Under floor flow & diffusers

[henra]

So basically, while I'm sure they do accelerate the air under the floor through a variety of techniques, I don't think your logic dictates that they must, simply because your premise is flawed.

Ouch!
You shouldn't try to set @Hollus straight at things where he clearly has knowledge about.
Moreover the principle behind a diffuser is really basic fluid Dynamics.
The underbody of an F1 car basically (and ideally, that is what ebd and directing of Y250 vortex to the outside of the sidepods tried to achieve)) is a closed System, btw, albeit not an ideal one since it has significant spillage effects at the sides.

[hollus]

Peace all around, Moose was just discussing the issue as fitting in a forum. And my knowledge comes mostly from F1T so there might still be lots to learn. My point, in case it wasn't clear (it wasn't), is that the diffuser should not be seen as a device to do anything to the air as much as as a device to bring air back to what it was, as back to normal. Well, the ideal diffuser in the infinitely efficient car would be back to normal anyways. That is, seen it in isolation, which is the wrong way to look at it, IMO.

[CBeck113]

The air is accelerated by displacement - remember the air is otherwise stationary, and the car is moving (but when you calculate a diffusor or use a wind tunnel it's the other way around). The diffusor is there, as hollus stated, to try and bring the pressure back to its original state, after the reduction due to the displacement. The key to the whole process is that the increase has to occur as close to the back of the car as possible to insure that the downforce, caused by the low pressure under the car, remains as long as possible. Theoretically, you want the pressure to go from the minimum to ambient (or even higher, but that isn't possible without a thruster) to occur at the exit edge of the diffusor.

[variante]

My point, in case it wasn't clear (it wasn't), is that the diffuser should not be seen as a device to do anything to the air as much as as a device to bring air back to what it was, as back to normal.

Which is weird from you, since i remember you giving a completely different explanation some time ago (was that actually you anyways?) that i believe was the correct one.
That was more or less: the diffuser plays an active role in the creation of downforce, forcing the incoming fluid to expand and thus creating partial vacuum. The partial vacuum creates both a low pressure area underneath the diffuser and an extraction effect underneath the floor.

If the floor has no rake, no diffuser attached and sharp leading edge, it will create no low pressure nor pressure differential.
If that very same floor has a diffuser attached, we will observe low pressure undeneath the entire floor, the diffuser and (especially) at the diffuser's throat.

edit: the conclusion is that the increase of pressure given by the increase of section of the diffuser is just an inevitable consequence of its primary function, not the opposite.

[mrluke]

The diffuser does not make a low pressure area. Just before the mouth of the diffuser is the lowest pressure area, the diffuser then slows the air down, increasing air pressure back to ambient as it exits the diffuser.

The diffuser reduces the turbulance caused by the high velocity air rejoining ambient after the car, the more this turbulance is reduced, the better the flow upstream, under the car, thereby working the floor harder.

[hollus]

I might have said many things as I claim that for all of this there is a Berniuli explanation, a Newton explanation and a gazillion billiar balls (ideal gas) explanation, and that they are all one and the same. In any case here I was referring just to velocity, there is no expansion in bernoulli's world.
I honestly think we are all more or less agreeing and getting muddled in different languages and in cause and effect, which is the way that language works but not the way fluids behave below mach1. I think that cause and effect are one and the same, and that as you well say, the diffuser would be useless without a matching underfloor, just as the flat underfloor is useless without the diffuser.

If we are anyways discussing rake... Does anyone have any idea of the contribution to downforce of the upper surface of the floor (when raked)?

P.S. Didn't we have a good discussion, somewhere, CFD included, of raked underfloors?

[chuckdanny]

Oh i have another funny explanation about the rake and how it improves the air flow under the car.
Imagine the air as a ball mounted on a spring vertically. As the front of the floor get in contact with the top of the ball it pushes it down and a little sideways, the spring flexes a little, then after this compression without loss the reacting force of the spring pushes it back to the surface beneath the floor. If this surface is not inclined then its reaction as no component in the direction of the floor but because it is inclined forward it pushes back also toward the rear of the car (no friction). Remember the spring did not only acquire potential energy in being pushed downward but also sideward so both the flexion momentum and reaction of the inclined floor push the air backward.
If the floor was horizontal then a gap would form only in the diffuser area because of the geometry of the displacement parallele to the floor but because it is inclined another triangular gap is forming (as seen from side) that connect the inlet to the outlet gap with backward momentum everywhere along the gap because of this inclination that create a backward component of the reaction force of the floor to the air molecules and because air has a per molecule momentum about 502 km/h at ambient temperature (it's a thermomentum) which develop in the gap.

