Slit exhausts -are they worthy?

Here are our CFD links and discussions about aerodynamics, suspension, driver safety and tyres. Please stick to F1 on this forum.
shelly
136
Joined: 05 May 2009, 12:18

Slit exhausts -are they worthy?

Post

Exahusts seem to be one of the hottest topic for 2011 (how hot? ask marekk or ringo for an estimate).

Design path taken by teams are different from each other, for positioning and shape of the pipes.

Some have kept a rounded pipe shape for the exit (Mercedes, Renault), others have gone for more stretched, rectangular shapes (Ferrari, RedBull). Toro Rosso and McLaren (in testing) have, or have tried, very thin and elongated exhaust exits: slit exits.

I think this could be a potential direction of evolution for exhausts exploiting, because thinner but wider exhausts could blow closer to the boundary layer, and could interact more exatensively with surfaces. I see potential for exploitnig coanda effect too.

Most probably from an engine point of view they are not an advantage, but I think that for aerodynamics slit exahusts could be more powerful than circular, or elliptical pipes.

What do you think?
twitter: @armchair_aero

marekk
2
Joined: 12 Feb 2011, 00:29

Re: Slit exhausts -are they worthy?

Post

As most of the teams use retarded ignition to keep blowed stream (and car balance) stable, exhaust can reach temps in excess of 1000 degree celsius.
So you can't use (as for current regulations) materialls other than inconel.
To let engine breathe, interface between rounded and squared parts has to be quite long.
Alltogether quite bulky and heavy solution, but if aerodynamic gains are big enough, we can see some implementations this year, with macca's obviously first to reintroduce it.

shelly
136
Joined: 05 May 2009, 12:18

Re: Slit exhausts -are they worthy?

Post

Rbr and Mclaren have already built their versions of slit exhaust, with MC realisation (obviously?) more extreme and maybe not satisfactory from an engine point of view.

In the forward exhaust thread, and partly on cars threads, exhausts have been discussed extensively. The possible advantage from their usage have been outlined to be mainly increased speed and increased mass flow in certain areas.

In this thread I would like to discuss the potential of the exhausts in tems of perturbation of the exaternal flow field, i.e. entrainement, boundary layer energisation, pathline deviation.

For example if we discuss ferrari exhaust positioning in australia, we can agree that there they are placed with a main aim of interaction with surfaces (rear wheel winglets, diffuser footplate), rather than with far from surface flow, even if of course both phenomena take place at the same time.

If blowing exhaust flush or nearly flush to aero surfaces is the main aim, I think that we will see an evolution towards slit exits, making exhaust start their blow as thin sheets.

Maybe exhausts gases plume (the first part, before they lose shape in mixing), could be shaped to create virtula fences or skirts, as someone as already suggested in other threads.

I know that we are a bit too much on the sci-fi side, but I would like to share opinions.
twitter: @armchair_aero

User avatar
horse
6
Joined: 23 Oct 2009, 17:53
Location: Bilbao, ES
Contact:

Re: Slit exhausts -are they worthy?

Post

According to autosport Williams are now taking on the slit exhaust design after rejecting the Renault and Mercedes system.

I am wondering whether is has something to do with interference from the exhaust on the operation of the beam wing and diffuser Gurney. I mean, they used to pump the exhaust up into the air and out of the way, right? Perhaps the turbulence of the standard low exhaust is outweighing the benefits of hot gases / exhaust ploom in that area.

Image

Perhaps then the slit exhaust offers the best compromise. The exhaust gases do not interfere with the aero about the beam wing and diffuser Gurney but they do offer benefits around the sides of the diffuser with "virtual skirts" and / or boundary layer effects, tire squirt, etc, etc.

Image

EDIT: Am I talking about the same type of exhaust as the thread is about or is this about this initial Macca concept?
"Words are for meaning: when you've got the meaning, you can forget the words." - Chuang Tzu

shelly
136
Joined: 05 May 2009, 12:18

Re: Slit exhausts -are they worthy?

Post

@horse: I have explained (and wanted to share) my point of view on melbourne mclaren exhaust in the mp4-26 thread, on page 147 IIRC.
That was also related to why ferrari does not have the famous 5 cm opening: I think that exhust exiting in that position roll themselves up around the tip vortices of the floor and enter he diffusre from below.

In this thread I wanted to push the discussion a bit furher about possible use of exhausts (if still possibel with recent contraints on materials).

As for now, exhausts blow let us say more ore less "along" the normal flow, adding energy to it; stiil it has pointed out in the fee thread already that they also act like virtual skirts, forcing the normal flow to take different direction.

So I would like to go deeper on this subject, and discuss the merits of the systema mclaren ran in testing (not the multi exit octopus, but the one with two exit on the sides) which is some how similar ot toro rosso's.

