Exhaust Blown Floor - Forward Exhaust Exit

[marekk]

Could you write some definition, explanation, example about those statetements? So we fix some points, focus the discussion and move on

From a study of air jet weaving machines (http://www.bmf.hu/conferences/cinti2009 ... ission.pdf), but should give us an idea:


Hot exhaust should grow wider and loose speed a little bit slower IMHO.

[shelly]

This is a good starting point; still it is more on the side of what I thought exhaust should behave, but raptor22 says exhausts are not a jet and behave differently, so I hope he will give his contribution too.

I have tried to find some references about exhaust runner speed; till now I have just found a very good phd thesis of a belgian student, but I have yet to go through it.

[ringo]

Could you write some definition, explanation, example about those statetements? So we fix some points, focus the discussion and move on

From a study of air jet weaving machines (http://www.bmf.hu/conferences/cinti2009 ... ission.pdf), but should give us an idea:


Hot exhaust should grow wider and loose speed a little bit slower IMHO.

This is a different situation. It cannot represent what is happening on the F1 car. Note that in F1 there is surrounding air moving at speed.
This cases does not have moving air around it.
Not saying this is not true for it's specific application, but this observation cannot be cut and pasted on a completely different situation.
The exhaust can behave differently in different environments.

Only God knows how an exhaust will behave in any environment. It's best to find a study or perform a study, to know these things. Jet or tractor beam, the words describing it don't say much for the behavior.

[shelly]

In this interview on Autosport.com

http://www.autosport.com/news/report.php/id/89968

Ross Brawn says that preformance -wise exhausts' impact will be comparable to duoble decker's impact. I don't know if it is the usual pr stuff for the press or something he 100% believes; still it is interesting to note is position.

@ringo: the situation described on marekk post is not that different, because you are mainly interested with the difference in speed between exhaust and outside flow.

We have already discussed a bit about speed, but we still need now to get a more precise esteem of the order of magnitude of the energy contained in the exhaust. Every body in the interviews, including Brawn, talks about this "energy": we agree that it is there, but how much is it? What part of it is exploitable? How will the ejection effect work? Is merc trying to use a blown flap/Coanda effect?

Agree with ringo that is not that important to baptise exhaust as a jet, but as raptor22 wrote that it is not a jet, and that he knows well how such things behave, I wanted (and still wanting) him to share something with us, so that we can build some fixed points in the discussion by giving some reasonable answers to the first questions, and then move on to the next.

[volarchico]

This is a different situation. It cannot represent what is happening on the F1 car. Note that in F1 there is surrounding air moving at speed.
This cases does not have moving air around it.
Not saying this is not true for it's specific application, but this observation cannot be cut and pasted on a completely different situation.
The exhaust can behave differently in different environments.

Only God knows how an exhaust will behave in any environment. It's best to find a study or perform a study, to know these things. Jet or tractor beam, the words describing it don't say much for the behavior.

Different, yes. Similar, yes. As a very simplified view of an exhaust as a jet, the diagram shows the basics. And it's all about picking a frame of reference. Instead of imagining watching the F1 from stationary ground, imagine getting to "sit" on the car's floor and go along for the ride. In that frame of reference, you effectively cancel out the motion of the vehicle and instead can watch the relative motion of the exhaust gases. Not sure if I'm explaining myself very well, but what I'm getting at is that the diagram isn't that bad of a starting point to get a general idea of how a exhaust interacts with its surroundings. It's no where near an exact answer, but as you said, only God or a lot of computer processors working for weeks (months?) can understand the exact nature.

[marekk]

@ringo: if you subtract the vector speed of your external flow from the vector speed of your jet, you have a situation (in reference frame of the car) almost exactly as described by this study.

There are 2 really significant and not so obvious points:

1. we have speedup of gases in initial section - about 40-50cm for RB31, and this means even lower pressure and more downforce.
2. Due to cone forming, it won't work as a squirt.