That's amazing !

[chuckdanny]

Remember, air can only be pushed not pulled, no push/pull dilemna here
Don't tell me the spring has a pull reaction

[variante]

the diffuser would be useless without a matching underfloor

I didn't say that though: only the opposite. As a diffuser without a floor is simply called "wing".
That's why I'm very skeptical of all those who consider the diffuser only as a mean to bring back pressure to normal. It's not!
The diffuser is the main responsible for downforce! There's a low pressure paradise under there, lower than most of the floor. If engineers had the chance, they would get rid of the underfloor in favour of a giant diffuser (=wing cars, raked cars,...)

[timbo]

the diffuser would be useless without a matching underfloor

I didn't say that though: only the opposite. As a diffuser without a floor is simply called "wing".
That's why I'm very skeptical of all those who consider the diffuser only as a mean to bring back pressure to normal. It's not!
The diffuser is the main responsible for downforce! There's a low pressure paradise under there, lower than most of the floor. If engineers had the chance, they would get rid of the underfloor in favour of a giant diffuser (=wing cars, raked cars,...)

You're talking a different thing. Diffuser have clear definition. Diffuser without the floor would itself be floor .
If you look at pressure distribution on this picture, you see that the lowest pressures are at the zone which is closest to the ground:

Now, I am not saying diffuser does not provide downforce. However, most of its effect is in enhancing the flow upstream of it.

PS, which is why it is possible to have to big a diffuser -- if the feeding flow is inadequate.

[variante]

I agree, timbo. I was playing with words to underline something else. Definitions apart, i found it necessary to underline the primary function of the diffuser over its consequences. Too many people here have inverted the cause-consequence relationship. Too many people here keep saying something like "diffuser's function is to slow air down" "...to bring back pressure to atmosferic". Wrong. These are consequences; and they are just a fraction of the entire story.

[chuckdanny]

Right, diffuser like his name suggest is there to diffuse the air that was made coherent and narrower, concentrated to make it's task. It brings back the air into it's original state quicker hence improve efficiency and it's an enabler a catalyser of the speeding reaction underneath the floor.
But the narrowing of part the entry section that are submited to acceleration imply that the other part lose contact with there ancient neighbours so that because at those mach number the fluid is still behaving in an incompressible way you must find other neighbours along the way to prevent them from coming from the back to the front in a reverse flow.
That's the mixing challenge of the diffuser to chose the right neighbours to keep things flowing. And in that process i suspect the vortices to constently provide those "good" neighbours...

Remember, the physics of fluid does apply to the "physics" of crowds.

[AlainProst]

Diffuser creates an aspiration of the car to the floor. It's the same principle of a wing but it isn't the same way to do it. Indeed, the principle is create a difference of pressure between the top and the bottom of the diffuser. It's same than a wing but the low pressure artificially increased by the diffuser because the air can't escape through the sides because the diffuser it prevents it. Ingeniors try also to increase the high pressure on the top but it's currently a big challenge with nose down who prevent air to go to diffuser.

[GPR-A duplicate2]

Mods, can you please extract all the diffuser related discussion and move into a new thread please?

[tok-tokkie]

You're talking a different thing. Diffuser have clear definition. Diffuser without the floor would itself be floor .
If you look at pressure distribution on this picture, you see that the lowest pressures are at the zone which is closest to the ground:
http://www.mh-aerotools.de/airfoils/images/wig_5.jpg
Now, I am not saying diffuser does not provide downforce. However, most of its effect is in enhancing the flow upstream of it.

PS, which is why it is possible to have to big a diffuser -- if the feeding flow is inadequate.

That 'inverted' aerofoil is not behaving the same as a F1 diffuser. Look at the streamlines ahead of the aerofoil - it is drawing air from way above the leading edge into the venturi gap that the aerofoil makes with the ground. In a F1 diffuser that air is not available so it does not behave as shown by this diagram.