Exhaust Blown Floor - Forward Exhaust Exit

[forty-two]

Actually, another question just popped into my head.

Presumably, if the intention is to produce an air curtain to increase downforce, surely if all the gas were heading under the car, maximum downforce would be at maximum revs (not good), so perhaps the positioning of the exits is crucial to ensure that some of the gases get "wafted" over the top of the floor at higher speeds?

[747heavy]

15,000 x 2.4 Litres = 36,000 litres/min / 600 litres/second = 300 litres per side.

But I think that is wrong because I've taken the capacity of the combustion chambers rather than the expected volume of the products of combustion, so HELP!

Keep in mind that a 4-stroke engine only "exhaust" it´s gas every 2nd revolution.
Then use the delta T of the gas from intake to exhaust and you will come closer to your answer. (ideal gas law)
For an order of magnitude estimate it should be close enough.

if interested, some reading on the subject can be found here.
http://www.eng-tips.com/viewthread.cfm?qid=104735

[forty-two]

Thanks Jumbo!

[xpensive]

...
Erm. Thats on two co-planar surfaces.

The important bit is me ballsing up the negative 0.5 *rho *V^2 (and its not the first time I've done that either!)

What I believe is the important thing to remember is that the total pressure is kept constant through a passage (bar the losses), such as between the F1 car's floor and the road, why the static pressure will decrease when dynamic pressure increases with the air-speed, which produces a static pressure differential vs the top of the car where air-speed is slower.

That static pressure differential times air-passage area, is what creates your down-force, why everything you do on and around the floor/diffuser, vortexes and what have you, aims at increasing air-speed under the same floor and really not much else.

I actually played with some numbers two years ago, here: viewtopic.php?f=6&t=6545&hilit=importan ... e+diffuser

Btw kil, remember to multiply your "centistokes" with density times 10^-6 before your calc's and you be ok.

[kilcoo316]

What I believe is the important thing to remember is that the total pressure is kept constant through a passage (bar the losses), such as between the F1 car's floor and the road, why the static pressure will decrease when dynamic pressure increases with the air-speed, which produces a static pressure differential vs the top of the car where air-speed is slower.

That static pressure differential times air-passage area, is what creates your down-force, why everything you do on and around the floor/diffuser, vortexes and what have you, aims at increasing air-speed under the same floor and really not much else.

I actually played with some numbers two years ago, here: viewtopic.php?f=6&t=6545&hilit=importan ... e+diffuser

Btw kil, remember to multiply your "centistokes" with density times 10^-6 before your calc's and you be ok.

Vortex cores are low pressure areas of their own - due to very high rotational speed, just be aware that that might not be reflected in their axial speeds.

[PlatinumZealot]

The exhaust is creating a physical barrier to separate the air from under the car from the air to the sides of the car.
You have to do some type of momentum analysis involving the jet from the exhaust and the oncoming air from the front and sides.

[Richard]

Forty Two rasied a couple of point I've been wondering about:

What difference on aero performance would there be between a hot air from the exhaust versus ambient temperature air jet of the same velocity? The hot air will contract as it cools, however that temperature loss is probably due to heat transfer to the adjacent air, which then expands. So no net change?

Secondly, how strong is that jet from the exhasut in terms of volume or speed? Compared to the airstream rushing past when the car is at top speed?

[forty-two]

Compared to the airstream rushing past when the car is at top speed?

Precisely when you don't normally want maximum downforce perhaps?

[Neno]

Compared to the airstream rushing past when the car is at top speed?

Precisely when you don't normally want maximum downforce perhaps?

then you have KERS, and ARW lol. this two thing together are powerfull for your top speed. You need forward exhaust exit for downforce of the car in corners, specialy in fast corners.

[horse]

Precisely when you don't normally want maximum downforce perhaps?

IMHO, additional downforce is not an issue as long as it does not lead to additional drag and generally gaining downforce from the floor is pretty low cost on the drag front.

[hud]

Precisely when you don't normally want maximum downforce perhaps?

IMHO, additional downforce is not an issue as long as it does not lead to additional drag and generally gaining downforce from the floor is pretty low cost on the drag front.

wouldn't TOO MUCH downforce on long straight increase tyre roll resistance? and also tyre wear?

[xpensive]

The exhaust is creating a physical barrier to separate the air from under the car from the air to the sides of the car.
You have to do some type of momentum analysis involving the jet from the exhaust and the oncoming air from the front and sides.

Agreed smik. The thinking is clearly that with the xhaust along the sides of the floor (with high speed air and low static-pressure inside), the hot gases with its low density does something with the pressure differential between the two sides, but I'm still at a loss to intellectually pin-point xactly what?

[myurr]

Precisely when you don't normally want maximum downforce perhaps?

IMHO, additional downforce is not an issue as long as it does not lead to additional drag and generally gaining downforce from the floor is pretty low cost on the drag front.

wouldn't TOO MUCH downforce on long straight increase tyre roll resistance? and also tyre wear?

Everyone seems to be forgetting that the traditional EBD works in the same way, higher the revs the more downforce produced.

[Paul]

I wonder how much whichever effect it produces changes with speed and consequently with air flow speed. I mean it seems logical that at lower speed exhaust gases are more prone to keep the angle they are are exiting at, while at higher speed they mix with air flow sooner. Which would mean that at lower speed exhaust gases would mix with air and flow closer to the edge or even outside the floor, while at higher speed they would mix with air closer to exhaust exit and flow closer to centreline of the car.

[forty-two]

I wonder how much whichever effect it produces changes with speed and consequently with air flow speed. I mean it seems logical that at lower speed exhaust gases are more prone to keep the angle they are are exiting at, while at higher speed they mix with air flow sooner. Which would mean that at lower speed exhaust gases would mix with air and flow closer to the edge or even outside the floor, while at higher speed they would mix with air closer to exhaust exit and flow closer to centreline of the car.

Precisely what I was hinting at above.