Exhaust Blown Floor - Forward Exhaust Exit

[JaymzVsTheWorld]

Does anyone know if there were ever experiments on race cars where the floor was deliberately heated to energise the flow and gain downforce? For example, I could imagine flat panel heat exchagers like the ones tried on Brabham BT46 installed in place of bare floor panels, thus also contributing to engine cooling. But that is just my imagination, so...

I remember a few years ago I hear a roumour about the bodywork having an electric current flowing over it, I think it was Ferrari that tried it out, or so I heard.

And this year almost all of the cars are equiped with more or less reliable sorce of high voltage current (KERS). In the aviation there are some planes, which use controll boundary layer separation by some plasma injection devices near leading edge of airfoils.

I wonder how many kiloplops it would sap from KERS to make a gain in aero performance from the current.

I remember when we were chatting about the rumour, it was said that it was something to do with yaw, and cost too much. Plus it heat up the bodywork too much.

I thought at the time that maybe they would need a good conductor and insulator mixed in with the carbon fibre and paint, gold maybe. But that's most likely complete mince I'm talking!

[segedunum]

Sheet 7 of this thread has some interesting images showing dynamic ride heights amongst the teams. Renault is is lowest by a good margin. It is consistent with the idea that they are running lower heights to make the exhaust block off the sidepod inlets to create, essentially a massive low pressure under the car, starting at the front of the sidepods.

Indeed. Renault are running no visible rake whatsoever and they are visibly closer to the ground. Fuel to the fire......

[ringo]

An interesting point marekk, aerodynamic drag is obviously proportional to density, when dynamic pressure being rho*speed^2/2. Didn't see that one coming, but if so why not route the xhaust to the tip of the nosecone or something?

As for the gasses quickly cooling off, the animations above are cute, but I don't think we should take them to the bank, at full song there's more than 250 kW(think 500 Sunday morning toasters) spewing out from each pipe at a speed much higher than the car itself. What were the input numbers in those images anyway?

Edit: Thanks marekk, these are more likely numbers.

The exhaust has as much power as the engine, so it's 750hp of hot air, 375 per side.
But it's still not enough to have any considerable heating of the ambient.
In thermodynamics the ambient is considered a thermal reservoir, you simply can't change it's temperature with something as small as an exhaust pipe.
Worst if the air is moving at very high speed. It's like resting a newly welded piece of metal in pan of water vs putting it under the Niagara falls.
The heat of that piece of metal will do nothing to make any considerable temperature change to the waterfall.
I am not saying that there isn't any heating of the air, but the temperatures don't stay up for any considerable distance or time.
In the test the car was moving at 70m/s the gases were 800 degrees celcius by the time they flow over the A arms they're about 60 or so degrees celcius.
That's still something, though it's not as high as i initially expected as well.

What renault is doing may be the equaivalent to what torro rosso is doin with it's raised floor.

If you look on this video, you will notice a blue area between the wheels leading to the top of the diffuser. It's a velocity plot, so we can see the air speeds are low in this area.
I think torro rosso's floor is simply a way of getting faster moving air directly to this area, they're not looking for down force directly from the second floor.

the video is without a blown floor.


Renault is probably getting a similar effect but their exhuast gases are more spread out, so they're getting more from the exhaust. The seal the floor slightly, add energy to the same place torro rosso is adding energy, and they also seal the sides of the diffuser.

[marekk]

An interesting point marekk, aerodynamic drag is obviously proportional to density, when dynamic pressure being rho*speed^2/2. Didn't see that one coming, but if so why not route the xhaust to the tip of the nosecone or something?

As for the gasses quickly cooling off, the animations above are cute, but I don't think we should take them to the bank, at full song there's more than 250 kW(think 500 Sunday morning toasters) spewing out from each pipe at a speed much higher than the car itself. What were the input numbers in those images anyway?

Edit: Thanks marekk, these are more likely numbers.

