Cory M

It's hard to generalize on a few pro cars, each with their balance optimized to different tracks and to different rule sets. Look at the article on the Paragon Porsche I posted earlier. Significant gains in front downforce were made over the factory design, that car has through hood venting and still benefited from a simple flat splitter. The 2011 RSR also has a bigger splitter than any other iteration of the 997 to date. The F430 and 997 RSR both have side venting in addition to the through hood ducts. That Ford GT is built to a more open FIA GT1 rulebook. Maybe it doesn't need a big front splitter because better balance is achieved by letting air through to the rear diffuser, maybe the rules limit splitter size, only Matech knows for sure.

We are probably seeing more through hood vented cars because it's a lower drag design when done right (see Katz, Carrol Smith, Racecar Engineering articles). These aren't crude louvers but fully optimized smooth ducts.

Aero is a compromise. Teams are limited by rules, time, resources, testing, etc. It's very hard to generalize...

J richard

Yes

stownsen914

I would disagree here. For a hood vent (or any other vent for that matter), there are a couple basic ways to make it work well:
1. As you have pointed out, it is ideal to have differential pressure between the upper and lower surfaces. Lower pressure on the outside of the vent will obviously encourage air flow in that direction.
2. Even without good differential pressure, a properly designed vent can work if there is high speed air flow over the vent (for example air flow over the side of a car. The flow over the vent creates the necessary negative pressure, localized over the opening of the vent, and air flows out accordingly. Louvers in particular work this way. Naturally differential pressure can help, but isn't necessary to make a vent work.

Scott

mark kibort

So, let me understand what you are saying. Are you saying that the air roling down the front of the cars frontal nose area, is traveling down and then hits the splitter adding downforce, and then of course, adding pressure? Because, as Cory pointed out the obvious, "F=PA". Ive measured the pressure and becauses of compresibility, that number doesnt go up as far as you might think. there is a change of momentium of this air, but the air doesnt get trapped. it finds a equalibrium point where it sheds air underneath and around the car. I guess I always like looking at extreme cases to better understand what ishappening. say you have a 10ft slanted wall faced HOUSE, with a huge splitter up front vs a car with the same size splitter. will the pressure be greater with the house ?

The point is, I agree. some change of momentium applied to the splitter creates force, but it will show up as pressure. you saw my tests of a fairly high nose area (capture area) where the pressure readings were .25psi at 130mph . are you saying that if my nose was taller and gave more of an volume area, that the pressure would be greater? and by how much? Do you agree that the net downforce as measured at the wheels is the difference of what is flowing over or through the car, vs under the car? that is a HUGE factor to consider.

So, if you have worked with restrictive flow examples as I have as well, you do know that as pressure build up, flow increases. sure compared to the incoming flow , the restriction can seem to "block" the air, but it still increases with every increase of pressure. and pressure increases with speed. for example. at 120mph, and only 120mph, the flow through the radiator is SO great that it back drives my cooliing fans to a pretty high speed, fast enough for me to hear them whine! (not at 90 or 100mph, but 120mph) when turned on, becuase they are drivein by DC motors, they spin much faster and current requirements to drive them go down. several proofs in point there.

yes, we all agree, no need to be an "axx". I have already shown the pressure that builds up at all sorts of different points around the splitter. my point is that there are more factors than just the "pressure which builds up" on the splitter face.
your quote here from the bmw marketing site is kindergarden compared to Highschool with talking about the actual concepts of why things work here. This debate or discussion is more akin to discussing how and why airplanes fly. is it change of momentium? is it differnetial pressure? tastes great, less filling!

Its not hard go generalize, but hard to get to the specifics. certainly we have discussed here the myriad of forces acting on the front and rear of the car due to altering the flow to, over , under through , the car. the debate here, i guess is what is effects downforce more and what is the theory behind the splitter. we know they work. But, its a combination of things. in the case of the GT you would probably get more downforce if you let the air at the nose entre the car, with no pressure build up and let it neutralize the vacuum on top of the hood area for where its flowing. you short circuite the lift forces of the flow. if it was better to blunt off the nose and put a splitter, wouldnt they do that instead? nascar has too, because there is no hood venting. other cars like Daytona prototypes , have no splitter , but an inlet that vents to the low pressure zone. The GT40 uses a little of both concepts and ferrari does all 3 of the major techniques. splitter, hood vent, and under body venturi tunnels, with rear diffusers and wings. again, the hood vent turns the front of the car into effectively a upside down wing to provide downforce. without it, the car is a wing with lift, but a front splitter counter acts this lifting force by two ways. taking air that would normally go under the car fed from the frontal nose area, and routes it to the side, through the hood vent, AND puts pressure on the surface itself , greater than what is underneath the splitter to create downforce.

mark kibort

scott,

1. yes, flow moves to differential pressure. the hood vents work because actually, we are venting positive pressure into a negative pressure zone, as the graph i posted shows. its a HUGE incentive for air to fill that void on top of the hood. any venting is purley for flow efficiency and turbulence reduction, not for pressure drop creation to pull air out. that is not its purpose.

