BMW exhausts

[gcdugas]

The F2002 was the first car to use the periscope design to get the flow as horizontal as possible....

Very funny.... The context of my comment is that the F2002 was the first of the modern F1 cars that have high exhausts that makes an effort to get the flow horizontal.

[gcdugas]

It would be usefull if those that do not know the fundamentals of fluid dynamics did not post there views on these subjects with such conviction. Others with no knowledge could easily mistake this conviction for being truthfull. In the other topic about high exhausts Reca in general terms explains it fairly comprehensively and correctly. So again, for those without proper textbooks on fluid dynamics: low pressure in sub-sonic aerodynamics is not caused by absence of air! (The continuity equation and the Bernoulli equation will show you that low pressure is caused by high velocity of the air and therefore by an abundance of air, as long as you have the pressure difference to maintain its high velocity) Absence of air would in general cause low velocities and therefore high pressure.
On the subject of the BMW exhausts, who knows.... If they were actually hitting the wing (for a reasonable amount of time), the wing would not survive due to burning up. If they were severely disrupting the airflow over the rear wing, their design would be changed. So either the effect is minimal, or benificial (Or least detrimental. After all, the exhaust has to exit somewhere). My guess would be slightly benificial, as the high velocity jet of the exhaust will entrain the air around it, resulting in a slight increase in downforce. (With a diameter of 80mm, and at 800degC, the velocity at 18000 rpm would roughly be 140 m/s, or 500 km/h)

Welcome mighty professor with all your text books! But please tell me how anything that fills a low pressure zone doesn't increase the pressure? I know about bernoulli and venturi effects too. But we are talking about a signifigant volume of gasses being introduced to a low pressure area.

My vote is for "least detrimental" which was implicit in my post that neglected to cite any textbook reference. Sorry. BTW, using your figures of 500 km/h, that would be a severe detriment of air flow to the rear wing if it was pointed anyway but horizontal. Try again genius. And try to be a little more humble.... OK? I appealed for comments on my "hack" aero thoughts, not a reprimand from a know-it-all. Moreover your reprimand is faulty. Back to the drawing board for you.

[pyry]

its a rookie fight

[Carlos]

Tom---Could it be a slightly later vintage Lotus?--the tires are a little wide for '63 and I think the following car is a BRM from '65. If I was well versed in Fluid Dynamics---I wouldn't state THE FIRST LAW---When On A Cattle Drive---Don't Drink Downstream From The Herd.

Regards Carlos

Manchild---can you help us out? What is the # designation and driver of that Lotus? Admirable aero though--the era of marvellous cars that looked like cigars.

[manchild]

It's Danny Hulme in Brabham-Repco, 1967 Monaco GP. The car behind is Jackie Stewart's BRM

[DaveKillens]

Dcdugas, then please explain why an eductor can work the way it does? It's because the high velocity fluid coming from the central nozzle entrains the fluid surrounding it, and draw it along. This is the concept I am trying to sell.
http://www.1877eductors.com/tank.htm
Just replace the central nozzle with the exhaust pipes, and the secondary, larger bellmouth with the diffuser, or wing structure.

[gcdugas]

Dcdugas, then please explain why an eductor can work the way it does? It's because the high velocity fluid coming from the central nozzle entrains the fluid surrounding it, and draw it along. This is the concept I am trying to sell.
http://www.1877eductors.com/tank.htm
Just replace the central nozzle with the exhaust pipes, and the secondary, larger bellmouth with the diffuser, or wing structure.

The exhaust isn't going that much faster than the normal wind because it isn't that focused by the time it nears the wing. Moreover it is still pointing up! It would have to go signifigantly faster than the air stream and it would have to be largely horizontal. The 500 km/h figure is highly dubious, but even if it were near that it, if it spreads from 80mm to just 160mm by the rear wing, then it would be going only 125 km/h (slower than the airstream), and it would be filling the low pressure zone under the wing element, thus negating it somewhat.

In your link they are talking about liquids which are different than gasses because gasses are compressable and liquids aren't to any appreciable degree. The effect described in that link is related to the ventury effect. However if it were possible for a focussed stream to be passing through a low pressure zone with a signifigant delta in velocity, then it could "punch through" the low pressure zone and drag surrounding air with it thus lowering the pressure even more but we are talking huge deltas (3X-4X minimum) in velocity and a very focussed stream in order to achieve the effect otherwise the low pressure zone will destroy the focus of the faster stream and increase in pressure. It has to be faster by enough to "punch through" the low pressure zone and it has to maintain its focus.

