Exhaust Blown Floor - Forward Exhaust Exit

[marcush.]

your graphics are correct manchild but i don't think your other assumption is. As you mentioned in the other thread, there's nothing you're allowed to do under the floor. That and the rules effectively forbid multiple exits as well.
@giblet, most of the heat will have dissipated by the time it reaches the tyres, if it were really that hot, there'd be more heat shielding in the front.

@ringo, i think you finally found something the rest of us hadn't seen. marcush.'s explanation is linguistically unintelligible so i'm discounting it

hehehe ..I guess the german tendency for not being able to produce a sentence shorter than 1km is just too much for you...
I will keep trying to improve ..(say more in less words).

[myurr]

Just a quick thought. I remember a while back a rumour on this board that McLaren's MP4-26 had side pod inlets shaped like an upside down L. That could be explained if they wanted to supply cooling air to the outside of the exhausts as they route through the car...

[imightbewrong]

Shows a bit better how the exhausts are pointed onto the edge of the floor

[FW17]

Is this allowed this year? The exhausts can still be taken below

[horse]

Some stuff from the Renault thread...

I wonder about one additional thing. If all that previous speculations are correct, how are they making hot air travel under the floor to the diffuser, when exhaust pipes are pointed sideways rather then directly to the back? Wouldn't it make it escape at sides instead of going to the back of the car?

The car has considerable foward motion remember, with a great deal of air being sucked under the car. I see no reason why the exhaust gases should not be being drawn under the car also. They have made the cut out to put it under the side pods, you can see that from the images.

Rather than the exhaust outlet being designed for aerodynamic flow purposes, could it be that the intention of placing the outlets where they are is to......heat up the track, to give a massive advantage on rear wheel adhesion on low track temperature circuits ?

I'd be surprised if you could make a significant impact on track temperature in such a short amount of time, TBH.

If it works the way I think it does (no guarantees implied or otherwise), then I think it's more like a wing turned sideways, drawing air from the sides rather than from above or below. So we're talking about just increasing the pressure on top of the diffuser/floor rather than decreasing it underneath.

Can you explain a bit more about how this works? Would such an approach not be easier just blowing over the top, or something nearby to the top of the diffuser?

I don't think there's a problem with the boundary layer detaching under floor, since the rake isn't all that. Maybe at the diffuser, maybe back when they were much larger. So I think blowing the exhaust under the floor would be entirely counter-productive.

I'm not talking about shedding, in fact the opposite. I was thinking that the hot air might stop a boundary layer forming allowing a more uniform air flow under the car.

Now from this thread.

It has splitter within, and it is positioned so that exhaust gasses are split by the floor, so part of the them goes below the floor, part above = virtual skirts.

How does that work though - how do you stop the two air streams mixing? It's not as if one is oil and one is water.

[n_anirudh]

Would it be fair to say that better scavenging occurs when exhausts are let out forward of the floor, assuming that the lower pressure sucks the gases out as compared to exiting at the rear?

WOuld the hotter gases decrease the density of the air underneath the car??

[SLC]

It's 100% an aero benefit.

By increasing the mass flow over the floor LE you end up with a stronger initial suction peak (where the air is accelerated around the allowed 50mm radius coming off of the step plane), and this sets the baseline pressure for the forward half of the floor. This increased strength suction peak will draw in more flow from the T-tray and BBoard lower edges - so in turn you end up strengthening the floor vortex system as well.

It will obviously have an effect on the diffuser and diffuser kink-line performance as well. In general the mass flow under the flow is set by the height of the diffuser exit (and therefore by the pressure recovery the flow experiences), but by artificially introducing more mass flow at the floor inlet you increase the general energy level available for the diffuser kink line to accelerate. Thus, the amount of suction generated by the rear of the floor increases (or, in other words, the more aggressive you are able to make the diffuser kink profile without danger of separation).

There is at some point a balancing act between the load generated by the forward portion of the floor and the rearward portion. The harder you push the floor LE vortex system the more total pressure is lost... leaving the rear of the floor with less energy to work with. This is an on-going battle between front and rear development.

The design aim has nothing to do with heating anything up, either the air or the tyres (or the track surface :p )

Oh and in general, boundary layer buildup on the lower floor surface is not really a big concern. Granted, the increased mass flow effect provided by the forward exhaust will be helping to decrease the adverse pressure gradient along the floor - but seeing as it isn't all that bad in the first place it's not a big deal.

[Raptor22]

from the renault r31 thread

n smikle wrote:
It has energy in the form of speed and temperature. When you impede it's motion or diffuser it the pressure will rise. Naturally it is going to drop to atmospheric after some time outside the pipe, but for example if ones uses their hand to block the pipe and you will see the pressure can get high.

There was a pdf on a CFD analysis of the blown diffuser floating around the internet. the pressure behind the exhaust was higher. I do not know how much higher though, but it can be significant if the exhaust is impeded.

