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dumb aero question

 
Old 03-15-2008, 06:37 PM
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chancecasey
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Default dumb aero question

I've noticed that with just about every front bumper/splitter setup, the designs always seem to "peel away" the layer of air very close to the ground, usually with a splitter that is parallel to the ground.

Often, though not always, the rest of the underside of the car is a little higher off the ground than whatever is at the frontmost part of the car, the splitter.

Seems to me that the relatively small ground-to-splitter distance, followed by the somewhat larger ground-to-rest-of-the-underbody distance, would create a reverse venturi effect, slowing the air flow under the car.

Since increasing the velocity of air flow under the car would seem to increase downforce / reduce lift, why is there no splitter that is a bit higher off the ground, then as you go back, there is a "gentle curve" downward to the level of the bottom-most part of the underbody, somewhere near the front of the tub.

Kind of like a rear diffuser but in the front.

Logically I would think this would grab a little more air up front, then as the air goes back under the car, it gets forced into a smaller area, increasing the velocity - the venturi effect.

I know aero often defies logic - and I'm wondering if anyone knows for sure this approach (sort of a "boat" shaped front, like a water ski, snowboard, etc.) just doesn't work, and why.
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Old 03-15-2008, 06:49 PM
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Oh this ought to be good !
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Old 03-15-2008, 06:58 PM
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If you can reduce the amount of air (slow air) under the car (via splitter) and accelerate it at the same time this will lower the under car pressure. The splitter reduces the amount of air travelling under the vehicle and forces more air over the top of the car. This accelerates the the air and reduces lift. Granted, lower ground effects, smooth trays and diffusers capitalize on the accelerated air by reducing high pressure air pools (lift).

In the "waterski or boat" frontend you are inviting slow air under the car. Slow air = high pressure (lift)= Bad!

I'm not a aerodynamicist but I did stay at a Holiday Inn Express last night. http://www.youtube.com/watch?v=lgX7i0C-IK4
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Old 03-15-2008, 07:29 PM
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BTW, Aston Martin used a splitter on their GT1 car that had a small curve upwards. What it did for the car is anyone's guess ( directed air for cooling ). Maybe someone from Advantage CFD can chime in.
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Old 03-17-2008, 01:24 AM
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JackOlsen
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I'm occasionally very dumb, so I think I'm qualified to answer a dumb aero question. Plus, I've built and tested a few different aero pieces for my car, some more useful than others. Some of them were really dumb. I spent a lot of time talking to experts, and then tried out some of my dumb ideas against their better advice. I built a diffuser. I built a front wing. The front wing was especially dumb.

The short answer, I think, is this: unless you can create a three-part underbody component that is always (at it's low point) just about 40mm above the pavement, and unless you can run springs heavy enough to pretty much eliminate any body roll, diving or squatting, then it doesn't matter. If you can't maintain the ground effects pieces in very precise relationship to the ground, at a distance of less than two inches from the pavement, you're not really going to be generating useful ground effect aero. At best, you'll porpoise. At worst, you'll lose all the stick you're generating whenever you brake or turn.

We're talking really stiff springs -- 2000 pounds or more, probably.

There are some things you can do that will have an impact on your car's handling. One is managing air pressure under the car -- keeping as much air from getting underneath it as possible. This is why people drop their splitters so low. The less air being crammed under there and squeezed down, the better. Absent a meaningful underbody kit, I don't think velocity is a factor --you're just jamming in high-pressure air, which will increase the car's lift.

The other thing you can do for the air that does get down underneath is to make sure that the car's underside is generating as little turbulence as possible. Smoothing out the underbelly of my car with a sheet of ABS plastic made a noticeable difference in my laptimes. Your 964 already has a pretty well-developed underbody design. Mine didn't. I had a big aftermarket AC condenser right under where I sat. But any 911 will benefit from smoothing things out down there. Turbulence means drag.

Simon McBeath's book does a great job of dissecting competition car aero. I'd like to say it was so simple that even I understood it. But if I'm wildly off base in this post, then I hope someone who's smarter about this stuff will set me straight.
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Old 03-17-2008, 01:49 AM
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I dont think drag is really all that it is cracked up to be on our cars. I think i proved that with my "Olsen" testing methods, i was able to show a 100lb additional downforce with my GT3 wing at 100mph (for which you only travel at, while at tracks such as Laguna and Sears, for only a few seconds at best), cost an additional 10lbs of drag, through a total gear ratio of 4.5:1 would only be like 2ft-lbs of engine torque.
I dont think this could be looked at as a measureable lap time improver, and certainly the equivlant drag reduction by using ABS under the chassis, would save only a fraction of the drag caused by a stock chassis. Sure, at the faster tracks, this might make a difference, but I think VIR, Mosport, RA, might be the only tracks around that would save you some time by an underside mod.