[joseff]

Actually, 500km/h isn't dubious at all. I'm not a mechanical engineer (photographer, actually...), but:

Assuming:
1.2 liters displacement (one bank)
18000rpm -> 300 revs/sec
intake pressure = exhaust pressure (just outside the nozzle) = 1atm
cylinder filling at 90% (wild guess here for a NA engine)
intake gas temperature = 27C = 300K
exhaust gas temperature = 800C = 1073K
exhaust nozzle diameter = 8cm = 0.08m
complete combustion of fuel: ie. no NOx, CO, only CO2
no oxygen in fuel

we have:
intake volume / sec = 90% * 1.2 * (300/2) liters = 162 liters
Vin / Tin = Vex /Tex
-> Vex = (1073/300) * 162 liters = 579.42 liters
exhaust nozzle surface area = 3.14 * 0.04^2 sqm = 5.024e-3 sqm

and therefore
exhaust speed = 5.7942e-1 / 5.024e-3 m/s = 115.33 m/s = 415.19km/h

I neglected dynamic pressure (airbox, exhaust pressure) and monoxides in the exhaust, both of which would increase the exhaust speed even more.

[gcdugas]

Actually, 500km/h isn't dubious at all. I'm not a mechanical engineer (photographer, actually...), but:

Assuming:
1.2 liters displacement (one bank)
18000rpm -> 300 revs/sec
intake pressure = exhaust pressure (just outside the nozzle) = 1atm
cylinder filling at 90% (wild guess here for a NA engine)
intake gas temperature = 27C = 300K
exhaust gas temperature = 800C = 1073K
exhaust nozzle diameter = 8cm = 0.08m
complete combustion of fuel: ie. no NOx, CO, only CO2
no oxygen in fuel

we have:
intake volume / sec = 90% * 1.2 * (300/2) liters = 162 liters
Vin / Tin = Vex /Tex
-> Vex = (1073/300) * 162 liters = 579.42 liters
exhaust nozzle surface area = 3.14 * 0.04^2 sqm = 5.024e-3 sqm

and therefore
exhaust speed = 5.7942e-1 / 5.024e-3 m/s = 115.33 m/s = 415.19km/h

I neglected dynamic pressure (airbox, exhaust pressure) and monoxides in the exhaust, both of which would increase the exhaust speed even more.

Why point that up? It would severely disturb the flow to the wing. And if it flares out to just 160mm, the speed is still 1/4 your figure, and slower than the air the car is passing through. Certainly slower than is needed to contribute to the low pressure zone under the wing element. That is why I still lean towards "least detrimental" as the reason for its placement. The upward direction is still a mystery to me. Maybe that is what is needed to get the heat to go above the suspension but beneath the wing??? That is all I can figure.

[joseff]

The 500 km/h figure is highly dubious, but even if it were near that it, if it spreads from 80mm to just 160mm by the rear wing, then it would be going only 125 km/h (slower than the airstream), and it would be filling the low pressure zone under the wing element, thus negating it somewhat.

You have to take the frame of reference into account - a 500km/h exhaust stream on a car travelling at 200km/h means the exhaust travels 300km/h *relative to* the airstream. If the stream really expands to 160mm across without mixing with air, it'll still be going 200 + (300/4) km/h = 275km/h not 125km/h.

Taking this idea to the diffuser, where there's a lot less air volume compared to above the car, the effect should even be more pronounced.

On the subject of the BMW exhausts, who knows.... If they were actually hitting the wing (for a reasonable amount of time), the wing would not survive due to burning up.

I agree with this... if you look at the exhaust outlet position, my guess would be that it's directed upwards simply to avoid the rear suspension, not to aim at the wing.

From Wikipedia on the Ferrari F2002:

(The periscopic exhaust outlets) was incorporated both to use the hot exhaust gases for aerodynamic effct but to raise these gases higher and out the way of the rear suspension. On previous occasion Ferrari's non chimneyed top exiting exhaust outlets had caused the rear suspension and other elements at the rear of the car to overheat or even melt when minor cracks occurred.

