[chillsworld]

Originally Posted by freebeard

My suggestion—take a well-proven form like the Volkhart-Sagitta, use a boxer motor for that low hoodline and then put twin radiators in front of the wheels Boxter-style.

Well I don't, nor do most people have, the ability to design a car from the ground up. That's great an all about the Volk and designing an optimal cooling system, but I'm talking about things we can do to the vehicles in existence... In our garages and driven every day

Originally Posted by freebeard

I'm with serialk11r. The difference between the P1 and the time attack cars is the former is organically curved (probably internally as well) while the latter look like an air conditioning duct. Easily fabricated but the un-filleted inside corners are not friendly to the air.

Originally Posted by serialk11r

Okay I've attached an edited picture where I drew a red line around where I believe the "effective area" of the vents on the P1 is. I might be drawing it too big, they might have designed the edges of the vents in some tricky way.

I think they DID design it in some tricky way SerialK11r, and you're 100% correct freebeard when you state that the designs between the attack car and the P1 are different. Most if not all wind tunnel tested designs (that I see) have rounded edges within the ducting. There is a big difference between something made in a garage and something made in a factory with millions of dollars in RnD. But design specifics aside... Is the concept not sound? Surely we don't need a perfect design to see benefits from it's usage? I mean if it can get us a marked improvement, even if it's not as good as it could be, I would think it would be worth the time to consider and design it the best we can? Try to determine the optimal inlet/outlet ratio, hood placement, internal ducting design, etc etc. Figure out what *should* work for what we want to accomplish, so that anyone who wants to try it has some sort of starting point. I understand the EVO might not be what we want to replicate.... But would the mitsu from post #1 be more in line with our goals?

Originally Posted by Gasoline Fumes

Thank you for this addition to the conversation!

Originally Posted by serialk11r

Yes it's still huge, I'd say maybe about the same size as the one on the hood of the Corvette or the Evo. But look at a picture of the front of the P1.



The intake for that radiator is half the height of the nose, and extends nearly all the way to the headlights. My eyeball guesstimate is that the vent is maybe 30-40% bigger than the intake. Plus, the nose is raked down sharper than the front engined Vette or Evo, so the pressure there is higher in the first place.

The Evo has a puny intake that's half the size of that. There is no way enough air is making it to the other side of the radiator to be reducing lift. I bet the air in that vent is moving too slowly and is causing lift and drag.

Again, the goals and system to which the rad duct partakes in, are different than they are for a supercar or ecomodder. But lets go with what you have brought up, inlet vs outlet size. Or rather lets start with inlet vs radiator size.

"We did some simple wind tunnel testing of a radiator with different size inlet ducts last year. Mass flow DOES increase with a smaller inlet vs radiator size (in our testing anyway and obviously only until separation begins)."

"Because the diverging duct increases pressure by slowing the air down, rather then slamming it against the radiator, which are basic pressure velocity fluid equations."

"A huge low pressure zone behind the wing and in front of the rad matches the low pressure behind it and cancels flow.

But then why wouldn't you just increase the bejesus out of your pressure in front of the radiator? I think you would, but not by using a huge converging scoop.

You want to increase your pressure in the area RIGHT in front of the radiator, not at the entrance of the duct. Say you have a CONVERGING duct. The pressure at the entrance will fall into equilibrium with the velocity pressure, like everyone has been saying. But then your pressure will decrease as the air moves towards the radiator. You will still have enough pressure and velocity to maintain flow through the radiator, and there will still be an even lower pressure behind the radiator. If you look at the pressure "continuum" throughout the process it will go from inlet=high, front of rad=mid, behind rad=low.

Ok, now use a diverging duct. The static pressure at the inlet will be atmo, the pressure right in front of the rad will be high, and the pressure behind the radiator will be low. So the pressure continuum goes: mid -> high -> low. The velocity pressure we have all been talking about will keep the flow moving forward through the mid -> high pressure region, aka the duct inlet to radiator. The optimal duct design will make the best use of this velocity pressure and flow the most air, more than a converging duct would. The highest pressure differential DIRECTLY across the radiator will produce the most flow (since the radiator is the restricting element). You need to find a balance however, between this pressure differential and the air velocity, to obtain the maximum mass flow rate. I believe the area ratio, A,rad/A,inlet, will be VERY close to 1, but less than 1.