One thing we know about energy of exhaust more or less precisely: for this type of engine there is more energy to exhaust then to wheels. So lets say 600-700kW.
If you mange to use 1% of it to generate downforce and reduce drag, you have effectively additional 6-7 kW engine working for you. That's what Ross Brawn is talking about: this concept has huge potential.
But is quite tricky to make it really work, i think.

[Raptor22]

Shelly, I am on a business trip at the moment so internet connectivity is poor. Once I'm back home i'll sit down and write something up.

Basically though, Exhaust gases behave as a jet when they are expanded through a diverging nozzle i.e. Rocket engine, afterburner in gas Turbine. Typically in the throat of the nozzle gases are moving supersonic. Even here by the time the gas leaves the boundary of the nozzle the kinetic energy bleeds off rapidly. Next time you watch a rocket launch have a close look at the plume. there is a just subsonic parabolic region behind the nozzle, a defined boundary and then a large cloud of hot gas moving very slowly but expanding rapidly. In a automotive application we have a mass flow of hot gas thats not expanded via a divergent nozzle and only expands once the gas leaves the boundary of the pipe.

If We look at it simplisticly we are dealing with Q=VA.

Q Exhaust pipe must equal Q environment.

With the Renault Ringo's images give a good idea of what is going on the only assumption I think he has made incorrectly is to assume the exhaust gas is behaving as a stream. It does for perhaps around 50mm from the exit but it is exhausting into a highly turbulent flow coming off the splitter and a more streamloned flow just above the road. It is hot gas and not at ambient and there are thermodynamic effects that are difficult to model without knowing the exhaust length, flow velocity just prior to exit etc.
So yes some of the gas is going to be pulled along the side of the pod (expansion + kinetic energy is going to result in rapid dispersion), the bulk of it is going to be pulled under the car probably along the streamlines he shows but in a highly turbulent manner Since the flowclose to the ground is flwoing for directly ahead ahile flow at a higher streamline is coming off the splitter and going around the pod.
Both flows are influencing the behaviour of the exhaust gas.

If Renault has included a divergent nozzle into the exhaust pipe outlet design then I would agree with you, we could assume a Jet stream like behaviour for possibly a foot from the exhaust if the temperature and pressure is high enough. I just don't see anything resembling such a device.

You're welcome to disagree but like I said, we are not dealing with a rocket motor even though it does go like the clappers.

Other examples are Oxyacetylene torches for cutting or welding. the gas flow (exhaust there is being expanded inside a diverging nozzle and we are achieving a supersonic flow or close to it. Its this gas flwo and the proximityof the combustion that results in a jet of hot gas. These exhausts are nothing close to this. They're just exhaust pipes.

[volarchico]

Maybe there is just a slight misunderstanding of the term "jet"?

You don't necessarily need a diverging nozzle to call something a jet. A garden-hose with no nozzle creates a "jet" of water. Maybe Shelly and Raptor22 are operating from different definitions of the same word?

There are also all sorts of images of jets with converging nozzles. There are others with no nozzle, just a difference in velocity from the ambient surroundings:

[Just_a_fan]

it is exhausting into a highly turbulent flow coming off the splitter and a more streamloned flow just above the road.

You've also got the pair of vortices generated by the barge board in front of this region to factor in too...

[godlameroso]

I have an educated guess/theory of what they're trying to do with the exhaust. A lot of people have the misconception of exhaust being a continuous flow, only that's not true. Exhaust can be thought of more as a pulse than a flow, because the ignition of gasoline in itself is a discreet event, so to will all resulting reactions. Despite exhaust headers converging, all the converging does is line all the pulses up in a single file.

The exhaust pulse is nothing more than a high pressure shockwave, this high pressure shockwave is followed by a corresponding low pressure one. Now, your race car is traveling at say 180kph, at this point aerodynamics are doing their thing, creating a pressure differential across the car. The exhaust pulses meet this already flowing air, and by process of least resistance separate. The high pressure shock waves are pulled upwards, along with the rest of the high pressure flow created by the upper part of the chassis, while the low pressure ones are pulled underneath the car along the low pressure zone.

