lending me the wind tunnel and letting me make a model free of charge.     
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« Reply #27 on: July 10, 2014, 09:11:59 PM »

Bratfink;

Interesting comments- thanks. "Pitch change" can do odd things. A friend who was an engineer with John Wyer Automotive Engineering in the '60 & '70s told me a story about wind tunnel testing a sports racing car in the MIRA facility. They were investigating the effects of ride height & attitude on downforce and found that under certain conditions, the nose of the car would start oscillating up and down above a critical speed.  shocked

The explanation was that as the speed increased, the body shape generated increasing downforce in the front, which compressed the front springs and dropped the nose down. At a critical speed, the nose dropped down so far that the airflow under the car dropped dramatically and the downforce also dropped, allowing the front springs to raise the nose back up and then the whole cycle repeated itself. Stiffer front springs solved the problem.

Regards, Neil  Tucson, AZ
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Regards, Neil  Tucson, AZ
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« Reply #28 on: July 11, 2014, 07:21:41 AM »

Bratfink;
They were investigating the effects of ride height & attitude on downforce and found that under certain conditions, the nose of the car would start oscillating up and down above a critical speed.  shocked

The explanation was that as the speed increased, the body shape generated increasing downforce in the front, which compressed the front springs and dropped the nose down. At a critical speed, the nose dropped down so far that the airflow under the car dropped dramatically and the downforce also dropped, allowing the front springs to raise the nose back up and then the whole cycle repeated itself. Stiffer front springs solved the problem.

Sports prototypes still do it! You watch footage of cars coming down the Mulsanne and see them bobbing along, people think it's the bumps in the track, and some of it might be, but the effect is way too sinusoidal for my liking. and it mirrors what I see in the wind tunnel with this sort of car. I have seen models get so out of shape they would end up grinding the front splitter on the belt!

My boss came from Mira (now he's a guy I'd love to get out on the salt, he'd have a field day helping people out), it's a fixed ground tunnel. The effect could have been worse had the "road" been moving. You can think of a front splitter as "a diffuser for the front of the car", the same air flow interactions that make diffusers work also make splitters work. And if you choke the flow to them they also stop working the same way.   

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« Reply #29 on: July 16, 2014, 11:21:42 PM »

In the old wing car era they called that phenomena "porpoising" and it would drive you nuts. grin
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