If you go fast enough, like in a WWII fighter, the diverging-converging duct will actually produce thrust, like a mini pulsejet. However, in FSAE we cannot hope to go this fast, but the same flow-maximization concept applies."

"Also note the radiator core takes up space in the air stream so there may be a ~30% reduction in the actual area for air to flow through the core. Therefore a diverging-converging setup will help to keep airflow through the radiator at the same speed as the free stream air around it. This all seems to be a drag reduction exercise IMHO."


A really interesting read here, where I pulled these quotes from: Diverging/Converging Radiator Ducting - Why? These are small engine cars that see low speeds... No super cars, no time attack, just efficient design! ANd of course they design them from the ground up to do things we can't necessarily do with our vehicles (like converging duct exits behind the car. But there is a lot that can be learned from them.

So in a time attack or hill climb car where cooling is of the utmost importance, forcing more air mass through the radiator would be more important. And doing so in a manner which avoids extra frontal pressure and drag would also be important. The inlet for the EVO is smaller than the radiator for a reason, or so it would seem. They utilize the remaining leftover space for intake to the brakes, oil cooler, air intake for turbo. Remember, their goals are not the same as our goals... We don't have to cool red hot brakes and transmissions with our available front intake space.

See additional cooling intakes where the space for a converging intake scoop would normally be for the radiator. Also note the extreme thickness of the core the smaller intake is feeding... I understand the "duct" probably isn't long enough to create a true convergent pressure trait, but I think it abides by the same principle in the short amount of space it has to diverge towards the face. Like I said before, you guys are getting really hung up an example that is about as far from "normal" as we can possibly get



Now lets talk about inlet vs outlet design, to see if your guess about the larger exit on the EVO is correct. Insight from the airplane crowd: Cooling

"Since the air is slowed before entering the radiator there is little necessity for streamlining the duct at the radiator. The fact that the air appears to have to make a right angle turn through the radiator in a horizontal layout does not cause problems...we are at this point working with pressure differentials not streamline high velocity flow.

Streamlining does come into play in the exit plenum/duct after the radiator. The "back" wall of the duct, the area into which the airflow exiting the back of the radiator will impinge should curve, gently or relatively abruptly but continuously towards the exit from the cowling. Sidewalls should also smoothly converge to the exit, meaning no square steps or direction changers, radiuses as gradual as possible but fitting the space available. You will hear rules of thumb that the exit openings, because the air is heated and thus presents more volume than the original intake air, should be 2.5 times the area of the intake opening. With good extraction of this heated air, allowed by the fact that it is exhausted into a low pressure area, the exit cross section can be as small as the intake in some cases (I believe this is the design of the P51 mustang right guys?). In others larger, but I have never seen this exit area have to exceed about 1.5 to twice the area of intake. Remember this air is compressing and accelerating producing a high flow rate."

You eyeballed the P1 and guessed 40% larger exit, so that would make sense for the car. Just as needing an even larger exit would make sense for a car with a MASSIVE cooling system creating what I would assume to be hotter than usual air exiting it like the EVO in question (like a prop driven airplane). I of course don't know any of this to be true. But given your example, and my examples, and the above reading material... It would make sense to me. I would also deduce that the thicker core is equivalent to a larger thinner core, there by giving the illusion that the exit ducting is far too large when compared to the cooling system and intake inlet. In reality it could be perfect given a higher rate of expansion due to heat exchange. But that is just speculation on my part.

Now with this 500lb gorilla in the room, downforce. As I have stated prior, the duct is part of a larger system which generates downforce. However, lets look more closely at the EVO's inlet/outlet ratio and then look even closer at the shape of the hood opening and duct work... Since we seem to be stuck on this idea.



You can see that the duct is as large or larger than the core... Then it actually converges, it becomes smaller before it widens again to meet the large opening of the hood. The pinched area of the duct looks to be about 1.5 times the inlet duct and maybe even closer to 1:1. This confines and speeds up the air as talked about by the airplane guys (right?), then it opens up bringing velocity down and creating a high(er) pressure zone like a rear diffuser or like the convergent inlet discussed above. Wouldn't this cause at least a small amount of down force? Again, I don't think this was designed for down force, but it's something you guys keep coming back to.


~C