Open ended questions about exhausts and downforce

[godlameroso]

I want to start a dialog, and for the purpose of dialoguing I believe it is best to ask open ended questions. So instead of asking what is "the" benefit of exhaust blown diffusers, I would like to know what "are" the benefits. I'm sure there are more than one. This way we can avoid trying to pinpoint exactly what exhausts are doing under a certain guise of inspection, and instead try to take a more systems approach.

I'll start, exhausts heat up the air substantially, this heat gives air the property of increased viscosity, along with some heating of the surrounding air by convection. It's also a property that air expands as it's heated, therefore this is a knock on effect.

So my question is what other factors may be being exploited by teams as they attempt to converge on that one superior exhaust design?

[flynfrog]

So my question is what other factors may be being exploited by teams as they attempt to converge on that one superior exhaust design?

Maximizing power out of the engine? Reducing reliance on blowing to save fuel reduce weight? There is no one optimum.

[olefud]

I’ll bite. The heated air forms thicker, better-adhered boundary layer that, in turn, attaches and directs the slipstream. Also, the exhaust has both kinetic and heat energy that, with provocation, tend to form vortices. Vortices serve to seal and direct less energetic streamline air flows. Given that the rules are attempting to frustrate use of these qualities to enhance diffuser/wing air flow, it will become increasingly necessary to use the unaugmented slipstream to influence exhaust flow. Which gets us back to the imponderables.

[shelly]

My take is: the main benefit of exhaust flow is speed. Pointing high speed exhausts in certain zones on the car accelerates by entrainement adjacent flow. If you manage to have the exhausts accelerating an existing vortex, you can exploit Helmholtz theorem and gain a significant reduction of the pressure in the core of the vortex.

In the diffuser-ebd application those two effects are both at work: the exhausts accelerate the rear wheels wake, thus making it narrower and reducing its negative effect on the diffuser, and accelerate the core of the tip/footplate vortex of the diffuser, creating a narrower vortex with a much loer pressure in its core

[hollus]

To the OP: there is no net expansion due to heating. The expansion of the heated mass of air around the exhaust flow is more or less exactly compensated by the contraction of the cooling gas that served as the source of heat: the exhaust gas itself.

As for benefits, i'll throw a curve ball: Making the exhaust exit somewhere useful eliminates the need of making it exit somewhere where it would be harmful, which is about enywhere. I am sure the aerodynamycists would have loved to design a car without exhausts, or at least they would have until 3 years ago.

[Wicker Bill]

To the OP: there is no net expansion due to heating. The expansion of the heated mass of air around the exhaust flow is more or less exactly compensated by the contraction of the cooling gas that served as the source of heat: the exhaust gas itself.

As for benefits, i'll throw a curve ball: Making the exhaust exit somewhere useful eliminates the need of making it exit somewhere where it would be harmful, which is about enywhere. I am sure the aerodynamycists would have loved to design a car without exhausts, or at least they would have until 3 years ago.

I don't agree with any of that. Why would high momentum flow be harmful from a load point of view?
Shelly seems to have it spot on for me. High energy flow being used to power up load generating structures.

BTW in your first para you're making a wrong assumption, it's not a closed system. You''re injecting heated as into a (let's say) isothermal flow which will cause net heating and therefore net gas expansion.

[hollus]

It is not a closed system, but the surrounding air is more or less at atmospheric pressure, and the exhaust, once out of the pipes, too. Both and their mixture will remain at atmospheric pressure, more or less, at all times. So they do occupy definite volumes.

Let's looks at a very simplified case, where two volumes of gas (exhaust gas and surrounding air) do not mix but just exchange energy.
In that case, in PV=nRT, n is constant and we can write, for each mass of gas, V/T=k with k being constant.
So V1 is the gas out of the exhaust, with temperature T1.
V2 is the surrounding air, with T2.
V3 is the final volume of mass V1, after cooling to T3.
V4 is the final volume of mass V2, after heating to T4.
I'll further assume the masses of each volume of gas are the same (n1=n2) and that then the final temperatures, T3=T4 are simply (T1+T2)/2 (I am not 100% sure that this holds water, but won't be that far from reality). In this case, also, k1=n1RP is equal to k2=n2RP. I'll call that just k.

Then, V1=kT1 and V2=kT2.