The exhaust has as much power as the engine, so it's 750hp of hot air, 375 per side.
But it's still not enough to have any consdirable heating of the ambient.
In thermodynamics the ambient is considered a thermal reservoir, you simply can't change it's temperature with something as small as an exhaust pipe.
Worst if the air is moving at very high speed. It's like resting a newly welded piece of metal in pan of water vs putting it under the Niagara falls.
The heat of that piece of metal will do nothing to make any considerable temperature change to the waterfall.
I am not saying that there isn't any heating of the air, but the temperatures don't stay up for any considerable distance or time.
In the test the car was moving at 70m/s the gases were 800 degrees celcius by the time they flow over the A arms they're about 60 or so degrees celcius.
That's still something, though it's not as high as i initially expected as well.

Of course, we are not going to change microclimate at grand prix venue with this, but the volume under floor its not that big, we can influence it quite a lot, i think.
And 60 degres plus on A-arms can be definitely considered something in F1, where quick driver is 0.2s quicker then slow one.

[ringo]

Yes of course, it's still worth something as small as it seems.

[xpensive]

When John Barnard introduced the coke-bottle rear end with full-width "canard wings" on the MP4-something in the early 80s, the thinking was that the rear-wheels were to reduce air-speed above the "canard wings" vs under the same.

This reduces the dynamic pressure and as a consequence creates a static pressure differential between above and under the surfaces, Bernoulli in action if you wish.

If the forward xhaust can reduce the air's density above the rear end, it might actually do the same, reduce dynamic pressure and increase static such?

Just thinking out loud of course.

[marekk]

When John Barnard introduced the coke-bottle rear end with full-width "canard wings" on the MP4-something in the early 80s, the thinking was that the rear-wheels were to reduce air-speed above the "canard wings" vs under the same.

This reduces the dynamic pressure and as a consequence create a static pressure differential between above and under the surfaces, Bernoulli in action if you wish.

If the forward xhaust can reduce the the air's density above the rear end, it might actually do the same, reduce dynamic pressure and increase static such?

Just thinking out loud of course.

Not sure if there's enough hot air to significantly change air's density for whole rear end, but blowing part of exhausts over the floor and farther under sidepods will certainly help with keeping this area's flow laminar by energizing boundary layers and preventing flow separation. Looking at R31's pictures form Valencia & Jerez, there's a lot of renault's interest to map precisely temps at the rear end.

[machin]

So as a bit of a summary of Ringo's findings:-

1, The exhaust doesn't stay under the car, nor appear to "seal" the floor.
2, The temperature of the gases by the time they get any distance from the exhaust pipes is pretty insignificant.

My thoughts are now that this feature simply increases the downforce of the aerofoil sections at the leading edge of the floor by blowing gases over them. my reasons, coupled with Ringo's findings, are:-

The exhaust blown diffuser from last year allows the creation of downforce even when vehicle speed is zero. This is good, except that it means the balance of the car changes with road speed, since the extra downforce is at the back (the centre of pressure actually shifts forward with increasing speed as the exhaust flow becomes a smaller and smaller % of the total flow). Last year this balance change could be countered, to some degree, using the variable front wing... but that's not available this year....

By blowing the exhaust gases over an aerofoil feature which is near the middle of the car you get (hopefully) a similar benefit as you would with the EBD except now it won't affect the balance....

[marcush.]

the density thing does only benefit when you direct to the correct surfaces.Put a wing profile into a windchannel lower air density and you got less drag but also less downforce-so just heating up is not helping at all.
Also many d/f dominated cars lose balance and total downforce with rising track temps or ambient temps .the exhaut blowing is a twosided sword.

[horse]

I'm still not convinced we can reach firm conclusions without getting the geometry right. I'm sure it will make a difference.



Note the lack of floor cut-out and position of the turning vane relative to the barge board.

[marekk]

the density thing does only benefit when you direct to the correct surfaces.Put a wing profile into a windchannel lower air density and you got less drag but also less downforce-so just heating up is not helping at all.
Also many d/f dominated cars lose balance and total downforce with rising track temps or ambient temps .the exhaut blowing is a twosided sword.

There are no wing profiles in hot air stream from FEE - just floor's and lower sidepod's skin, diffuser and rear suspension. All of them could benefit from decrease in air density.