I think you give some reasons on your post of how splitters work.

I think the direct answer to the question, is that if the splitter was further from the ground, even if you then had the rest of the body closer to the ground, would cause a much larger high pressure area under the front of the car that would cause lift. the rest wouldnt matter after that. (not to mention, the drag ).


mk
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Old 03-17-2008, 12:33 PM
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One other thing to think about is that most cars (even Porsches) have too "dirty" unaerodynamic an underside to be of much use for getting much venturi effect. For the most part, air that gets under our cars just gets "stacked" under there and creates lift and drag. So in most applications for production-based racers, the best bet is to have a really low spoiler/splitter up front to keep as much air out from under the car as possible.

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Old 03-17-2008, 01:05 PM
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Yes, its the pressure underneath that produces the pressure for lift, however, if left unmodified, its basically ambient pressure. the car just runs over standing air. The key is diverting the air that normally could run under the car as it moves forward, and making it travel to the top of the car. This is easy to do, because air moves via differential pressure. so, a nice big hole in the hood, allows for air molecules that could have been "run over by the car" at ambient pressure, to be now forced up to the top of the car to meet the low pressure area. (usually in the middle 1/3rd of the hood) via a lower splitter and path to that hood hole. Cars have lift, because they are shapped like a crude air foil (wing) where the air on top has to go faster to go over the car to meet at the rear, and that air that is traveling the shorter distance under the car. (where they meet, they create drag)



Originally Posted by stownsen914 View Post
One other thing to think about is that most cars (even Porsches) have too "dirty" unaerodynamic an underside to be of much use for getting much venturi effect. For the most part, air that gets under our cars just gets "stacked" under there and creates lift and drag. So in most applications for production-based racers, the best bet is to have a really low spoiler/splitter up front to keep as much air out from under the car as possible.

Scott
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Old 03-17-2008, 01:17 PM
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Old 03-17-2008, 02:01 PM
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Mark,

I'll have to dig out my Joseph Katz racecar aero book, but I am quite sure that he addresses how cars with "dirty" undersides (basically anything other than a flat-bottom) cause slowing of air that passes under the car. Granted, the car is moving, not the air, so let's just think of it as movement of air relative to the car. This is compared to, say, air passing over the top of the car, which does not slow all that much over most newer Porsches, anyway, since they are well-designed aerodynamically. So a car travelling at 100 mph would have air flowing over the top at something close to that speed relative to the car, while the air underneath the car would travel at somewhat lower speed relative to the car. Essentially some of the air under the car is getting dragged along for the ride by all the little protrusions under the car. In summary, the slowing of the air under a non-flat-bottomed car can have a tendency to create drag and (I thought) some meaurable lift.

You are certainly correct that most aero lift on a car comes from the shape of the topside of the car. Getting rid of any drag and/or lift caused by dirty airflow under a car is desirable, but arguably would not have as much impact as having good flow over the topside of the car (possibly one of the reasons most manufacturers emphasize topside aero and largely ignore underside aero).

Scott
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Old 03-17-2008, 03:23 PM
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I can help explain some of this. Let's start with the upturned 'splitter'. What would happen is that you would force some air under the car, which is generally a bad thing, and create a high pressure area at the front.

A production car has a dirty aerodynamic underside, so the best thing to do, if you can't clean it up, is to keep air out of there. The airflow will be turbulent, and will stall - creating high pressure (lift) and drag. So the first thing you can do is an air damn to keep air out.

The next step up is to make that air damn a low splitter. That will keep most air out and provide a horizontal surface for the high pressure to act upon, creating some downforce.

Now, if you can clean up the bottom, then airflow can be used to create substantial downforce. You need a venturi shape - either tunnels or a flat bottom with diffuser. The low point needs to be about an inch clearance and you need VERY stiff springs to maintain the ride height so the aero works all the time. When you have ground effects, then you start to see splitters that have raised sections to pull more air under that car to feed the tunnels, but that is pretty much beyond what you can do with a production based car without huge modifications.

Just as a reference, my car wieghts ~800 lbs and at 130mph, in a medium downforce trim, makes ~1400 lbs of net downforce.
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Old 03-17-2008, 03:38 PM
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Would you add a splitter to this front bumper?
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Old 03-17-2008, 05:09 PM
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kurt M
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Just an internet opinion but I would as there is little down side to it. You are already have a blunt faced air dam and pay for it with increased drag. You might as well keep more air from under the car and produce some down force to boot.
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Old 03-17-2008, 09:36 PM
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JackOlsen
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Originally Posted by chrisp View Post
Would you add a splitter to this front bumper?
Yes. You'll see benefits up to about 5 inches.
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Old 03-17-2008, 10:16 PM
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chancecasey
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I think my wife would argue a little more than that.....
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