Sorry but thats not correct at all. What you are feeling is the pressure rise against your hand not the exhaust pressure. When thehot gas leaves the boundary of the pipe it moves to atmospheric pressure instantaneously. Your hand is feeling pressure due to the velocity of the gas and is not a pressure that will be felt by the underside of the car.

The airflowingunder the car will be accelerated by the faster moving exhaust gas. It is this acceleration of the normal airflow due to the added kinetic energy and thermal energy loss to expansion that accelerates the air flow under the car. That drops the pressure under the car increasing downforce under the stepped bottom.
it is this alone that is doing the magic, nothing else.

Using a exhaust exit in or near the diffusor is slightlyless effective because you impart less kinetic energy to the freestream due to loss of knetic energy due to skin friction. Both concepts allow for accelerating the air flow under the car.

Lotus Renaults solution may have a slightly higher expansion ration resulting a few kilo more downforce but that expansion is now taking place over a much bigger surface area which could negate the benefit.

[Raptor22]

Would it be fair to say that better scavenging occurs when exhausts are let out forward of the floor, assuming that the lower pressure sucks the gases out as compared to exiting at the rear?

WOuld the hotter gases decrease the density of the air underneath the car??

yes the hot gas could decrease the density. The hot exhaust gas has kinetic energy and thermal energy which causes the gas to expand before it cools. The exoansion sppeds up the air flow, lowering the pressure.

wrt to improved scavenging, probably marginal effect since the exhaust exits at or ahead of the stagnation point so the surrounding gas will be at a higher pressure than under the car.

[mith]

I had this very same idea posted in the R31 thread...does two fools thinking the same create a truth? (In that case it was one fool ...I cannot judge for your state of mind)

to me the thing looks like they are blowing the cup holders with the hot exhaust gas .These are airfoil shaped and in side view of the car you can even see how the profile forms a gurney -even if it´s longitudinally orientated.
With the exhaust blowing this wing section this generates downforce ,downforce close to the leading edge of the sidepod,near the CoG of the car this cannot be a bad thing even though ,admittedly the span of that wing is not big.

But what about the outer boundaries of this airflow? will it follow the undercut of the sidepods ?

If nothing else hot air is considerably less dense than cold air .So putting hot air into areas of positive lift or not usable for producing downforce due to their shape this could be used to reduce drag?

I couldn't understand it yesterday, but with Ringo's drawing it became clear. I'm not the expert here, but I think you both are right.

[HungryHebbo]

I had this very same idea posted in the R31 thread...does two fools thinking the same create a truth? (In that case it was one fool ...I cannot judge for your state of mind)

to me the thing looks like they are blowing the cup holders with the hot exhaust gas .These are airfoil shaped and in side view of the car you can even see how the profile forms a gurney -even if it´s longitudinally orientated.
With the exhaust blowing this wing section this generates downforce ,downforce close to the leading edge of the sidepod,near the CoG of the car this cannot be a bad thing even though ,admittedly the span of that wing is not big.

But what about the outer boundaries of this airflow? will it follow the undercut of the sidepods ?

If nothing else hot air is considerably less dense than cold air .So putting hot air into areas of positive lift or not usable for producing downforce due to their shape this could be used to reduce drag?

I couldn't understand it yesterday, but with Ringo's drawing it became clear. I'm not the expert here, but I think you both are right.

I'd thought along these lines also when the pictures were released, it does look as though the upward curve could have something to do with it. So then I got to thinking that surely when the exhaust velocity is high, it'll have some 'blocking' effect on the air flow underneath the floor? It is, after all, pointing near-as-damnit perpendicular to the flow...

I also wondered whether other cars also had the upturned area of floor near the sidepod entrance - turns out they do and have done for quite some time. I can only assume that's some sort of vortex generator on other cars though, so I'm not convinced anymore that the Renault's upturned bits ARE acting as a wing.

[Giblet]

@giblet, most of the heat will have dissipated by the time it reaches the tyres, if it were really that hot, there'd be more heat shielding in the front.

A car in F1 can not follow another car for too long or it will overheat. I think you might be underestimating the amount of heat that comes out of those pipes at full song. It might not be enough to set CF alight downstream of the exhaust grasses, but I think it's quite reasonable to see Renault's tires in the rear to be up to temperature faster than those with rear facing pipes. Enough of an advantage to have heat in tires quicker on restarts.

[Lurk]

Overheating is mainly due to less air on sidepods.

[Giblet]

Good point Lurk, thanks.

The drivers can also feel the heat coming off the cars in front however. In a sport of lots of minor effects making a car faster, this is just one more thing to add to the list.

Can anyone shed any light on how much heat would be going towards those rear tires, if any?

I am merely speculating, but I would be surprised to find the exhaust gas to be devoid of heat by the time it hits the rears.

[imightbewrong]