Exhaust Blown Floor - Forward Exhaust Exit

[BreezyRacer]

While I agree that the bottom itself is flat I also believe that they take advantage of the generous 50 mm radiusing to flow the exhaust under the floor to isolate the undertray. BTW, here's a review from an F1 aero designer on the car as well as some others .. worth the light read.

http://www.f1fanatic.co.uk/2011/02/23/m ... s-rinland/

[ringo]

edit: On second thought, let's not go off topic. Since we're not done dealing with the R31.

[shelly]

I think that the most convenient way to imagine the effect of forward exhaust is this.

The interface between exhaust gas and air comig from up front is like a virtual wall,dome shaped, protruding from the leading edge of the floor.

This virtual wall is not 100% solid, because of mixing; still is partially seen as an obstacle from the upcoming air, which has to turn around it;air coming from front in fact sees a virtual leading edge shape much more aggressive than the real carbon leading edge, subject to 50mm radius rule.

The acceleration of air around the virtual leading egde causes pressure to drop and deflects part of the jet (most of it) under the floor, which is fed by this additional mass flow plus the suction enhancement.

I think that the shape of the exhaust outlet will evolve to a flatter, more squared shape to change the shape of the virtual wall and make it more 2dimensional and more effective.

The idea came to me from potential theory. WHat do you think?

[bot6]

The idea has some merit. Essentially, you're talking about using the exhaust flow as a thermal version of the old skirts...

The only thing is that the flow surrounding the exhaust flow will have influence on that exhaust flow. Even if they don't intermix to a big extent, they will still interact, and the normal and exhaust flows will follow each other according to the pressure gradients caused by the floor and diffuser. So it might work to some extent, but I don't think it's the dominant factor.

[shelly]

I am not thinking of old skirts, but of leading edge of the floor.
If you take a lateral view of the car, you would see the mixing line as a bow shaped curve, with the apex slightly bended downward.

The mixing line is a line of zero average velocity in the reference of the car; intense mixing takes place across this line, but the outer flow sees it as boundary.

If you imagine the car exhausting thick colured smoke like aerobatic planes you would have a visulisation of the added virtual bodywork like a cloud that is thick near the leading edge (where the effect is dominant) and then strands away under the floor and towards the rear of the car as complete mixing takes place.

I hope that now I have explained myself better.

[bot6]

Basically like a local "virtual" lowering of the floor near the leading edge?

[marekk]

I am not thinking of old skirts, but of leading edge of the floor.
If you take a lateral view of the car, you would see the mixing line as a bow shaped curve, with the apex slightly bended downward.

The mixing line is a line of zero average velocity in the reference of the car; intense mixing takes place across this line, but the outer flow sees it as boundary.

If you imagine the car exhausting thick colured smoke like aerobatic planes you would have a visulisation of the added virtual bodywork like a cloud that is thick near the leading edge (where the effect is dominant) and then strands away under the floor and towards the rear of the car as complete mixing takes place.

I hope that now I have explained myself better.

I'm afraid fluids don't work this way:


There is simply no reason for both flows to not intermix.
Of course the momentum from both flows are conserved, so resulting flow is bended accordingly, but there will be no virtual wall. Both flows just follow pressure gradients.

My understanding of how it works is basically that as you add this highly energetic exhaust flow (both temp and speed are in fact energy), you allow low pressure area starting at the leading edge of the floor to reach further rearwards, creating more downforce due to greater area it acts on. You need every joule of energy to fight against this increasing pressure gradient.
It achives it probably by reducing the thickness of floor's boundary layers of (more or less) stationary air, so you achive more mass flow to the rear. But it's just my imagination.

[forty-two]

But that plot shows "fluid" entering a box, not a narrow slot, which is formed by a flat (apart from the plank) floor travelling over the ground.

I think the "virtual skirt" idea has a good deal of merit.

[volarchico]

I'm afraid fluids don't work this way.
...
There is simply no reason for both flows to not intermix.

+1

[shelly]

@marekk: I know that fluids mix. But in this situation the will always be a small zone before the exhaust in which the fludi is 99% from the exhaust, and mixing will take place a upstream and then on the upper and lower side.

For example, if you thin of dye injection in water, the zone near the injector will always have 99% dye and then mixing will take place , but in the nearby location.

The picture you posted is misleading, because it it not referred to a jet pointed opposite to average flow.

So, marekk, of course fluids mix, but here we are thinking of what appens in the zone where they are yet to mix.

[Raptor22]

@marekk: I know that fluids mix. But in this situation the will always be a small zone before the exhaust in which the fludi is 99% from the exhaust, and mixing will take place a upstream and then on the upper and lower side.

For example, if you thin of dye injection in water, the zone near the injector will always have 99% dye and then mixing will take place , but in the nearby location.

The picture you posted is misleading, because it it not referred to a jet pointed opposite to average flow.

So, marekk, of course fluids mix, but here we are thinking of what appens in the zone where they are yet to mix.

some seem to assuming the exhaust flow is a jet, it is not. It is only a jet withing the confines of the exhaust pipe. Once it leaves it is just hot gas with rapidly dissipating kinetic energy. Yes within the short distance form the exit to the leading edge of the floor you have an increased gas flow velocity due to the slowing exhaust gas still having more kinetic energy than the air flowing from the splitter.

SO yes your thinking has merit but only due to the action of the exhaust gas on the leading edge. The bulk of the action is the increased mass flow to the diffusor where the magic happens.
The increased mass flow is what increases the diffusor eficiency with the long throat created byt the cars long bfloor.

Red Bull add mass at the satrt of the diffusor Renault are doing it further upstream

[shelly]

@raptor22: why do you keep saying that exhaust is not a jet? It is.

[Raptor22]

it is not a jet because the gas looses kinetic energy very rapidly once it is outside of the exhaust pipe. It is only a jet of hot gas within the pipe.
Granted it the loss of momentum is not instantaneous but it is very rapid and it is expanding immediately once it is beyond the boundary of the pipe.

the fact the car is moving at 300km/hr does not make the exhaust gas a jet. It cannot be focussed and channeled over a relatively large distance without significant change in cross section.

[shelly]

@raptor22: I agree that th efact that the car travels at 300kph does not make exhaust a jet.
It is the fact that exhaust speed is more than 600 kph that makes it a jet.

[Raptor22]

600kph at the exit, and 10kph an inch further downstream.