2012 Exhaust Blowing & Coanda

[Pup]

I agree with Shelly - the whole idea of the blown diffuser is to mitigate that airflow.

Looking again at smikle's CFD, I'm really surprised at how effectively the system works. It's nice to be proven right for once.

[Richard]

Hi T-bone

This tread is a general conversation about exhaust blowing. It was created by moving posts from the W03, RB8 and Mp4-27 threads. We have to do this with some topics each year because we end up with 3 or 4 car threads all talking about the same thing and it kills any discussion of the actual car.

So please do compare cars.

[Pup]

1. If the exhaust is deflected more at low speed does this mean that the exhaust flow hits the rear tyres (and hence heats them) more at slow speeds? If so, then presumably this is a real advantage when preparing for qualifying runs, race start and even in getting traction out of low speed corners? If this is the case then might this not present overheating issues during extended safety car periods?

Well, I was thrown into the ringer earlier for saying that was part of the design, but I believe it's a very important element of their concept. I think the fact that the exhaust hits the floor and then disperses around the tire fairly evenly is the key. Also, the exhaust isn't right up against the tires like last year.

[shelly]

@pup: interation of exhausts and footplate vortices is not about mitigation, but about incrasing the intensity of the vortices, and at the same time sweeping away tyre squirt.

[Just_a_fan]

This is fascinating stuff - afraid I'm rather new to this so please excuse the lack of understanding but I have the following questions:

1. If the exhaust is deflected more at low speed does this mean that the exhaust flow hits the rear tyres (and hence heats them) more at slow speeds? If so, then presumably this is a real advantage when preparing for qualifying runs, race start and even in getting traction out of low speed corners? If this is the case then might this not present overheating issues during extended safety car periods?

It was suggested last year (and raised by me and then poo-pooed by others) that RedBull were using an exhaust strategy to heat the tyres for qualifying and the first lap of the race. It would seem that this strategy is open to McLaren if the analysis presented here is representative of the flow that McLaren have attained on the car.

As for safety car periods, there are two things to remember - the tyres get cool during such periods (hence the drivers weaving etc. behind the safety car in order to maintain tyre temps and thus pressures) and suring saftey car periods the cars are usually run in a low-fuel setting. This will reduce the energy in the exhaust anyway. It is conceivable that McLaren will turn up their engines earlier than normal at the end of the safety car period in order to maximise heating and so grip at the rear. Who knows.

[Pup]

@pup: interation of exhausts and footplate vortices is not about mitigation, but about incrasing the intensity of the vortices, and at the same time sweeping away tyre squirt.

I don't quite get that. Not that I'm doubting you; I'm just a bit foggy-brained about it.

Granted, I've never quite understood the idea of 'sealing' the diffuser with very energetic, low-pressure air, but that seems to be the consensus around here so I've run with it.

[shelly]

The lower part of the tyre has a wake that starts from the contact patch and grows wider towards the back. If you blow in proximity of the side of the tyre at high speed you will make the expansion angle of ths wake smaller.

So slow air fom the tyre wake won't interfere with the diffuser footplate, an more air will come from the top of it (where the temp sticker was spotted on mp4-27) and will curl around its edge, forming a vortex.

If you manage to accelerate this vortex along its axis, it will be narrower too, so it will have lower pressure, i.e. downforce, collected by the footplate itself. The upwash inside the diffuser could also be exploited.

Maybe word explaination is obscure, please tell me if so.

[Crucial_Xtreme]

The exhaust is less effective at getting to the floor at higher speeds. The pictures of the CFD sim show more flow spilling over the foot step and getting under the floor which says to me it's current configuration is definitely helping in the low-medium speed corners. Question is if there's less exhaust getting to the floor at higher speeds how is the car recovering the df compared to what it has at a lower speed??

**Edited for grammar

[tok-tokkie]

I have been eagerly looking forward to that CFD. Many thanks Smirkle.

The tire warming: It seems Vettel was using it last year enabling him to do his banzai Q3 qualifying & also his great race starts. Anyone have an idea why Webber did not seem to benifit? Question is vaguely linked to the thread but move it where it belongs mods.

[Pup]

The lower part of the tyre has a wake that starts from the contact patch and grows wider towards the back. If you blow in proximity of the side of the tyre at high speed you will make the expansion angle of ths wake smaller.

So slow air fom the tyre wake won't interfere with the diffuser footplate, an more air will come from the top of it (where the temp sticker was spotted on mp4-27) and will curl around its edge, forming a vortex.

If you manage to accelerate this vortex along its axis, it will be narrower too, so it will have lower pressure, i.e. downforce, collected by the footplate itself. The upwash inside the diffuser could also be exploited.

Maybe word explaination is obscure, please tell me if so.

Oddly enough, that makes sense to me.

[amouzouris]

that makes sense to me too!

[hollus]

Thanks a lot for that n_ smikle.

I think your exhausts are modeled too oblique. The rule says that they should blow +/- 10 degrees from the car's centerline, your looks more like 20-25 degrees away. But that would probably just result in a slight change of position of the washout more towards the center of the car without changing the overall trends. It would also reduce the somewhat excessive amount of hot air hitting the tire at 30m/s.
Impressive how the exhaust gas just tries to break through the floor in the lateral 30m/s pic!
It also looks like the vertical cutout is there not only for legality reasons, but also to deflect the exhaust further down!

[hardingfv32]

n smikle, Thanks for the effort.

1) How is the flow axis from the exhaust pipe specified?

2) What are the details of the drawing describing the outlet and shape of the channel?

3) Why do we not see the general air flow around the body? You can hide this flow and only show the flow of the exhaust?

Brian

[PlatinumZealot]

The exhaust is less effective at getting to the floor at higher speeds. The pictures of the CFD sim show more flow spilling over the foot step and getting under the floor which says to me it's current configuration is definitely helping in the low-medium speed corners. Question is if there's less exhaust getting to the floor at higher speeds how is the car recovering the df compared to what it has at a lower speed??

**Edited for grammar

Also, the effect is much weaker than last years. You can see how much the exhaust has spread out and slowed down. My guess it might only be a few tenths of gain rather than that honking 2 seconds that the EBD was giving last year.

[hardingfv32]

In this illustration, using the scale provided the exhaust flow leaves the pipe at around 100 m/s while it was specified that the exhaust flow used was 348 m/s. What is the reason for this?



Brian