Exhaust Blown Floor - Forward Exhaust Exit

[shelly]

Could please give more evidence?

[shelly]

Basically like a local "virtual" lowering of the floor near the leading edge?

Yes, like adding a virtual lip in front of the leading edge, that fades away downstream because of mixing.
Different shapes of hte exit (flatter,oval, rounded) will change the shape of this virtual lip.

[ringo]

@marekk: I know that fluids mix. But in this situation the will always be a small zone before the exhaust in which the fludi is 99% from the exhaust, and mixing will take place a upstream and then on the upper and lower side.

For example, if you thin of dye injection in water, the zone near the injector will always have 99% dye and then mixing will take place , but in the nearby location.

The picture you posted is misleading, because it it not referred to a jet pointed opposite to average flow.

So, marekk, of course fluids mix, but here we are thinking of what appens in the zone where they are yet to mix.

some seem to assuming the exhaust flow is a jet, it is not. It is only a jet withing the confines of the exhaust pipe. Once it leaves it is just hot gas with rapidly dissipating kinetic energy. Yes within the short distance form the exit to the leading edge of the floor you have an increased gas flow velocity due to the slowing exhaust gas still having more kinetic energy than the air flowing from the splitter.

SO yes your thinking has merit but only due to the action of the exhaust gas on the leading edge. The bulk of the action is the increased mass flow to the diffusor where the magic happens.
The increased mass flow is what increases the diffusor eficiency with the long throat created byt the cars long bfloor.

Red Bull add mass at the satrt of the diffusor Renault are doing it further upstream

How do you make these things up?
Where is your proof? What merit do you have for your reasoning.

Increase diffuser efficiency how?
Why does the exhuast have to go to the diffuser, why not into the tyres or wide of the car.

These things cannot be answered by imagination. It's either you see it, or have experience with it.
willem toet the aerodynamicist is unsure, so how can anyone else say what is happening?
External flow is a very complicated thing.

[marekk]

@raptor22: I agree that th efact that the car travels at 300kph does not make exhaust a jet.
It is the fact that exhaust speed is more than 600 kph that makes it a jet.

From fluid compression thread:

No experimental data, but quick math (2.4ltr * 18000 /60 /4) gives us for current F1 engine 180 ltr/s at intake, and assuming 300K ambient and 900K after combustion 300 ltr/s to each of pipes after expansion. Assuming incompressibility and pipe with cross-sectional area of 0,00785m^2 (round, 10 cm diameter), it will be about 38 m/s (137 kmh).
To achive 600 kmh, you need about 3600K after combustion.

[shelly]

marekk, I think that there are some errors in your math.

For example, you divide by four when relating mass fow to capacity and rpm: you should divide by two.
I do not know what hypotesis you have made for temperature.

Biggest mistake lies in the fact that you consider air density to be costant, and divide 300 l/sec (which we have seen, it should have been 600) by 0.0078 to get 38 m/s.

The point is that being air hot (600K maybe) and density is much lower than 1.2, it is a 0.something.

So exhaust speed is probably around 600kph, or more.

[ringo]

2.4 lt engine
exhaust stroke every 2 cycles, at 9,000 cylces per minute is, 150 exhausts per second.
for half the car it's 1.2kg per side. so it's 150 x 1.2 kg 180 l/s

which is .18m3 x 1.2kg/m3 = 0.216kg/s using density at ambient!
mass flow does not change, se we use ambient to find mass and we stick with the mass from now on.

at 850 degrees density is 0.31kg/m3 which is not ambient!
for a diameter of 2.5 inch, area = 0.00316 m2
using our known and constant mass, not density!

velocity = mass flow/density (at 850C!) x area = 0.216 / 0.31 x 0.00316 = 220m/s

this is 790 kph so shelly is right.

density makes the difference.

[marekk]

marekk, I think that there are some errors in your math.

For example, you divide by four when relating mass fow to capacity and rpm: you should divide by two.
I do not know what hypotesis you have made for temperature.

Biggest mistake lies in the fact that you consider air density to be costant, and divide 300 l/sec (which we have seen, it should have been 600) by 0.0078 to get 38 m/s.

The point is that being air hot (600K maybe) and density is much lower than 1.2, it is a 0.something.

So exhaust speed is probably around 600kph, or more.

