gcdugas wrote:
The exhaust never filled the diffuser.
In this pic you can see the diffuser of the Ferrari F1/87 (V6 turbo) and of the F1/89 (V12 N/A) :
[IMG:150:128]http://img56.imageshack.us/img56/4821/d ... lt6.th.jpg[/img]
To have the exhausts to blow inside the diffuser was a quite usual solution from end of ground effect era till roughly 1995-1996. Then the reduction in length of the lateral channel itself (that since mid 1994 had to stop at rear wheels axle) and stepped bottom rule forced teams to completely re-think the design of rear end and that solution became less popular, still there were cars having exhausts in the diffuser (sometimes in the central channel) till 2000-2001.
gcdugas wrote:
Au contraire. Anything that fills the diffuser would only detract from the low pressure zone and coresponding downforce.
Let’s see the theory of how a diffuser works then.
Most important law of fluid dynamics is mass conservation, meaning density * velocity * area = constant. The flow is incompressible so density is constant and we can simplify it to velocity * area = constant.
This applies at the throat and at the outlet, at the throat (and upstream it) the area is small consequently velocity will be high, at the outlet the area is large hence velocity is low. (then obviously where velocity is low pressure is high and viceversa so we have the wanted low pressure area under the car, upstream the throat)
Second important thing often forgotten (or ignored) is that the car underfloor is exit driven, meaning that it’s the condition at the outlet that controls the mass flow passing under the car, it’s not a matter of how much air I can push from the front, but of how much I can suck from behind (unless the car reaches supersonic speed, but we can safely assume that’s not going to happen in the next couple of years... there’s the engine freeze
).
Consequently it doesn’t matter if at the front there’s a “mouth” as big as Texas (FW26 anyone ?) under the car it will go only the mass of air that the diffuser “asks” for and that’s, ideally, density * outlet area * velocity at the outlet.
Once the geometry, hence the ratio between outlet area and throat area, is set, the ideal velocity jump in the diffuser is set and consequently the ideal pressure jump is set. (then to be able to achieve a velocity jump as close as possible to that ideal is 99% of design work, but that’s another matter)
Incidentally, lowering the car changes the geometry of the diffuser, in particular it increases the ratio between outlet area and throat area hence increasing the ideal pressure jump.
Now that we understand all the above it’s evident that for a given diffuser geometry pretty much the only parameter left is the velocity at the outlet. Increase that velocity and that will induce an increment of velocity also at the throat and under the car, hence an increment of downforce.
To obtain that result for example you can add a wing near the outlet exploiting the increment of velocity caused by nose curvature. In that case the wing acts de facto as a prolongation of the diffuser.
Another way is what we are talking about in this thread. Blowing exhausts gasses in the diffuser you mix them with the airstream that passed under the car hence increasing a lot its velocity. That will induce an increment of the velocity at the throat and under the car => more downforce.
Then if the exhausts are blowing just above the upper edge of the diffuser outlet the mechanism changes little, it’s still a modification of boundary conditions at the outlet. Actually, given what I said about the beam wing acting as prolongation of the diffuser one can argue that also in that configuration the exhausts are still blowing inside the diffuser, the “big” diffuser formed by lateral channels + beam wing.