From F1Technical on the Ferrari F2002:

The exhaust pipe is covered with a 'fin' that also functions as an air outlet. The fact that these two are put out together, makes, thanks to a local low pressure area after that fin, 'suck' the gases out of the car, thereby helping each other to flow faster, and thus lowering engine temperatures.

Just like the eductors.

[joseff]

Why point that up? It would severely disturb the flow to the wing.

If you see videos of engine blowups (Manchild, help me with Taku blowup vids here!) you'll see that on slow moving cars the exhaust stream does in fact contribute to airflow under the rear wing.

Ergo, extra downforce at lower speeds

The F2001 heat damage comments suggest that at higher speeds the exhaust gases won't come near the rear wing, going instead almost horizontal.

[Reca]

It was here... Clicky

During the thread someone was under the severely mistaken impression that the exhaust went directly into the diffuser and added? downforce...

I’m under the impression you didn’t read the whole thread.
viewtopic.php?t=3277&start=15

Another thing then.
You say that gasses and liquids are very different because the former are compressible while the latter are not. Well, that’s not entirely correct.
First, Navier-Stokes equations work for both gasses and liquids (or any fluid as long as it’s Newtonian like water, air, oil etc etc, not good for example for mayonnaise...), the basic dynamics is exactly the same.
Then, gasses can be easily considered to be incompressible as long as the velocity is low, well in the subsonic range.
Typically under Mach = 0.3 the compressibility effects are neglected because the error is minimal quantitatively and has no influence qualitatively. Mach number is ratio of actual velocity and speed of sound, speed of sound depends by temp of the flow (proportional to square root of absolute temp). With temp in the order of 15°C speed of sound is about 340 m/s, meaning that Mach 0.3 is about 100 m/s = 360 km/h, higher than speed the car reaches so we are definitively working in a range were we can consider the flow incompressible.
For the exhausts the speed is lot higher but also the temp is. Using Navier results that are correct given the data (that sound quite close to actual ones to me), a speed of 150 m/s with a temp of 800°C would correspond to Mach in the order of 0.23-0.25, hence, again, we can consider the flow definitively incompressible.

At the end, they test with blowing exhausts in the wind tunnel.

[gcdugas]

Joseff wrote:

You have to take the frame of reference into account - a 500km/h exhaust stream on a car travelling at 200km/h means the exhaust travels 300km/h *relative to* the airstream. If the stream really expands to 160mm across without mixing with air, it'll still be going 200 + (300/4) km/h = 275km/h not 125km/h.

I think you are figuring this wrong... In one equation you add the 200 but in another you do not. Think about it. And the one that is wrong is the one where you do add the 200 to get 375 km/h. The 125 km/h figure is correct *relative to* the air flow.

If you see videos of engine blowups (Manchild, help me with Taku blowup vids here!) you'll see that on slow moving cars the exhaust stream does in fact contribute to airflow under the rear wing.

Ergo, extra downforce at lower speeds

I have never seen even one video where the exhaust is moving faster than the airstream except maybe for the first 30 ft. at the start or leaving the pits.

The F2001 heat damage comments suggest that at higher speeds the exhaust gases won't come near the rear wing, going instead almost horizontal.

In the F2001 the pipes were cut off at the bodywork level which looks really slick standing still but the exit angle for the gasses was still about 35° above horizontal. The heat damage is because the gasses are getting knocked down by the air flow and this can only happen if the gasses are going slower than the air flow. So we have the air getting disturbed before the wing which is bad, and the low pressure zone is "filled" with a volume of gasses that also negates the effect of the wing.

The exhaust is a necessary evil. It has to go somewhere. In the diffuser is bad because it too "fills" the low pressure zone. (and yes I have seen those pics on the high exhaust thread but they were all very old cars with the gearbox taking a large space. Modern cars have the largest part of the diffuser in the center, not the sides, and use smaller gearboxes. Those pics were as relevent as the pic of the old Lotus.)

The ideal place for gasses to exit would be just above the diffuser at the level of the rear axle but there are too many parts there unless you made the pipe real long and snaked it through. If you did that you would ruin the frequency of the pipe, increase overall backpressure, and suffer a loss in power.

So up it goes into the "least detrimental" position. Now if the flow is pointed straight up you will have this large invisible plume disturbing the flow before the wing. If you were able to have it purely horizontal, you would protect the air flow to the wing maximally but cook all your suspension parts. So the exit angle is a trade off. And "least detrimental" wins out....... as I see it.

