Exhaust Blown Floor - Forward Exhaust Exit

[Sayshina]

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

Yes, but also irrelevant. Take a hacksaw to your road car right now, open it up right after the collector and see what happens to your engine tuning.

As for the expanding chamber idea, what are you going to do when it comes time to decrease your radius back down? I assume someone would have noticed and photographed a 30 cm megaphone exhaust sticking out of the McLaren sidepod inlet, and you wouldn't normally expect much useful work from an exhaust like that anyway. If on the other hand you're decreasing your radius at the end you've created an expansion chamber, which will send a pressure wave back to the exhaust valves. You can tune it or the engine so that it's not a stronge wave at engine speeds you care about, but it's going to have an effect on your engine tune, almost certainly a negative one.

Probably not in a way that's going to burn your valves in a lap or 2, so it's possible they tried it just to make sure their simulation had it right and they weren't missing a trick. For testing purposes you could just run a bit rich.

The Renault could be in a very different position. Are there published photo's of the sidepod internals? I haven't seen any, but then I only occasionally pay attention to F1. In any case, it's entirely possible, in theory, that Renault made an engine modification change request to cover any tuning issues. It's legal as long as it's for reliability only, subject to FIA aproval, and we usually don't hear about them until months later. I don't honestly know how that would be handled on an exclusive basis, as it would only be Renaults own cars effected, and I've never heard of that case coming up before.

However, why would they run their exhausts like that in the first place? If you committed to this concept during the drawing phase, and all indications are they did, you could design your manifolds to put the collector almost exactly where they have their exhaust outlet. You'd most likely have to move some electronics, and you might still want to isolate your manifold from the radiator flow, but that was generally not the practice when the entire manifold was downwind from the radiators, and most of it would still be.

In either case, my other point still stands. It seems to run counter to modern racing engine design. You try to send as much of your waste heat as humanly possible out the exhaust specifically so that you can reduce your cooling system to its minumum capacity. You then want to get this exhaust away from the cooling system as soon as possible, because exhaust manifolds radiate a LOT of heat. This is why when they were allowed chimneys became the preferred exit, as they allowed the exhaust heat a short path out of the sidepod, carried the heat far enough above the sidepod to not interrupt the sidepod flow, and even managed to create a little thrust.

If you want that exhaust heat to do a little work, like say directing it into the diffuser, and in order to do so it has to stay under the bodywork, you will inevitably bleed some of the exhaust energy back into the cooling system. Splitting the sidepod airflow isn't really going to change that. As a simple example, if McClaren did run those long pipes back up to the front of the sidepods they would get a respectable percentage of that radiant exhaust heat soaking back into the block, which would send it into the cooling system.

The cooling system isn't just the size of the radiators, though they may be the most draggy part, but all of the airflow under the bodywork. Since you don't have any good way to stop the exhaust from dumping tons of heat into the area around itself, you have to have a very good reason for keeping it in the sidepods any longer than strictly necessary. Routing it to the back of your car as normal, and then all the way back to the front, well you would have to realize some impressive gains to make that worth doing. Either that, or you would have to have some impressive top secret means of insullating your exhaust flow. And by top secret I mean unknown to any of the other teams as well.

[Raptor22]

@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.

Does 4 years in Aerospace designing rocket motors count as experience in what is a jet and what is not a jet?


btw a 4stroke engine exhausts on every 4th stroke, not every 2nd stroke. If you are considering the engine as a whole then you need to account for the firing order.
Only 2 strokes exhaust every 2nd stroke.

If you are refering to revolutionsof the crank then yes it exhausts every 2nd rev of the crank.

[shelly]

@raptor22: can you explain why you do not consider exhausts a jet, and why you predict them to slow abruptly after exiting exhaust pipe?

[segedunum]

You've still got a pretty large volume of hot gas being ejected faster than the speed of the car. Of course it's a jet - it's gas being directed in a confined space that's travelling faster than the air around it. The gas will cool reasonably rapidly, but in this hypothetical scenario it's also being ejected into a much more confined space.

