Airflow/Aerodynamics

[Rook]

Here's something that's never made sense to me.

 

Let's look at airfoil design. The basis of which is Bernoulli's principle. An airfoil's convexed section (typically the upper half on an aircraft wing) creates a higher speed of airflow and thusly lower pressure. The lower, straighter half of the airfoil is subject to lower speed/higher pressure air. Higher pressure below the airfoil and lower pressure above creates lift. Which is to say that you need faster moving air in the direction you want lift...or something like that.

 

Now on a car, downforce is somehow created by inducing a higher amount of airflow over the TOP of the vehicle, and minimal airflow beneath. Where I'm confused is that this is essencially how a wing functions.....so how does this NEGATE lift?

[white_2kgt]

Airplane wing for LIFT,

 

 

Car wing for DOWNFORCE,

 

The 'airplane' wing is upside down to create downforce. Both use the same principals, just opposite reactions. Is that what you are asking? There are some VERY good books out there about racecar aero which will contian more information that you will ever find here.

[Rook]

Not exactly where I was going. Let's take a look at your first image.

 

 

Imagine the above is a give performance car. An R&S Mk3, Mosler MT900, Factory Five GTM or something else hoplessly out of my financial reach. The body structures of these afore mentioned supercars take on a cross-section quite similar to an airfoil. See the path of the upper airflow? Since it has a further distance to travel in the same given time as the airflow beneath the object, it travels faster, and the pressure on top is lower. NOW, on an airfoil, as above, this creates lift. If this is so, with all things being equal...why is the same effect not had on a car? I.e. Less airflow beneath the object creating downforce instead of lift. I'm referring to aerodynamic properties of vehicular shape, here, not just wings added for downforce.

 

What I'm getting at is that, though it has been proven, the idea of less air beneath a car creating downforce just doesn't make sense.

 

Man I'm gonna get a headache from all this thinking.

[DriverGT5]

It's not as simple as "less air under a car". There are actually two ways to get dowforce from the bottom of a car. First, you have to understand that the ground changes things a bit. Don't think of the bottom of the car as the bottom of a wing (or top since you flip it over to get downforce), think of the bottom of the car as more a tube or tunnel.

 

Now, there are two ways you get downforce. The goal is to create a lower pressure under the car than on the top. Given the shape of the car, that's tricky. You do this one of two ways, speeding up the air under the car, or creating a vaccum under the car. Either way will reduce the pressure and lead to downforce.

 

Protype cars, Formula cars, etc, use the accelerate the air methode. If you see a car with defusers on the rear, they are trying to decelerate the air coming out of the car. By scooping a lot of air, then choking the flow against the ground, you speed up the air, lower the pressure, and create downforce. Most race cars do this to get front downforce, some really well built cars have tunnels along the underside to create huge amounts of downforce. There are some pictures of this in the link.

 

http://e30m3performance.com/myths/splitter/venturi.htm

 

The other way to create downforce is to run a very low nose. You see this in NASCAR. These airdams do two things, one, prevent the car from scooping too much air and lifting off the ground, and two, it creates a low presssure zone behind the airdam. The confusing part is two different ways to get to the same result, more air = low pressure, less air = low pressure.

 

Personally, I think it's to hard for the majority of us to set up a nice ventury design, so the best solution to me is a simple splitter. It seems the easiest and most effective way to get downforce.

 

I hope that made sense and that I understand the concepts enough.

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[DriverGT5]

I may have gone into detail you didn't want. The short answer to your question is less air under the car creates suction against the ground. A wing in flight has noting to suck against. My guess would be if you flew a wing low enough to the ground that it interupted the air flow on the bottom such that it was redirected over the top, you'd have a similar situation where the wing would suck itself down. Basically turning the high pressure underside into a lower pressure zone, reversing the effects of the wing.

[Rook]

That, sir, was EXACTLY the answer I was looking for! (*smacks forehead* Duh! Forgot about the ground! lol)

 

Not only that but you've provided me with a most interesting website!

 

Thanks alot!

[DriverGT5]

One of the great things about building my monstrosity is that there are no easy answers, so I have to research and find EVERYTHING. The up side to that is I have a nice collection of informative websites.

 

How to build a splitter.

http://e30m3performance.com/installs/installs-1/splitter/split_1.htm

 

Huge regestry of many race cars, some with wind tunnel data.

http://www.mulsannescorner.com/

 

And here's my favorit find of all!!! A Java based air foil simulator that allows you to build a virtual wing and see how much downforce it puts out. Neat to see what matters and what doesn't. I was surprised how little the wing profile matters and how much the angle of attack matters in comparison.

http://www.lerc.nasa.gov/WWW/K-12/airplane/foil2.html

[Rook]

Sweet! An excuse to sit lazily in my offtime on the internet!

 

V8 RX-7....Miata IRS......tell me that's not a handfull to keep hooked up?

[DriverGT5]

I'll let you know after CMP this weekend. So far all my driving has been on 4 year old slicks in 35 degree weather. It spins up when you roll on the throttle in 4th gear...