My comments have to do with that angle. Last year, the Hondas and the Renault in particular, had their pipes almost horizontal. And that was with the 3.0 litre engine producing 125% the volume of gasses that this year's engines produce.

I am just puzzled at the upward angle of the BMW. It is much more vertical than all the other cars, period.[/quote]

[Navier]

Just a couple of notes; there's no reason for the flow to expand to double the radius. Forgetting density changes due to the cooling-off of the exhaust for a moment.... a 140 m/s jet will be slowed down if the air that surrounds it is moving at a slower speed (thereby speeding up that air, and expanding the jet at the same time)... but never to something slower than the surrounding air! When the jet slows down it will increase in size (Bernoulli again), but this is somewhat irrelevant as it has passed its momentum and energy to the surrounding air. At the same time the jet cools off (and heats up the surrounding air), increasing the density and that would actually reduce the size again. To get a feel for the relative importance of all these factors (Jet speed, car speed, temperature gradients, size of the jet), a couple of CFD runs would give a nice idea.
This all means that you can't 'fill' a low pressure zone (filled with a higher pressure I presume), if the jet has a higher velocity to start with. At their level, they will surely try to use the energy of the jet to its maximum.
The angle could disrupt the airflow over the wing, but my guess is that it doesn't or they would have made a more horizontal tailpipe like some other teams.
BTW, my number for the exhaust jet is somewhat higher than the calc of joseff, as I anticipate a 20% increase in massflow due to the volumetric efficiency of the airbox.

[Reca]

The exhaust is a necessary evil. It has to go somewhere. In the diffuser is bad because it too "fills" the low pressure zone. (and yes I have seen those pics on the high exhaust thread but they were all very old cars with the gearbox taking a large space. Modern cars have the largest part of the diffuser in the center, not the sides, and use smaller gearboxes. Those pics were as relevent as the pic of the old Lotus.)

I put these pics in particular only because those were the ones I had easily available on my hard drive but I also explained that till 2000-2001 there were cars with the exhausts blowing in the diffuser, also in the central channel, simply the solution was less popular due to the radical reduction of dimensions of the diffuser itself. Actually the latest example is even more recent, in the original design of the Mp4/18, 2003, the exhausts were blowing in the lateral channels of the diffuser, although after the first tests they moved them up mainly to solve reliability issues, with little success since the car eventually never raced. But the simple fact that they tried that solution in spite of the fact that certainly it wasn’t the easiest possible design to me sounds like they really wanted to put them there for a reason and I doubt it was to reduce downforce.

Besides F1 didn’t start in late 90s and usually there’s little new under the sun, for example the chimneys that you say McLaren first introduced in 1999 were already used by Shadow in late 70s and the gills Ferrari “introduced” in 2003 are as old as the automotive world and used since several decades.
In this case in particular these pics are very relevant anyway because you said that the exhausts blowing in the diffuser would reduce the downforce and that’s what people here question because it goes against experience and theory. Even if a car was designed in late 80s, physics laws were the same back then, and even if the knowledge wasn’t certainly as good as nowadays, the effect of exhausts blowing in the diffuser was back then the same as today, and well known by all drivers and designers, downforce was increased while going full throttle and reduced lifting. It happened at the time and unsurprisingly happens today too when someone drives the same car, doesn’t matter when it was designed.

In spite of the fact that many people told you so and also explained in details how the diffuser works hence why the exhausts blowing in the diffuser leads to more downforce, you continue with a “you don’t want exhausts gasses to fill the diffuser low pressure area” repeated as a mantra non substantiated by a more extended explanation. Given that the exhaust flow pressure is mandated by the local ambient at the outlet I fail to see how can you raise the pressure of a given area by “filling” it with a mass flow at the same low pressure. Still you don’t explain it, just repeat again and again the same phrase insisting that it would reduce downforce and saying that people believing otherwise got a “severely mistaken impression...”.

Then again you contradict yourself while saying that the best possible position for exhausts would be blowing just above the diffuser. As I already explained in that same post, that area right under the beam wing is still a low pressure area that can be seen as prolongation of the diffuser. If to blow the exhausts in the diffuser was so bad I can’t see how putting them in that position just under the wing would be the best possible solution because they are conceptually the same thing and in both cases you are blowing straight in a low pressure area.

There are a few more mistaken concepts in your posts but I think you got the picture.