By placing the exhaust pulses in the stream of air, you have created a virtual wing, by using the front wing, end plates, and to some extent the turning vanes and nose.

Pretty clever....if that was their aim.

It would also explain the funky exhaust notes we hear, possibly trying to exploit the pressure differential through exhaust tuning.

[marekk]

The high pressure shock waves are pulled upwards, along with the rest of the high pressure flow created by the upper part of the chassis, while the low pressure ones are pulled underneath the car along the low pressure zone.

Flow will always follow pressure gradient - if you have lower pressure region under floor, it will go this way. No exceptions.

[volarchico]

And I'm not too sure how accurate it is to refer to the exhaust gasses as high and low pressure shock-waves. Maybe this is just an issue of translation of a highly technical aspect into English?

[shelly]

@raptor22: as you wrote, we are not dealing with a rocket motor, so maybe, as volarchico wrote, there has been some misunderstanding on basic definition of jet.

I think we all agree that exhaust flow in 4 stroke engine is subsonic; therefore subsonic nozzle shape is the opposite (converging instead of diverging) of supersonic nozzles of rockets, afterburners and the other examples you cited.
I think also that the behaviour of gas is different between sub an supersonic jets( let us use this abbreviation word for the moment, while we are not yet converged on precise definition of what a jet is).

Still it has to be seen if and how a subsonic nozzle could be engineered at the end of a 4stroke engine exhaust (I think that simply making that last part convergent does not work, but I am not expert of exhaust acoustics).

I hope that you will discuss further and help in claryfing things.


@godlameroso: each cilynder exahust produces a pulsating train of waves of high-low pressure, at 150Hz frequency (for 18000rpm) for each cylinder. I do not know how much exhaust form different cylinders mix and interact in the collectors, and what is the shape and frequency of the final output pressure signal.
I think that going into this analysis at the moment is premature, and that is better now to focus on time-averaged behaviour.

Probably as volarchico wrote there have been some translation issues, beause in subsonic range shock waves do not form; maybe is some translation of "high pressure waves".

[godlameroso]

@godlameroso: each cilynder exahust produces a pulsating train of waves of high-low pressure, at 150Hz frequency (for 18000rpm) for each cylinder. I do not know how much exhaust form different cylinders mix and interact in the collectors, and what is the shape and frequency of the final output pressure signal.
I think that going into this analysis at the moment is premature, and that is better now to focus on time-averaged behaviour.

I was simply stating that exhaust pulses are discreet events, and not a continuous jet with continuous pressure. If you've ever stood behind a running car and had the exhaust hit your leg, you notice that it's not flowing like wind, but rather a series of pulses. Those super annoying and loud motorcycles illustrate the nature of exhaust perfectly.

Newton's 3rd law, for every action there is a reaction, if an exhaust pulse is a high pressure wave, then it has to be followed by a low pressure one.

So you have alternating and discreet high and low pressure waves being injected into the airstream and naturally they will follow the path of least resistance. I don't have a wind tunnel or CFD to prove this, but it should work in principle, I think a lot of the Mercedes GP improvement has come from the exploitation of this effect.

[shelly]

@godlameroso: I think perceivable fluctuations bahind a car exhausts are due to big scale turbulent obscillation in the mixing flow (issue that has already been treated above). I think the custom bike example is not relevant for f1, because it is about engine with one or two cylinder per bank, revving at much lower rpm (maybe 2000 instead of 18000?).

As I wrote, I think that obscillation effect maybe is important, but I do not think it is the key factor. For example as far as merc w02 is concerned, I think that the idea behind their exahust positioning is to exploit Coanda effect like in the blown flaps of some airplane, to energise and ben inwards in the coke bottle the flow around the sidepods.