 

Don't even ask about the time I drove it on damp roads.

[Rook]

Oh hey you're in SC, eh? I'm gonna have to run into you when I get back stateside and see that thing. You'll see by my car that I have a taste for the "off the wall"

[DriverGT5]

Oh hey you're in SC, eh? I'm gonna have to run into you when I get back stateside and see that thing. You'll see by my car that I have a taste for the "off the wall"

 

What do you drive? I'll be in the area, heck, I might even be living in Charlotte in 5 months, never know. One thing is pretty sure, if I'm still around, I'll be at all the NASA-SE events. Let me know when you get back, I'd love to talk cars.

[Rook]

I'm driving a 2000 V6 Mustang with 04 Cobra IRS amongst other bits. Weighs in just over 3000lbs and running a best of 15.2 in the 1/4. Though I think I can hit 14s powershifting. In a couple weeks I'm having the full-on Max Motorsports grip witchcraft done. Tubular K Member and all.

[DriverGT5]

Sounds like you're building a pretty sweet car. You going to do some HPDE's this year? I hear that K-member install sucks BTW.

[Rook]

I'm hoping...VERY strongly hoping to catch the April 14-15 HPDE at Rockingham, but it depends on what I end up doing when I get back (long story, but I'm kinda voluntarily deployed right now)

 

I'm sure the K-member install will suck, but I decided to treat myself and have it all installed in my absense. Ahh, the luxury of not being around for the downtime and "crap...need another part..." Only downside (besides the rediculous labor cost) being that I'll have to re-learn the car when I get back, as I'm sure the combination of bushings (urathane and aluminum), full length subframes, coilovers, tubular everything, and castor/camber plates will change things up like day and night.

[white_2kgt]

I'm sure the K-member install will suck, but I decided to treat myself and have it all installed in my absense. Ahh, the luxury of not being around for the downtime and "crap...need another part..." Only downside (besides the rediculous labor cost) being that I'll have to re-learn the car when I get back, as I'm sure the combination of bushings (urathane and aluminum), full length subframes, coilovers, tubular everything, and castor/camber plates will change things up like day and night.

 

If you can, get it on a lift. You just need to support the engine, then drop the 4 bolts and transfer everything to a work bench. From there bolt up the new k-member and get the bolts started, leave about 1/2" gap so you can 'wiggle' it into place and square it up. Do that first then put on the A-arms and spindles, etc. I've got a few pics on my site when I put mine on,

 

http://www.2kgt.com/item.php?itemid=119

 

I actually had the engine out already when I swapped.

[XAIX]

Not quite right. I believe the air flow starts to accelerate as it enters the area of the rear diffuser where the horizontal surface starts to transition/angle up from the road surface creating the most downforce in that general area. You can move the point of highest downforce front to rear of the car by moving the transition point front to rear.

[DriverGT5]

Not quite right. I believe the air flow starts to accelerate as it enters the area of the rear diffuser where the horizontal surface starts to transition/angle up from the road surface creating the most downforce in that general area. You can move the point of highest downforce front to rear of the car by moving the transition point front to rear.

 

Not saying I disagree with you, but how does a diffuser accelerate the air? I think of it as a river. I've never seen a river get 5 times wider AND faster at the same time. Usually when you open up a huge volume, what ever is flowing through it slows down. Of course, that isn't to say that a diffuser can create downforce, I just don't under stand how. Can any one else shed light on that particular topic?

[Rook]

As I understand it..and I think I do...I think....a diffuser creates downforce, or more accurately suction, by venting out air underneath the given vehicle, thereby increasing it's speed and creating a resultant lower pressure area underneath.

 

Or something.

[Gfaules]

Diffusers work in conjunction with other aerodynamic components of a car to help produce downforce. Because a car displaces air as it moves forward, the air molecules in the rear of the car get disrupted causing a vacuum of low pressure air.

 

To help channel the high speed air flow coming from underneath the car, diffusers redirects the high speed air flow to transition smoothly transition into the low pressure vacuum in the rear. This encourages a smoother flowing air flow underneath the car and allows better performance from other aerodynamic components such as aerodynamic wings.

[XXAIX]

DriverGT5

 

The air is initially accelerated as it reaches the transition where the diffuser’s horizontal surface starts to curve upwards. The air expands and accelerates. Eventually it does slow down. It is why we had to heat the downstream side of a natural gas regulator we designed for automobile applications. Under wide open conditions the gas would accelerate sufficiently to remove the surrounding heat as it exited the metering orifice and entered the larger diameter downstream tubing. Remember that energy is required to accelerate a body/gas molecule and heat is energy.

 

The air accelerates as it moves over the top side of an airplane wing in the same way it does with the diffusers horizontal surface which is nothing more than an up-side-down wing.

[DriverGT5]

DriverGT5

 

The air is initially accelerated as it reaches the transition where the diffuser’s horizontal surface starts to curve upwards. The air expands and accelerates. Eventually it does slow down. It is why we had to heat the downstream side of a natural gas regulator we designed for automobile applications. Under wide open conditions the gas would accelerate sufficiently to remove the surrounding heat as it exited the metering orifice and entered the larger diameter downstream tubing. Remember that energy is required to accelerate a body/gas molecule and heat is energy.