No one has yet been able to explain why ejecting hot gas under a floor would be desirable and why any [obvious] negative effects of that will be negated somewhere down the line. The fact is that you're pumping lower density gas that will decrease dynamic pressure under the car, in a confined space no less. That will have to have a net positive affect somewhere else in terms of increasing dynamic pressure under the floor somewhere otherwise this whole line of discussion is not going anywhere. Over the floor I could understand. Under it? I still can't see what that's achieving overall.

[shelly]

@segenudum: making exhaust under the floor is beneficial for at least three reasons:
-it increments the suction peak on the floor leading edge
-forces acceleration of externel flow air under the same leading edge (virtual wall idea I discussed above)
-increments mass flow rate under the floor

As a result, you have lower static pressure and more mass flow under the whole floor, plus a more intense suction peak under the leading edge: in two words, more downforce.

We have to read the explanations about exhausts not being a jet, and I hope raptor22 will provide them.

[marekk]

Exhaust gas is hot, but not THAT hot. Just looking at this table of steel temp vs color:

C F Color
400 752 Red heat, visible in the dark
474 885 Red heat, visible in the twilight
525 975 Red heat, visible in the daylight
581 1077 Red heat, visible in the sunlight
700 1292 Dark red
800 1472 Dull cherry-red
900 1652 Cherry-red
1000 1832 Bright cherry-red
1100 2012 Orange-red
C= Centigrade, F= Farenheit

we don't see pipe's end glowing red visible in daylight, so it's safe to say actual gas temp at exit is < 600C (900K).

So (looking at our previous math, even ringo's ones) is safe to say actual speed of gases is high, but not THAT high. At best comparable to average F1 car speed.


That said, there is still a LOT of energy, both directed, carrying momentum from the speed of mass flow, and undirected, "stored" in high temp gas (which is basically kinetic energy of gas molecules bouncing, rotating, oscillating in every possible direction).

If you look at pressure distribution map of F1 car's floor, there are 2 main downforce contributing areas: near diffuser at the rear, and near leading edge of the floor (due to car's rake floor forms sort of another diffuser).
There is a a large area between both low-pressure regions at almost ambient pressure, so your flow from floor's leading edge has to fight on it's way to the rear against this pressure gradient. It will loose energy for that (think speed and mass flow decrease).

There are many reasons for this behaviour, one of them being boundary layer formation along the floor:



it's a layer of air at speed lower then free flow (down to no speed at all just next to floor's surface), effectively "chocking" this space, and increasing drag (you have to carry this slow air with you).

Blowing this region with high temp, quick stream of exhaust gases adds energy to your "primary" flow, which in turns helps in this fight against increasing pressure. Boundary layer's thickness decreases, "unchocking" this channel to main diffuser.

Momentum conservation still valid, so if you blow perpendicular to "normal" flow, resulting summary flow will be directed a little sideways (virtual turning vane) with increased speed, possibly forming sort of virtual skirt.

Last but no least: As exhaust gases quickly cool going rearwards, volume decreases quickly as well (going from 600K to 300K results in half a starting volume), causing local pressure drop and speed up of the flow.

If this system works this way, it will increase downforce generated by floor and diffuser and decrease drag.

[ringo]

Does 4 years in Aerospace designing rocket motors count as experience in what is a jet and what is not a jet?

Jet or no jet, the name is irrelevant, because if we look on the definition of a fluid jet, it doesn't really sway the discussion one way or the other.

It doesn't matter which words you want to describe it in. As a rocket motor designer you should know about momentum change. What you are proposing is that all the exhaust turn 90 degrees to the normal of the exhaust outlet and go parallel to the car center line.
Have you ever had a rocket where the air blows it's jet 90 degrees as it exits the nozzle?
The air flow under the car is not parallel to the center line either, so there is still no incentive for the exhuast to orderly go along the car.

So it follows that the exhuast cannot force itself to go along the center line and come out the diffuser. The angle of the pipe is simply too wide.