2. flow over vents usually creat just the opposite. you get flow INTO the vent, not out of it. NACA vents are not vents, they are inlets. they are designed to create a GREATER pressure on the outside to push air into the car, not pull air out of the car. Someone said if you reverse the direction of the NACA inlet, it becomes a vent and creates a low pressure zone, but I dont think that is true. I dont know, never tested it. I have tested NACA inlets though. they are desined to create flow into the car, and not disturbe the laminar flow around the area of the bondary layer of flow. I want to see an example of a "vent" that can actually lower pressure around it. the only one I know, are the lovers you speak of. yes, they can create additional pressure drop on the back side of the loover. its like a "bump" before the hood vent. creats more exit flow. but the vents we are looking at for the FordGT and other hood vent examples do not incorporate those flow exiting assistance features.

as a side note, i also agree that going bigger and bigger with splitter will add more downforce, but to a limit to the buldge of the pressure zone in front of the car. that is dependent on car frontal nose area shape. at some point, its a diminishing return.

2BWise

I really don't want to get involved in this conversation. Just a quick question/observation. On the DPs, which are already wedged shaped in the front and provides lower drag. Why would they want to vent thru the hood? The air has to make a more drastic change of direction than flowing strictly over the front, increasing drag. In this case aren't the hood vents strictly there to promote flow out of the front mounted radiator?

J richard

Mark,

Yes, the more frontal area that can be directed into a high pressure (to a point you don't want to be pushing a house as you said) will impart downward pressure on the splitter due to 1. the ram effect, 2. the relative lower pressure you are creating under the car. The pics of the NASCAR or Andersons 928 are indicative of this.

It's hard to discuss the results of your tests, in reality there are so many variables, like sensor placement relative to openings and that once the splitter begins to deflect it spills air which imediately reduces it's capacity to develop a higher pressure, it then is acting more like an air dam rather than a splitter. You really want them so stiff they have no deflection at all and to be a true splitter you want them flat to create the planform for the pressure to act on. If they are sloped downward the airflow will be at an incident angle and not develop the force vector you want from the airflow.

And as has been said many times in this thread all cars are different, some are easier to develop that high pressure area, some like the prototypes have the profile that can utilize the downward sloping frontal area for easy downforce in which case the splitter is doing more of just controlling airflow above and below so it doesn't need to be as large. Then there are mixed bags like the Ford GT's that are blending intakes, splitters and air dams, which is way to complex to parse out in simple terms or have any clarity as to the contribution of individual elements.

Was the OP for this thread the most efficient wings? got a little off topic....

winders

Mark,

It is this type of comment that endears you to everyone so much. This is what has to stop and why you ruin every thread you participate in. People won't post because of your crap. You ruin the experience for many other people and add little value while you **** on every one else.

The quote may have come from a marketing site but it is from the head of BMW M aerodynamics development. You were discounting the pressure built up on top of the splitter. That is wrong to do. You completely discount information from others that suggest there is a lot of pressure on top of splitters because you have not measured it on your car. Your car is not every car. It's a frickin' 928 for crying out loud....

Scott

mark kibort

That is a great question. the reason is that the air, even though now more laminar, with little stagnation at the nose area, will speed up vs the the air traveling under the car. this is why they have to be so low to the ground. they then have an opening, for air flow that circumvents the rise up the nose. remember, first part of the nose will be high pressure and second part will be low pressure . with a wedge shape, the high pressure is very short, so you DONT want air to travel over the car and start to speed up, thats low pressure. so, it goes in the inlet, and out the vents, where the end of the low pressure zone is, flinging this air to the sides to avoid the abrupt cocpit windshield and roof. this creates downforce, and provides the air to the outer portions of the rear wing as well.
To your last point, no. even 911s with no front radiator , have hood vents that direct flow from nose to hood low pressure zones for downforce.

stownsen914

It depends on the shape of the vent/inlet. NACA ducts (facing in the normal NACA duct direction for use as an air inlet) work by skimming the boundary layer of air passing over the duct. If you turn a NACA duct around, you obviously get something completely different -- you would not be able to count on it being an inlet. I don't know if they make good vents though ...

