Investigating exhaust aided diffusers

[DaveW]

When I find it rather obvious from the above pictures that exhausts are located on the sides, just ahead of the rear wheel
and below suspension parts, I find it difficult to see that this would aid the diffuser in any significant way?

A group working at Leafield (Super Aguri), I believe, discovered that channelling air from beneath the undertray could improve the performance of an F1 diffuser significantly. The idea was implemented most successfully in the Brawn vehicle last year, & gave the team a performance advantage over the first half of the season.

How does the "double diffuser" work? I'm not an aerodynamicist, & regard aerodynamics as a weird & wonderful black art, particularly so in complex & constrained regions to be found at the back of an F1 vehicle. For what it's worth, therefore, here are my thoughts:

1. Drawing additional air from beneath the undertray helps to reduce the mean pressure under the vehicle.
2. It will increase the mass flow of the upwash behind the vehicle.
3. It may help to keep the flow in the main diffuser "attached".
4. There was some talk of it helping to "connect" diffuser flow to rear wing flow, presumably implying that the whole becomes more than the sum of the parts, as it is in a well-designed bi-plane aircraft.

Now, how might a high energy exhaust stream help, augmented, as it is, by a mixing region immediately aft of the exhausts?

1. It appears to be directed at the trailing edge of the undertray, & that could help to improve air extraction from the undertray itself, thus lowering the pressure under the vehicle.
2. Some of the flow appears to be channelled into the diffuser region (look closely at one of the photographs posted above). This is where the Coanda effect might be relevant.
3. The increased mass flow through the diffuser complex (upper or lower, it matters not, I think) could amplify the "connection" between the diffuser & the rear wing (if that is relevant).

Much speculation from a layman, I know. The fact is, RBR have an observable performance advantage this year (probably more than Brawn had last year), & one obvious feature that is different is the position & direction of the exhaust flow. I have been hoping that aerodynamicists might help to clarify my rather un(in)formed ideas.

[xpensive]

What it's all about is to speed up the air under the car relative to the air-speed on top of the same, all according to Bernoulli where;

Total pressure-pt, is static pressure-ps plus dynamic pressure-pd. Then if total pressure is held constant:

ps1 + pd1 = ps2 + pd2

The above gives a static pressure differential, ps1 - ps2 (which creates Force), equal to pd2 - pd1.

When pd is density * speed squared / 2 and Force is static pressure differential times Area, the resulting force is:
area * density * (speed2^2 - speed1^2)/2.

This force can be substantial, if you can arrange the air-speed under the car to be 100 m/s, while the speed on top is only 60,
downforce would be about 7700 N with a floor-area of 2 sq. meters.
If you find a way, through a novel diffuser, to increase speed under the car with 10%, downforce jumps to 10200 N, or with 32%.

Question here is to what xtent the RBR xhaust helps speeding up the air under the car?

[marcush.]

maybe it is just the position of the exhaust leading to NOT increasing the speed of the flow on the top of the car .
my understanding is that the flows over the car need to be slower than those below ,but obviously the distance to cover is longer over the top....so the lets call it hull will more than likely produce lift instead of downforce.so blowing the exhaust directly towards the trailing edge of the difusser and part of it into the upper deck you will counter this..?

brings me to another idea.
why not open the trailing edge of the bridge wing profile and blow out exhaust stream on the trailing edge of this profile? of course heat would be a big issue but sure you could build a slit type exhaust and shroud it with carbonfibre wingsection ....

[xpensive]

That's a very interesting point marcush, perhaps placing the xhaust on the sides avoids speeding up the air on top. Good thinking!

[DaveW]

That's a very interesting point marcush, perhaps placing the exhaust on the sides avoids speeding up the air on top. Good thinking!

X, your slip is showing....

[xpensive]

Don't corrupt my posts Dave!

[marcush.]

I do have a soft spot for swedes I have to admit...

[DaveW]

Ouch! Apologies.... Anyway, you were right to acknowledge Marcus' idea. I suppose the abolition of chimneys means that using exhaust kinetic energy to generate down force directly is no longer a "clean" option.

[xpensive]

Just kidding Dave. But I guess you can learn quite a bit from a picture, no?

[f1ssk]

hey ev1 ... i posted this a month bak on the Red Bull RB6 thread ...

I think im late on the discussion on the exhaust gas being used in the diffuser.
Anyways, The buoyancy of the hot gas delays the transition to turbulence in the diffuser and this can enable RB6 to have higher expansion. I got this idea from a video
I came across some ancient videos which proved interesting.
http://web.mit.edu/hml/ncfmf.html
watch the turbulence video.almost towards the end there is a part where in a channel they demonstrate if the hotter fluid is made to flow above the colder fluid it delays turbulence.

I think this is the most crucial part of the RB6 performance.

from billshoe : turbulence video - 22:30

another thought of...
the exhaust gases by itself is very fast...(faster than the flow of the air through the diffuser)
for the previous discussion to work, the velocity of the two gases have to be relatively equal... otherwise other instabilities (helmholtz instabilities -- watch the instabilities video in the previous post) will again disturb the flow.
the exhaust gases are hot and fast and radiator gases are slower ... an optimal mixing for the two gases can be used to make the velocity relatively constant for the hot and cold air.

I predict the ferraris to be bak with a bang at Canada, but will truly show their potential only at the next high downforce track with their blown diffuser.
Im a mac fan. btw. hope macs realize it n get it too.

[DaveW]

hey ev1 ... i posted this a month bak on the Red Bull RB6 thread ...

I did read & respond to your original post, f1ssk (I think). I also pondered over your fascinating link (thanks for that), & liked especially the "Pressure Fields & Fluid Acceleration" video (& notes). Remarkable stuff...

[f1ssk]

I posted it again coz I thought ppl were making it far too complicated.
ill check out the video.. thanks DaveW

[xpensive]

I posted it again coz I thought ppl were making it far too complicated.
ill check out the video.. thanks DaveW

I agree, this is why members need to reminded of the basic theory before being lured into catchword-based opinions and speculations.

[DaveW]

I agree, this is why members need to reminded of the basic theory before being lured into catchword-based opinions and speculations.

(Ouch)^2. I don't wish to be overly controversial, X, but you might like to take a look at the opening sequence of the Shapiro video I referenced & consider what Daniel Bernoulli had to say about that.... Shapiro does explain, if you have the patience. It was, for me, a reminder of the interesting possibilities offered by a source of high (kinetic) energy gas, & the theory turns out to be fairly "basic" too.... You might observe that Shapiro failed to "catch" the ball at the end of his demonstration. Perhaps that is what you meant... I plead guilty to the charge of (circumstantial) speculation.

[xpensive]

Easy now Dave, what I meant to say is that among all this talk about vortexes, stalling and boundary layers,
it would be useful to repeat what it all is aiming for, namely speeding up the air under the car and nothing else.