I you take a look at ringo's excellent top view of skin pressures, there is clearly visible an airfoil formed from both sidepods - and a big one, something in the range of commercial airliners, but with much bigger camber. There is a low pressure at trailing edge gradually increasing to the rear. What we can't see on this picture is boundary layer of slow air, sticking to sidepods skin, worth a few inches at the rear end.
With this gradient of pressures you have some air near the skin traveling forwards (drag!) with great risk of detaching (generating a lot more drag and preventing clean air's flow to the top of diffuser and beam wing). What one can do about it is to inject some more energized air in this layer, to help it travel longer against this increasing pressure. You can use some vertex generators (turbulent flows in boundary layer stay attached longer then laminar ones) - thats what all of them are doing, but at the cost of more skin drag (and vortex generating isn't free either).
Or you can throw some very turbulent and hot (more energy) gas from exhaust ...

As for downforce generation from exhaust blowing directly on some airfoil, i think quick answer is "no way". There are no airfoils in this region, there is no significant gas volume (after expansion maybe 0.6 m3/s form both pipes), there is no speed either (roughly 38 m/s). And there's no lift form turbulent flow's anyway.
And this is not a google-translation - it's realy my written english

[ringo]

I'm still not convinced we can reach firm conclusions without getting the geometry right. I'm sure it will make a difference.



Note the lack of floor cut-out and position of the turning vane relative to the barge board.

You want to see what you believe.
Ok i'll try and turn it to reality. What i will do is place the pipe under the car completely, completely ignoring the fact that renault's temp stickers indicate the air is flowing on top after curling around the floor and that the upwash from the airfoil shape of the edge of the floor would naturally turn the gases up.

This is like myth busters, i'll go extreme and put the pipe under the floor facing straight back. See if the underfloor this is plausible or not.

[ringo]

So as a bit of a summary of Ringo's findings:-

1, The exhaust doesn't stay under the car, nor appear to "seal" the floor.
2, The temperature of the gases by the time they get any distance from the exhaust pipes is pretty insignificant.

similar benefit as you would with the EBD except now it won't affect the balance....

Well i don't know if they're insignificant, they're just not blazing hot. 50 60 degrees celcius, maybe a little issue for the tyres??

The floor sealing thing seems to be more the job of the airfoil shape that most teams seem to have. I guess the FEE enhances the effect.

I'll try the underfloor thing to appease the supporters and then run a the Normal FEE at a monaco like corner speed.

[ringo]

When running with the pipe under the step plane, the gases stay under the floor but they still divert to the outside. Some escapes and curls over the floor and the rest that is underneath does go under the diffuser.
I wasn't expecting it to stay under.
keep in mind this is not actually what Renault have on their car. Their pipe is not bellow the step plane.
This is just an extreme exercise to see what happens when the exhaust is fully underneath the car.


Either way there is a tendency for the exhaust to move outward from the centre while conforming to the surrounding conditions.

The angle of the pipe also determines where the gases go and how much time they spend under the floor. In this case when the pipe is fully under the floor it doesn't make full use of the airfoils.



From this picture, the pipe seems to be over the step plane, judging from the background. I guess depending on how high it is, there will be a portion going under and over though even when the pipe is fully underneath 1/2 of the flow still comes to the top of the floor.

[machin]

there is no significant gas volume (after expansion maybe 0.6 m3/s form both pipes), there is no speed either (roughly 38 m/s)

And yet it is exactly this "insignificant" gas volume and velocity which creates sufficient downforce when blown over/through the diffuser...???

You can see from this plot that the majority of downforce is generated by the leading edge and diffuser sections of the floor since since this is where there is the greatest change of momentum. The leading edge of my model is pretty basic, and yet it produces a nice reduction in pressure:-

My thinking is that increaing flow in either the diffuser or leading edge region should increase this effect due to higher mass flow.... Doing it in the leading edge portion means that the increase won't affect the balance so much.

The volume flow rate becomes more significant as vehicle speed decreases since it becomes a higher % of the flow.

Maybe?