A better example of how this works for a vent is with louvers. The lip in a set of louvers "trips" air that passes over, creating a small low pressure area just aft of the lip, right where the next opening in the louvers is. This encourages flow from under the louvers to fill in the low pressure area, and creating outward flow from the louvers.

Air can still flow the "wrong" way through the louvers under certain conditions. I guess the key is to place the louvers (or any vent for that matter) properly, in order for it to work well.

mark kibort

Scott, You seemed to forget your TOTALLY RUDE comment of "STOP"! I was responding to that. stop the rudeness yourself. we are having a discussion here. the comment i made was in response to your commen which was an attempt to prove you are right? im not sure. My point was , yes, we know the basics of downforce due to splitters. The bmw aero guy, im sure is brillant. he was giving a very general discription of how splitters work. We are talking about the more esoteric reasons they work. lots of other factors thats all. read your post again and then read mine.
to your point, yes, i have measured the pressure on the 928, it is a car with a nose and a huge splitter. it is a data point. based on its physical qualities, it is relevant. my points are based on the potential of pressure on the splitter itself. if you say 1000lbs for example, that is near 2-4psi for most splitters. this is not possible, so, im saying that the downforce could be due to other factors, that being, air is not going under the car now, and going around the car and/or through the hood vents. the scales dont tell you where the downforce is coming from, the pressure sensors do.
Thanks,
Mark

mark kibort

sure, as i mentioned , if you have a "bump" at the front of an inlet, it disturbs the flow to create a low pressure zone at the rear of it. it can then be responsible to create exit flow. as far as I know, i dont think just turning a NACA duct (used for positive pressure inlet) around, and then it is a negative pressure vent. I havent experiemented with that. I can easily test that with my windo naca duct. I have a feeling air will be coming in just the same, but thats a guess.


Now we are getting somewhere. the ram pressure. you know it has a max potential at any given speed based on a set of variables. because we are talkking compressible, flow, things are not as intuitive as they may think.

In our years of studying this stuff in the aero books, you can go back and see specific examples of why the house and car with similar sized splitters might have the same downforce surface pressures. hey, if you are looking at Andersons car, you can look at mine too. it won't hurt! mine actually has a better splitter, in shape, but not in height to the ground, and that is important too. mine doesnt release the air to the sides until its past the body work, anderson does release it, just in time to hit the sides of the chassis and tires. width is important .

now, to your second point, splitters always spill air . my tests were conducted and measured pressure with out any bending ofthe splitter. it always remained horizontal (besides the one that failed without support) So, as another note. i found it interesting (and you can see the black sensor tube at the center of the splitter) that all the pressure readings were the same, all the way to the face of the radiator. (very close) it was the same, all the way to the leading edge, and vs the point where it attached to the body. this proves that the pressure curves we all see for cars are true (imagine that) there is a buldge of pressure leading the way and it is fairly uniform.
my results are .25psi at and around the splitter which is not deflecting. my splitter is 50" by 6.5", so its about 70lbs of downforce. but, to my main point, the net effects are quite abit more. for example. I could have that same value and not route air to the hood vent, and release air to the under carage at the sides, like anderson and reduce the NET downforce substantially..

interesting you mention air dam. when the splitter failed downward, i did an experiment to see what happened behind it and under the car. it did reduce pressures under the car slightly , where as before it was ambient. (I measured in 1 ft increments backward behind the splitter to about mid engine.)

mark kibort

I was talking about the are above the splitter, not the splitter itself. sure , you get more downforce with a larger surface area, up until you get out of the high pressure area that is buldging out in front of the car. again, the force on the splitter itself is one factor for downforce. when I extended my splitter from 3" to 6.5" , the pressure didnt change, but the surface area sure did, and that certainly changed direct downforce. however, there is an even greater effect of the that air that was splilling off the splitter going under the car, now, going to the hood vent and around the sides.
mk

mark kibort

cool video of air flow in the windtunnel over a GT40
http://www.youtube.com/watch?v=3EjFkPL_H_4

onefastviking

Look closely in every one of those air cooled, no radiator vented cars Mark and I bet you will find an oil cooler in every one.