V1       V3     V3         2V3              V2    2V4
-- = k = -- = --------- = -----. Similarly, -- = -----
T1       T3   (T1+T2)/2   T1+T2             T2   T1+T2

.....V1(T1+T2)   kT1(T1+T2)   k(T1+T2)                  k(T1+T2) 
V3 = --------- = ---------- = --------. Similarly, V4 = --------
        2T1         2T1          2                         2

Total initial volumes = V1+V2 = kT1+kT2

..............................2k(T1+T2)
Total final volumes = V3+V4 = --------- = kT1+kT2 = V1+V2
                                  2

The volume of a mixture of gases at the same temperature (T3=T4) is the same as the sum of their volumes if they had stayed separated.
I suspect that the case can be generalized, but I don't want to spend the time on it.

So there is no such thing as a free lunch, to heat and expand some gas, you have to cool other gas, which contracts. Overall, you are injecting a lot of high speed gas into free air, but after that, there is no net expansion.

[No Lotus]

So there is no such thing as a free lunch, to heat and expand some gas, you have to cool other gas, which contracts. Overall, you are injecting a lot of high speed gas into free air, but after that, there is no net expansion.

And that's the law, thermodynamically speaking.

[olefud]

So there is no such thing as a free lunch, to heat and expand some gas, you have to cool other gas, which contracts. Overall, you are injecting a lot of high speed gas into free air, but after that, there is no net expansion.

And that's the law, thermodynamically speaking.

While not a free lunch, you can take a portion of the atmosphere, mix it with fuel, combust the mixture, extract a portion of the resulting heat energy and exhaust the remaining heated gas into the atmosphere with a net resulting heating of the atmosphere. More important for our purposes, the exhausted gas temperature is well above ambient even after expansion to atmospheric pressure. For the short period that it is adjacent the car, it will have rather different properties than the ambient air and will tend to form a flow distinct from the ambient slipstream air. Only well after it departs the car’s surface will the exhaust equalize with the atmosphere with the mentioned heating of the atmosphere as a system.

Of course, sans overrun fuel input and with extraction of energy by electrical compounding, the heating will not be of a magnitude as in days past.

[Wicker Bill]

So there is no such thing as a free lunch, to heat and expand some gas, you have to cool other gas, which contracts. Overall, you are injecting a lot of high speed gas into free air, but after that, there is no net expansion.

And that's the law, thermodynamically speaking.

Not unless it's a closed system and it isn't.
PV=nRT being constant is an incorrect assumption given that one gas has different properties to the other.

In any reference volume you might care about (e.g. a volume around the car) the net effect of the interaction of the heated exhaust plume with the freestream volume will be an expansion.

[rjsa]

Too rusty to come around with the math, but I have one of these at home and it just works. All bout energizing the surrounding air

[youtube]http://www.youtube.com/watch?v=gChp0Cy33eY[/youtube]

[Wicker Bill]

Too rusty to come around with the math, but I have one of these at home and it just works. All bout energizing the surrounding air

Simple air amplifier, been around for years in industrial applications. They are a nice example of flow entrainment using a coanda slot but not the same problem as exhaust blowing.

Oh and they are the noisiest fans I've ever heard

[rjsa]

Much to the contrary, it is the exact same principle.

The hot air from the exhaust heats the cold air coming in from the underfloor, making it expand and exit the diffuser faster than it originally would. This in turn accelarates the flow under the floor right before the diffuser entrance reducing it's pressure, hence giving bigger downforce.

Ohh, and I just love the noise of it winding down, sounds just like a big turbofan

[Wicker Bill]

Much to the contrary, it is the exact same principle.

The hot air from the exhaust heats the cold air coming in from the underfloor, making it expand and exit the diffuser faster than it originally would. This in turn accelarates the flow under the floor right before the diffuser entrance reducing it's pressure, hence giving bigger downforce.

Ohh, and I just love the noise of it winding down, sounds just like a big turbofan

No, I don't believe that is true. The heat transfer is too localized to make that reality, the increase in loading comes from adding higher momentum flow into an area that will benefit from it - see shelly's explanation above.

Glad you like the noisy fan, maybe you should install a gas turbine in your lounge room to make it really relaxing

[Tommy Cookers]

Only well after it departs the car’s surface will the exhaust equalize with the atmosphere with the mentioned heating of the atmosphere as a system.
Of course, sans overrun fuel input and with extraction of energy by electrical compounding, the heating will not be of a magnitude as in days past.

currently engines have on the overrun continuously managed power settings to minimalise backdrive torque/aka engine braking (in conjuction with recovery action)
this effect will be increased in 2014
also, on the overrun, compounding would be suspended to retain turbo rpm as much as possible ?

so there would be quite a lot of exhaust heat around ?