Sidepod design and the Meredith effect

[tomislavp4]

I think most of you are familiar with the Meredith effect, used in the P-51 aircraft where it reduced the drag of the cooling system by around 90%. Although its effect is negligible below 300mph (according to Meredith) we can observe the sidepods of todays F1 cars are shaped to exploit the effect so that may not be true.

Meredith wrote in his paper that the thrust developed by the system would be greatly increased if exhaust gasses were also vented in the ducting, somewhere after the radiator. The exhaust pipes of an F1 car are inside the ducting so the exhaust system does increase the effect, but the gasses exit outside the sidepods. What I’m wondering is, has someone experimented with exhaust exits inside the sidepods on a F1 or a prototype car?

If nobody did, why not? Is generating DF with the exhaust gasses more beneficial then reducing drag? Is the effect negligible? Are there other potential issues? I'm thinking such a system could be advantageous on high speed tracks such as Monza.

[DaveW]

Interesting historical review from NASA's perspective here, particularly Section 5 for the current topic (but not F1).

and here is the original report, I think.

[flyboy2160]

Thanks Dave. Great finds! +1

[flyboy2160]

..What I’m wondering is, has someone experimented with exhaust exits inside the sidepods on a F1 or a prototype car?

If nobody did, why not? Is generating DF with the exhaust gasses more beneficial then reducing drag? Is the effect negligible? Are there other potential issues? I'm thinking such a system could be advantageous on high speed tracks such as Monza.

I don't recall ever seeing the internal exhaust duct attempted in F1. The closest I can recall was Ferrari's recent (failed) attempt at surrounding the exhaust pipe with an annular side pod exit.

I suspect that the use of the exhaust to help seal the diffuser is considered much more important than any potential thrust benefits.

[Just_a_fan]

Agreed. I don't see any evidence of the designers making use of the Meredith effect on the cars. I would expect that the radiators are probably less of a drag source than the wheels and the rear wing to be honest.

[tomislavp4]

Interesting historical review from NASA's perspective here, particularly Section 5 for the current topic (but not F1).

and here is the original report, I think.

Yep, been through those

I suspect that the use of the exhaust to help seal the diffuser is considered much more important than any potential thrust benefits.

Yeah, It's all about downforce these days. I was expecting at least someone to had tried it in the early days but no one did, it seems. I did search but found nothing so I figured I'd ask here.

Agreed. I don't see any evidence of the designers making use of the Meredith effect on the cars. I would expect that the radiators are probably less of a drag source than the wheels and the rear wing to be honest.

Well, yeah, but they cannot do much about the drag from the wheels now can they? Everything is optimized in todays cars, including the radiator ducting. Just open two cutout pictures, one of a F1 sidepod and the other of the P-51 ducting and you'll see how similar they are.

[gixxer_drew]

Been in use for ages. The trick stuff is boundary layer suction.

[DaveW]

Just open two cutout pictures, one of a F1 sidepod and the other of the P-51 ducting and you'll see how similar they are.

They probably are, but not the dynamic pressures, I think.

Is generating DF with the exhaust gasses more beneficial then reducing drag?

Probably - DF is everything, whilst drag simply requires more power in the current regulations. I suspect that Meredith would have had proverbial kittens if he was presented with a current F1 vehicle to study.

[Just_a_fan]

Just open two cutout pictures, one of a F1 sidepod and the other of the P-51 ducting and you'll see how similar they are.

A big difference being, of course, that the P-51's radiator was a variable nozzle device. Remember that an aircraft has a much, much, lower drag than an F1 car so the radiator's drag would be a huge percentage of the total. It's therefore worth the effort of trying to reduce it. On an F1 car, the radiator's drag will be a pretty small percentage of the overall drag. Finding reductions elsewhere (such as the rear wing endplate slots, or the outflow front wing endplates) will give bigger drag reductions whilst also improving downforce.

[olefud]

Meredith wrote in his paper that the thrust developed by the system would be greatly increased if exhaust gasses were also vented in the ducting, somewhere after the radiator. The exhaust pipes of an F1 car are inside the ducting so the exhaust system does increase the effect, but the gasses exit outside the sidepods. What I’m wondering is, has someone experimented with exhaust exits inside the sidepods on a F1 or a prototype car?

If nobody did, why not? Is generating DF with the exhaust gasses more beneficial then reducing drag? Is the effect negligible? Are there other potential issues? I'm thinking such a system could be advantageous on high speed tracks such as Monza.

With aircraft, direct thrust was realized from the exhaust stubs. Same with cars, dragsters for example. But diffuser sealing no doubt trumps thrust.

[Blackout]

Been in use for ages. The trick stuff is boundary layer suction.

Give us some examples did the R31, for example, use it in 2011 ?

[stez90]

If you speak italian check "Motori ad alta potenza specifica" book, written by an ex Ferrari F1 engineer. There's an in-depth analysis of this subject. I remember that you DON'T want the exhaust to exit inside the bodywork (altering the mass flow rate), you need only the exhaust pipe to travel in the nozzle behind radiators to heat up air and increase its energy.
Even without exhaust, the shape of the sidepots (divergent-radiator-nozzle) help to maximize the radiator heat exchange minimizing the drag. The effect can't give you positive thrust in an F1 car due to various limitations (speed, shape etc.), but it's still effective enough to produce near zero overall drag.
So yes, almost all F1 sidepots use this effect.

[tomislavp4]

If you speak italian check "Motori ad alta potenza specifica" book, written by an ex Ferrari F1 engineer. ...

Unfortunately I don't speak Italian but thanks for the useful information. So the effect is exploited in F1 cars with both the radiator and the exhaust piping dumping energy into the flow, as I suspected. Positive thrust generation from ducting is a myth. The thrust generated is not greater than the drag, at least if the vehicle is doing a fair bit under Mach 1.

What I don't really understand from your post is why you wouldn't want the exhaust altering the flow rate after the radiator. I'd guess that doing so will accelerate the flow even further, decreasing pressure after the radiator, further increasing the generated thrust

A big difference being, of course, that the P-51's radiator was a variable nozzle device. Remember that an aircraft has a much, much, lower drag than an F1 car so the radiator's drag would be a huge percentage of the total.

Yes, the P-51 has a variable nozzle but it also has to operate in a wider speed range. I'd guess that in a race car you can do without the variable nozzle and optimize the system for the speeds they operate within. I agree on the second point you make.

[stez90]

You can use exhaust jet to furter accelerate the flow (placing the exhaust near the outlet of the nozzle for example), but you don't want to send exhaust gases behind the radiators. I don't have the book right now to check the correct explanation, but the result was that.
The system works like:
-a diffuser that reduce speed around the radiator and increase pressure (with diverging angle limited by boundary layer)
-radiators (low speed=less drag, high pressure=better heat exchange)
-radiators heat up air, increasing its energy (amplified with hot exhaust pipe)
-nozzle that accelerate the airflow, converting pressure and thermal energy in kinetic energy (flow speed)
If you match the outlet flow velocity with freestream velocity you have a zero-drag system. If you exceed you have positive thrust (meredith effect). Otherwise you have drag.

[eyalynf1]

The application of exhaust to this system is an example of induced airflow. In heating ventilating and air conditioning, there is such a thing as a powered induction unit where a fraction (primary air) of the design airflow (primary + secondary air) is nozzled at high velocity and pressure into a supply duct, thereby inducing a volume of additional secondary air flow.