I think that interest in exhaust blown diffuser is driven by three factors:
-rear downforce hunger
-improvements in high temp materials
-improvement on engine management (now that engine developement towards higher power is blocked)


Moreover, ebd is a newey classic (I remember mp4-16)
twitter: @armchair_aero

User avatar
horse
6
Joined: 23 Oct 2009, 17:53
Location: Bilbao, ES
Contact:

Re: Slit exhausts -are they worthy?

Post

Mhmmm, I saw your post in the McLaren thread, and I think I have decided to 100% agree with you now (maybe 99.5% - need to leave some room for maneuver) about the exhaust entering the diffuser at the edges. I guess that the advantage of this is boundary layer excitement? Drawing in from the sides would have negative effects on the pressure, no?

It's a shame that we can't discuss the RBR exhaust some more as it seems to becoming the preferred option with 3 possibly 4 teams having similar solutions. Note that the Gurneys are greatly reduced at the edges of the McLaren where the exhausts are now positioned (although there's probably not much point having a Gurney out there anyway).

Image

My feeling for this is that this is because Gurneys might not work with exhaust flow. If it's too turbulent then the separated flow before the Gurney will spill over the top with negative effects.

Anyway, I guess that is off-topic. Another thread mods?
"Words are for meaning: when you've got the meaning, you can forget the words." - Chuang Tzu

marekk
2
Joined: 12 Feb 2011, 00:29

Re: Slit exhausts -are they worthy?

Post

We are talking 3 different scenarios IMO.

1. Blowing over the floor in rear direction.
Possible gains: skin friction decrease, boundary layer thickness decrease, flow speedup to the gurney and beam wing...
Risks: low pressure area on the top of the floor (lift), very turbulent flow (decrease in gurney's and beam wing's effectiveness)...

2. Blowing over the floor to the sides
Possible gains: exhaust gases attaching to the flow from outside to diffuser, helping with skin friction and boundary layers under the floor and in diffuser, separation of flows over and under the floor.

3. Blowing under the floor to the sides of diffuser.
Jet pump with added benefits of thermal reduction of friction and BL's thickness.
This is probably simplest scenario - you have to use maximum of allowed 5cm and variate only outlet's hight to find best compromise between exhaust's speed and engine's breathing requirements. Can be very effective, if they manage to get serous speeds in this exhaust flow, both from direct interaction with outer parts of diffuser and jet pump effect.

shelly
136
Joined: 05 May 2009, 12:18

Re: Slit exhausts -are they worthy?

Post

@marekk:

I think mclaren and ferrari follow your 2. solution. The point is that with that they have a downforce increase on the tip vortices trapped under the floor footplate.

Maybe rbr is on 3. (haven't seen a picture yet of their 5cm opening) and want to use exhaust like an ejector through a 5cm throat; that could be very effective.

What I wanted to discuss here is: let us take tororosso shaped exhaust and put them on the lateral edge of the diffuser: it will end up looking like mclaren's slit run in tests (see scarbs blog for photograph).

In his blog, scarbs elaborates on this adding that those exhausts would nee bending to blow under the floor; I think instead that they could blow horizontally, in lateral direction, and then get turned a relevant amount of degrees(30? 45 more?) by coanda effect around an upperly rounded floor edge.

This would create a thin virtual skirt, which I think it could be very effective.

In the end actual mcs and ferrari create a simialr virtual skirt n the diffuer lateral edge betwen the wheels; I think that the solution I eplained could b more effective.

What do you (horse and marekk, atm, but I hope more people will come) think?
twitter: @armchair_aero

User avatar
horse
6
Joined: 23 Oct 2009, 17:53
Location: Bilbao, ES
Contact:

Re: Slit exhausts -are they worthy?

Post

Ok, you mean this concept and potential explanation here (Liubomir Asenov, Scarbs):

Image
Image

It's a little hard to see the true route of the exhausts here but lets assume that scarbs is right in that the exhaust drops through the floor and then you are saying that it will round the side of the floor and go above the floor?

It doesn't seem like a very stable solution to me as you are kind of running up hill against the natural inclination of the flow to enter the diffuser, if I am interpreting your meaning right?
"Words are for meaning: when you've got the meaning, you can forget the words." - Chuang Tzu

shelly
136
Joined: 05 May 2009, 12:18

Re: Slit exhausts -are they worthy?