This pic is with the pipe bellow the step plane, which is illegal, and i think i posted this already.
I went to all lengths to get the thing to blow under the diffuser, i even turned the pipe more downstream than Renaults interpretation and still the air wont stay underneath completely.
And this is with a very low velocity exhaust, to make things easier.



It wont stay under. Hard to believe otherwise until there is some proof.
Too much evidence out there , Renault using temperature stickers, high temp chalk, on the top of the car and the CFD.

[ringo]

Exhaust gas is hot, but not THAT hot. Just looking at this table of steel temp vs color:

C F Color
400 752 Red heat, visible in the dark
474 885 Red heat, visible in the twilight
525 975 Red heat, visible in the daylight
581 1077 Red heat, visible in the sunlight
700 1292 Dark red
800 1472 Dull cherry-red
900 1652 Cherry-red
1000 1832 Bright cherry-red
1100 2012 Orange-red
C= Centigrade, F= Farenheit

we don't see pipe's end glowing red visible in daylight, so it's safe to say actual gas temp at exit is < 600C (900K).

Not really. You have to know how much the metal is absorbing. Aluminum cylinder heads don't see the high temps of the burning gas, so they don't melt.
600 is still hot though.

[marekk]

Exhaust gas is hot, but not THAT hot. Just looking at this table of steel temp vs color:

C F Color
400 752 Red heat, visible in the dark
474 885 Red heat, visible in the twilight
525 975 Red heat, visible in the daylight
581 1077 Red heat, visible in the sunlight
700 1292 Dark red
800 1472 Dull cherry-red
900 1652 Cherry-red
1000 1832 Bright cherry-red
1100 2012 Orange-red
C= Centigrade, F= Farenheit

we don't see pipe's end glowing red visible in daylight, so it's safe to say actual gas temp at exit is < 600C (900K).

Not really. You have to know how much the metal is absorbing. Aluminum cylinder heads don't see the high temps of the burning gas, so they don't melt.
600 is still hot though.

Aluminium cylinder heads do see the high temps of flame, but heat is quickly transferred to water and radiators. Try to run engine without cooling fluid as i did once, and if you are lucky, you can even see spark plugs flying around due to melted windings

And exhaust pipes do not have any cooling except ambient air, which as we now is not very good heat transfer agent. And those particular ones on r31 are extra insulated.

[ringo]

Ok more proof the thing blows between top and bottom and at the edges:



notice where the yellow lines are?
They are outside of the floor.


more temp stickers.

[horse]

notice where the yellow lines are?
They are outside of the floor.

For sure, and the darkened tarmac starts under the car. Anyway, the direction of the exhaust while the car is stationary is not really relevant, right?

[ringo]

Yes the darkened tarmac starts under the car, it has to. But what is certain is that it's not staying under.
Up to now i don't see any discrepancy with my amateur CFD.

What we are seeing with the wide blowing angles at stand still, is no different than the the CFD video that was posted or any of the pictures, save for minor details such as the exact pipe angle and floor design.

No evidence at all that it's braking back 90 degrees and going down the middle of the diffuser.
Some of it does thanks to the high pressure in front of the rear wheels, but majority of it isn't going there. At low speeds the gases blow very wide.

[horse]

At low speeds the gases blow very wide.

Oh yeah, no doubt, you can certainly see that the exhaust is easily clearing the floor at standstill, you are right. I wonder then, if a "tuned" response is possible where at low speed the system promotes drag reduction by improving flow about the side pods and at high speed more downforce is created instead.

In fact, that gives me an idea. Surely the system is most wanted in yaw (ish), when taking a bend? So in this case, will the flow from one side of the floor (on the inside of the turn) be more likely to go below the floor than the other?

[Raptor22]

those stains are from oil and fuel not exhaust gas.

[shelly]

@raptor22: in previous post you have written that exhausts are not a jet, and that they quickly lose speed outside the pipe.
Could you write some definition, explanation, example about those statetements? So we fix some points, focus the discussion and move on