 

The air accelerates as it moves over the top side of an airplane wing in the same way it does with the diffusers horizontal surface which is nothing more than an up-side-down wing.

 

Are you sure you really understand what is going on in this case? I know that compressing air heats it, but I've never heard of accelerating air as a source of heat. As a matter of fact, how do you accelerate air without changing the pressure or volume. I know the Ideal Gas law has no variable for velocity. I suspect the reason you need to heat your regulator (given I have no idea what it is or how it is used) is because the gas is expanding thus stripping heat according to the Ideal Gas Law.

 

For the air to be accelerating at the point you describe (given it's been 6 years since physics), you either need reduction in volume, or an increase in temperature (from an outside source of energy). What I still suspect is occurring is the air starts to expand at the point you describe given the increase in volume as the diffuser starts to expand.

[Gfaules]

As I understand it..and I think I do...I think....a diffuser creates downforce, or more accurately suction,

 

 

Exactly!

 

Aerodynamic downforce genereated by diffusers or "ground effects" works in a different way. As with a wing, the air going under the car is displaced downwards, but beyond that, things are different. The underside of the car is not flat, it's shaped like an inverted airfoil, creating what is essentially a venturi tube.

 

The air flowing through the gap between the bodywork and the ground is accellerated considerably to at least half again vehicle speed. Bernouli's principle says that the faster a fluid like air flows, the less pressure it exerts. There's a fair ammount of downforce by itself.

 

Then the underside sweeps up in a curve, allowing the compressed, accellerated air to expand in an enclosed space in a much larger volume than it originally had. This creates a further vacuum and more downforce.

 

The closeness of the bottom of the car to the road and the accelleration of airflow are why race car wind tunnels have to take the road surface into consideration.

 

When a fluid and a solid object move past each other (all motion is relative), the fluid does not move at a uniform speed. Owing to skin friction, among other things, a "boundary layer" of slow-moving fluid builds up against the skin, while fluid farther away moves unabated. This is why the water in the middle of a river can be boiling along in white water rapids while the water at the river's edge is relatively still.

 

The same thing happens under the car. The skin friction of air moving over pavement (which is pretty rough stuff) builds up a boundary layer a few inches thick above the pavement.

 

This doesn't affect an airplane wing, since it is several feet up in the air and trying to make lift. It does affect air flowing under the car, since the pavement's roughness slows the airflow down, it affects how fast it flows throught he tunnels, and thus the downforce. The skin friction of air flowing through even a relatively smooth wind tunnel is enough to affect the boundary layer flow through the model's tunnels and thus the downforce it creates. The moving ground plane in the wind tunnel takes the roughness and relative motion of the road into account and gives more accurate numbers.

[DriverGT5]

Good stuff. That's the best example I've read yet.

 

Just to take it a step further, many pro cars have venturi tunnels built into the underbody. That's why if you watched the Runoffs this year they made such a big deal about the "flat bottom" DSR's being so uncompetitive. The newer cars have some nice tunnels that further increase the downforce (suction) generated under the car.

 

Also, from what I've read in various engineering articles online, undertray modifications don't really lead to a huge benifit unless you can get creative. In otherwords, beyond the means of 99.5% of us. Things like splitters and wings are the best way for us to make use of downforce.

[Gfaules]

 

Also, from what I've read in various engineering articles online, undertray modifications don't really lead to a huge benefit unless you can get creative. In otherwords, beyond the means of 99.5% of us. Things like splitters and wings are the best way for us to make use of downforce.

 

That's absolutely correct. When team engineers develop and test under tray mods and under car tunnels, which by the way are the single biggest required factor to truly achieve good suction, they spend a whole bunch of Franklins testing and perfecting them in a wind tunnel. From what I have read, without a wind tunnel, 95 percent of the development is wasted. Based on those facts, I don't believe the average racer can achieve the extream results we want on time spent on under car mods. However, learning how to make wings and splitters work well is positively a plus.

 

Keep in mind the Nascar Car of Tomorrow is now using splitters and they are for the first time learning the exact same science we are discussing and they are scratching their heads as much as we are. It will be interesting to see what they learn and how fast they come to terms with it. Nascar has already passed it's first splitter rule; that being, if a car looses a splitter during a race, the team will not be required to come in and install a new one. However, they will suffer a large amount of handling. Rear wings on the other hand must be reinstalled if they come off. Some may not like Nascar racing but there's a whole lot of money going to be spent in a short period of time and we could all learn a lot from them at their expense.

[XXAIX]

DriverGT5

 

I never said that accelerating air or any other gas was a source of heat. Quite the contrary. I think if you read my post I said we needed to heat the regulator because as you correctly stated the gas was accelerating after it passed through a restrictive orifice and expanded into a larger diameter tubing. The down-stream side of the regulator would actually ice up under full throttle conditions without the heater. As there is no such thing as a free ride when something accelerates, as you stated the gas was “strippingâ€