Post

horse wrote:Ok, you mean this concept and potential explanation here (Liubomir Asenov, Scarbs)
Yep.
horse wrote:It's a little hard to see the true route of the exhausts here but lets assume that scarbs is right in that the exhaust drops through the floor and then you are saying that it will round the side of the floor and go above the floor?
Scarbs drawing is just hypotetical: his opinion is that for exahusts to work, they would have to bend downwards with their pipes. I agree with scarbs that they will be blow downwards, but I think this could be achieved by blowing horizontally around a curved edge, exploiting coanda effect. After coand induced rotation, exhaust will blow no more horizontally, but from up to down (even if not exactly vertical. So with coanda: horizontal pipes and vertical (diagonal) exhaust gas sheet.
horse wrote:It doesn't seem like a very stable solution to me as you are kind of running up hill against the natural inclination of the flow to enter the diffuser, if I am interpreting your meaning right?
You are nto uphill: you are putting some kind of "exhaust waterfall" or virtual skirt pouring up to down form the side edge of teh diffuser. Air entering from the sides will have to go around it, and it will push some of it under the diffuser.

Hope that I have explained better
twitter: @armchair_aero

User avatar
horse
6
Joined: 23 Oct 2009, 17:53
Location: Bilbao, ES
Contact:

Re: Slit exhausts -are they worthy?

Post

Ok, yes I see what you are saying now, it's an interesting concept, indeed. I guess it is made more complicated by the exhaust exiting perpendicular to free stream, whereas in a lot of other applications the effect is applied tangent to the free stream.
"Words are for meaning: when you've got the meaning, you can forget the words." - Chuang Tzu

shelly
136
Joined: 05 May 2009, 12:18

Re: Slit exhausts -are they worthy?

Post

Yes the idea of somehow "go against" or deflect normal flow to sme seems something a bit different comapared to current applications, with probably r31 fee being a partial exception.

Afrter some googling I think that curving exhausts the way I explained above with coanda effect could generate a significant lift around the rounded edge: it could be that this disadvantage could not be compensated.

But apart from the particular idea of blowing 90° to car direction to create a virtual skirts, I see more potential for flat streched exahust shapes.

For example the same warping effect we discussed for mclaren could be obtained with mid position exhaust (merc position) around the outer lateral edge of the floor.

If intead merc want to focus on sidepod blowing, with a streched exhaust they could blow a bogger height compared to their actual.
twitter: @armchair_aero

marekk
2
Joined: 12 Feb 2011, 00:29

Re: Slit exhausts -are they worthy?

Post

shelly wrote:@marekk:
What I wanted to discuss here is: let us take tororosso shaped exhaust and put them on the lateral edge of the diffuser: it will end up looking like mclaren's slit run in tests (see scarbs blog for photograph).

In his blog, scarbs elaborates on this adding that those exhausts would nee bending to blow under the floor; I think instead that they could blow horizontally, in lateral direction, and then get turned a relevant amount of degrees(30? 45 more?) by coanda effect around an upperly rounded floor edge.

This would create a thin virtual skirt, which I think it could be very effective.
What do you (horse and marekk, atm, but I hope more people will come) think?
To get 30-45 degree of flow bend you need 3-4 slotted flaps in addition to main wing profile.
Even for wing with very small aspect ratio like 3 (aspect ratio is wing_span^2/wing area) flow start to separate at 20 degree. More common used airfoils stall at < 15 degree. So i don't see it work like a skirt, even if positioned just next to the edge of the floor.

edited to add:
As i wrote in many threads earlier, we tend to overestimate kinetic energy of exhaust flow. Even at 100 m/s our calculated mass flow of 0,24 kg/s pro pipe will give us just 1,2kW of power (m*v^2/2). At 200 m/s, which i consider extreme, 4,8kW. It's only a very small percent of total energy of exhaust gases (we agreed on 300 kW pro pipe), not worth to bother at all IMO.

User avatar
horse
6
Joined: 23 Oct 2009, 17:53
Location: Bilbao, ES
Contact:

Re: Slit exhausts -are they worthy?

Post

shelly wrote:Afrter some googling I think that curving exhausts the way I explained above with coanda effect could generate a significant lift around the rounded edge: it could be that this disadvantage could not be compensated.
Yes, that's a good point. By redirecting the flow down you are effectively creating a lifting surface (like many applications of coanda).
shelly wrote:If intead merc want to focus on sidepod blowing, with a streched exhaust they could blow a bogger height compared to their actual.
Hmm, yes, that's an interesting one, sort of hugging the side pod, I guess? Keeping the gases closer to the centreline might well be beneficial (like a hybrid Merc + Sauber). Like marekk says, though, the influence from your gases can only go so far, whereas you get a strong local effect, their impact downstream is most likely less.
"Words are for meaning: when you've got the meaning, you can forget the words." - Chuang Tzu

BreezyRacer
2
Joined: 04 Nov 2006, 00:31

Re: Slit exhausts -are they worthy?

Post

IMO, the Red Bull type of layout is all about controlling the rear tire aero wake and keeping it out of the diffuser box. The various upper ventings thru the floor in front of the rear tire are about minimizing that same wake.

You guys really need to understand just how much wake/turbulence comes off those rear tires.