How does air from above the car get into the diffuser?

[Tozza Mazza]

Due to the rules changes for the 2011 season, how is it possible to get more air into the diffuser, to generate more downforce, I know that there is the starter motor hole, with an area of 3500mm squared, but how does air from above the car get through the hole into the diffuser itself? I also know that the end 50mm of the diffuser can be opened, but most teams seem to be starting with a narrow diffuser which widens to the full metre allowed, not exploiting this rule (bar Newey and RBR). If anyone could provide some clarity on this, it would be much appreciated!
TM

[fastback33]

It doesn't necessarily....Air is all around the car, generally air is moving slower over the top of the car then it is at the bottom. My understanding is that the diffuser doesn't actually generate downforce, but merely uses the pressure on the top of the car to generate all of the down load by increasing the airs velocity under the car.

[marcush.]

Interestingly the route over the top is longer than taking the way straight under the car .wouldn´t the air need to flow quicker over the top creating lift ? I´d think one of the concepts of the high nose is to direct more air under the car and elongating the path of the air going that way and at the same time shortening the path for the airflow moving over the top to reduce lift from the main hull.

[mike_dangerous]

Interestingly the route over the top is longer than taking the way straight under the car .wouldn´t the air need to flow quicker over the top creating lift ? I´d think one of the concepts of the high nose is to direct more air under the car and elongating the path of the air going that way and at the same time shortening the path for the airflow moving over the top to reduce lift from the main hull.

The length of the path has nothing to do with generating lift as the whole "packet" theory is wrong and definitely doesn't apply to a diffuser. The flow through the underbody and diffuser is very rotational and in close proximity to the ground. This rotation must be a lower pressure than if the flow were irrotational due to energy conservation. By channeling more air under the car, the area under the car through which the air must flow will not change therefore the volume flux (rate at which air crosses a control surface) will have to increase which causes a lower pressure. Nothing to do with path length under the car...

With regards to getting more air through the diffuser, the McLaren approach of increasing air flow over the rear beam wing will generate a lower pressure on the beam wing. This lower pressure is at the exit of the diffuser so by lowering the pressure here the air flow through the diffuser can be increased as the the air in the diffuser doesn't need to expand as much the expansion causing the air to slow...

[PlatinumZealot]

Due to the rules changes for the 2011 season, how is it possible to get more air into the diffuser, to generate more downforce, I know that there is the starter motor hole, with an area of 3500mm squared, but how does air from above the car get through the hole into the diffuser itself? I also know that the end 50mm of the diffuser can be opened, but most teams seem to be starting with a narrow diffuser which widens to the full metre allowed, not exploiting this rule (bar Newey and RBR). If anyone could provide some clarity on this, it would be much appreciated!
TM

It seems you have some misconceptions about how the floor of the car works, but that's very common.

Your first instinct is correct, the air at the top of the car cannot go into the throat of the diffuser.

Adding more air underneath the car generally does not increase the down-force. More air can be bad, because more air in the same space is more pressure under the car. The target is to increase the speed of the air under the car. Some time this may mean adding more air flow to certain spaces though. But in general the teams try to prevent exessive air from going under the car, that is why the cars don't have the front higher than the back, the car would just lift off like a kite.
Same explanation to why the car has front splitters. The front splitter stops excessive air from getting under the car, and to increase the speed of the air which does go under it. The diffusers job is make it easier for the air that is under the car to escape.

In simple terms, the more easier it is for the air to escape from under the car the faster it goes. That's the main objective, fast flow, not neccessarily high flow.

[mike_dangerous]

You seem to have a few misconceptions about the basic laws of mass conservation increasing the mass flow under the car will increase the velocity of the air as the effective area is still the same assuming the same ride height and compressible effects can be ignored.

[speedsense]

You seem to have a few misconceptions about the basic laws of mass conservation increasing the mass flow under the car will increase the velocity of the air as the effective area is still the same assuming the same ride height and compressible effects can be ignored.

Does mass conservation apply to the bottom of a race car? I ask because, the bottom is not enclosed and is in constant "flux" with regards to pitch, heave, warp, and roll. And what about intervention of side and frontal turbulences entering the enclosed space?

[mike_dangerous]

Mass Conservation will always apply. It isn't actually applied on an enclosed space, but the technique for using mass conservation in your analysis is to create a "control volume" where mass in = mass out + mass stored and calculate the flux accross the "control surfaces". This could be done in a single instant of time so pitch and roll etc. simply change the "control volume". Turbulence is obviously more difficult and is really an experimental science but I believe the rotationality of the underbody flow would reduce the effects of turbulence ala a delta wing. A rotational flow is NOT always a turbulent flow...

[speedsense]

Mass Conservation will always apply. It isn't actually applied on an enclosed space, but the technique for using mass conservation in your analysis is to create a "control volume" where mass in = mass out + mass stored and calculate the flux accross the "control surfaces". This could be done in a single instant of time so pitch and roll etc. simply change the "control volume". Turbulence is obviously more difficult and is really an experimental science but I believe the rotationality of the underbody flow would reduce the effects of turbulence ala a delta wing. A rotational flow is NOT always a turbulent flow...

Consider the front wing wake, wheel "air pumping" (in and out), and turning tires producing a downstream chaos of turbulence, that can't be neglected.....the very reasons why they have sidepods scooped out, barge boards, FEE, EBD, small diffs (williams).
The change of volume occurs in a frequent 3-5 hz cycle in MSeconds, at all times in movement. With literal chaos of air flow happening in front of it....

Let alone the ground underneath, at some tracks changing every hundred feet.

V

Sorry, but it ain't that simple.....

[mike_dangerous]

I didn't say it's that simple but it's all about making the right idealizations such that the theory can be applied and the general operation of the underbody can be understood. When taking into account the finer details of performance of such a component, yes these things need to be taken into account but this can only really be done through meticulous analysis of experimental data. This is not, however the level of discussion for that as it is very component specific but from my experience the application of the theory to a simplified case results in a good understanding of the real world case even though it does not supply the correct numerical results.

[Tozza Mazza]

whoa, this is very useful.
what you guys are saying basically shows why the DDD left the single diffuser teams in there wake in '09, its all about pressure, not sheer volume! and EBD is to do with accelerating the air in the diffuser, rather than increasing pressure (a side effect).
So, if i'm getting the gist of what you're saying correctly, the high pressure in the top level of the DDD pushed the single deck down further, creating more downforce, as well as having the air above the car also pushing the car down, generating much more rear downforce than teams running single diffusers. correct me if i'm still wrong

[mike_dangerous]

The volume flow rate and rationality of the flow affect how low the pressure is under the car. The lowest pressure is that in the undertray the pressure actually increases in the diffuser as the air expands. The ebd increases momentum of the flow giving a higher volume flow rate.