Agree with divide by two. My mistake. Let's do it one more time:

Intake will be 2.4*1800/60/2=360 ltr/s.
Ideal gas law:
pV = nRT
V = T(nR/p); we assume constant pressure p, don't take into account additional molecules of fuel, so just n (amount of air), r is Boltzmann's constant - so is constant

Volume at T=300K (intake) = 0.360 m^3
Volume at T=900K (after combustion) = 3*0.360m^3 = 1,08m^3
Volume to 1 of 2 pipes: 1,08m^3/2 = 0,54m^3
Speed = 0,54/0,0078 = 69,23m/s = 248,4 kmh
gases quickly cool and compress, so it will be far less on pipe's exit.


@ringo: at this density your flow is already hypersonic, you have to be carefull, sonic boom 9000 times a second can be dangerous

A hint for your next calculation: this gas is at 850C when compressed to about 57 atm.

[ringo]

You are misusing using the equations.
You can't know the volume at any time, you only know how much the engine displaces. The mass flow is the only definite thing.

but speed of sound increases with temperature and 220m/s is way bellow that even at atmospheric conditions which is 340m/s.
At 850 degrees the speed of sound is 671m/s

[manchild]

Math, I think I'll do what I've done in school - skip it.

Some people use numbers while some like me are as I've read somewhere "visual mathematicians". Sticking to visualizing of how flow of air stream and exhaust functions is what I'll do viewtopic.php?f=12&t=9187&start=431

[BreezyRacer]

Could this pic be a hint at what's going on?



Note the red strips along the front of the sidepod floor, where you would expect exhaust to be a factor. Is that some kind of temp paint? I didn't see any pics of it on the other side of the car but I'm a continent away

[manchild]

Yes, it seams like temp paint or temp tape. If you brighten and zoom in, you'll see that orange stuff is applied on both top and bottom of floor on same spot.

That undoubtedly confirms that their intention is to direct part of the exhaust fumes above, and part below the floor.

I'd say this is a fine tuning to get proper ratio.

[BreezyRacer]

Yes, it seams like temp paint or temp tape. If you brighten and zoom in, you'll see that orange stuff is applied on both top and bottom of floor on same spot.

That undoubtedly confirms that their intention is to direct part of the exhaust fumes above, and part below the floor.

I'd say this is a fine tuning to get proper ratio.

I would be more likely to say that want to see where it's going, not that they necessarily want it to go top and bottom. In this whole thread I'm a bottom man myself, but only in this thread ..

[Sayshina]

2.4 lt engine
exhaust stroke every 2 cycles, at 9,000 cylces per minute is, 150 exhausts per second.

Ringo, a racing engine spends most of its life at maximum throttle, but very little of its life at maximum rpm. 8,000 would probably be more representative.

Breezy, you can't really draw any conclusions from that photo, even if the gasses were intended to go someplace else entirely you'd expect the team to at least check that area.

To everyone else, you've largely ignored everything else and focused on the exhaust exit as though that were the starting point. I have no idea what's going on with the Renault, but I do know you're going to need to see what's going on in the sidepod as well as with the exhaust flow itself.

Also, in that same vein, I know Scarbs has claimed that "u" bend in the McClaren exhaust is/was for their version of a forward exit, but I just don't see it. You can't just tack on a long tail pipe without effecting the engine tune, and if you are going to run a long pipe in there you're going to dump a lot of heat out of your exhaust and into your sidepod, which ultimately will mean it's part of the cooling system. That's directly counter to modern engine design.

[manchild]

You can't just tack on a long tail pipe without effecting the engine tune, and if you are going to run a long pipe in there you're going to dump a lot of heat out of your exhaust and into your sidepod, which ultimately will mean it's part of the cooling system. That's directly counter to modern engine design.

If pipe gradually widens engine can be set up just as with shortest pipe with much smaller diameter. It could even have identical diameter at its ending just as short one, if it is wide enough in between to allow expansion/reduction of pressure to a level that wouldn't disturb normal breathing of the engine.



When it matters R31 sidepod, I thought we've agreed on fact that it is split longitudinally in two chambers. The one next to tub is the one where exhaust pipe is located and trough which pipe is being cooled. Second one is were the radiators are located. That way exhaust heat doesn't obstruct the cooling of radiators.

[PlatinumZealot]

The pipe is still very short when compared to street cars too.