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Old 11-21-2014, 01:16 PM   #11 (permalink)
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Quote:
Originally Posted by freebeard View Post
Air in at the stagnation point.

Highly evolved cooling systems have a fixed intake and a flap in the exit. (P-51)

Best location for the exit is at the widest part of the body, where the air is moving the fastest. (Edison 2)
I don't really have a choice of exit, the engine is very close behind the radiator so there is no way to do anything other than let it go over and around the engine and then out the bottom of the engine bay. I would also have a lot of cooling issues if it didn't, for example the transmission is air cooled. The undertray extends a little passed the radiator then stops.

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Old 11-21-2014, 06:25 PM   #12 (permalink)
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best place

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Originally Posted by Nigel_S View Post
Not quite the right image but it shows the grill area:




Yes, I realise that most people don't completely block the grill, and most people try to avoid using the fan, but I have a fan that I am happy to use and which will only use the amount of energy needed so I have a choice of where to draw the air from. The question is, which is the best place to draw the air from? Maybe it is from in front of the grill adding to the amount that ends up under the car, maybe not...
*If you're going to leave the radiator as it is,the ideal configuration is to create an airtight inlet duct which is as deep as the height of the radiator,as wide as the radiator,and 1/6th the height of the radiator,with the inlet right above the splitter,at the forward stagnation point.This might require a nose extension.
*If you were to lean the top of the radiator forward,again,with an airtight inlet duct,you could pull all the air from under the nose and at the forward stagnation point of an airdam,as older Corvettes and Pontiac Trans Am Firebirds utilized.This WOULD probably necessitate a nose extension in order to accommodate the exotic inlet.
*If you don't want to compromise the appearance of the car,then you'll have to settle for something less than ideal.You just may not have enough distance between the radiator face and the grille to allow proper air diffusion over the entire core face.
*Whatever your choice,you'll want to pull the air from the stagnation area.Otherwise you'll have single-fault failure mode for cooking all the mechanicals.
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Old 11-21-2014, 08:23 PM   #13 (permalink)
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Quote:
Originally Posted by aerohead View Post
*If you were to lean the top of the radiator forward,again,with an airtight inlet duct,you could pull all the air from under the nose and at the forward stagnation point of an airdam,as older Corvettes and Pontiac Trans Am Firebirds utilized.This WOULD probably necessitate a nose extension in order to accommodate the exotic inlet.
i figured it was all just working off the higher pressure in front of the air dam, and lower pressure behind
all my air goes under my bumper, using stock vertical radiator. older vettes / 4th gen camaros/ firebirds leaned back. the radiator leans forward on the latest vettes, but they duct the air off the top 1/3 or so of the radiator out the hood
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Old 11-22-2014, 01:54 PM   #14 (permalink)
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Originally Posted by 2000mc View Post
i figured it was all just working off the higher pressure in front of the air dam, and lower pressure behind
all my air goes under my bumper, using stock vertical radiator. older vettes / 4th gen camaros/ firebirds leaned back. the radiator leans forward on the latest vettes, but they duct the air off the top 1/3 or so of the radiator out the hood
yes,the frontal stagnation point is nothing but barometric pressure plus the ram component of the dynamic pressure.
By the position behind the radiator core,any pathway that the exiting air can communicate with is at a lower static pressure by virtue of Daniel Bernouli's principle of the pressure/velocity tradeoff.
Your Saturns nose has the grille cut right into this stagnation area and is harvesting cooling air with high efficiency.It would embarrass many 'new' cars.
The '82 Trans Am picked up a Cd 0.015 drag reduction ( 4.75% ) with the bottom-breather setup combined with a 25-mm lower (1-inch) airdam.
'Bummer about these configurations is that so many of these airdams were torn from the cars while negotiating 'normal' driveway inclines.
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Old 11-23-2014, 01:27 PM   #15 (permalink)
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Here's a picture to go with aerohead's analysis:



Quote:
I don't really have a choice of exit, the engine is very close behind the radiator so there is no way to do anything other than let it go over and around the engine and then out the bottom of the engine bay. I would also have a lot of cooling issues if it didn't, for example the transmission is air cooled. The undertray extends a little passed the radiator then stops.
Turn the undertray into a partial bellypan, and have the flap slide instead of hinge.
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Old 11-23-2014, 02:19 PM   #16 (permalink)
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Quote:
Originally Posted by freebeard View Post
Here's a picture to go with aerohead's analysis:



Turn the undertray into a partial bellypan, and have the flap slide instead of hinge.
That's a nice idea but I don't want to prevent air circulating around the engine bay, a belly pan seems risky. It was designed and tested with a large airflow keeping everything cool, having put in the grill block, at times in hot weather I am opening the grill in order to cool the engine bay more than the coolant in the radiator.

I like the image of the Corvette in post #2 but I don't have an air dam and the bottom under the radiator is flat. I'm not sure what effect adding an air dam would have, there is a bit of a tunnel running under the grill which may be intended to give highest flow under the centre of the car and shape the flow further back, or maybe it is just styling! I'm sure adding an air dam would have a significant effect though, seems like another project.

The original question hasn't really been answered but I think I'll take the easy option and just put a vent in the centre of the grill block with an iris to control the flow and control the iris based on the coolant temperature at the exit of the radiator and override it when engine compartment temperatures get too high. Then I can try to test whether opening the iris or turning up the fan power uses least fuel.

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Old 11-24-2014, 12:15 AM   #17 (permalink)
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TL;DR: Right where the factory put it.

Quote:
Question - When the radiator does need some air for cooling, where is the best place to get it?

I could open a flap in the grill but that increases the amount of air flowing under the car and that appears to be bad for fuel economy...Has anyone tried sourcing the cooling air from anywhere but the grill? Are there any figures available for the results? Any knowledge if it is a good or bad idea?
"The original question hasn't really been answered"? I think you are disrespecting aerohead's responses. You will need to particularize the general responses for yourself in any case. What kind of car is that in the picture from WaWaWoum, anyway.

As for myself, the point I was trying to make, apparently not particularly well, is that you could leave the grille completely unblocked, put sliding louvers in a belly pan and gain dynamic control of the airflow from there. You should be able to seal the engine compartment completely, pipe in coolant and intake air and have it work just fine. If it doesn't, perhaps an oil cooler is called for. How are your transmission temps?

Quote:
It seems widely accepted that there is an aerodynamic advantage to keeping the grill fully blocked until cooling air is needed.
This is your real question. Be wary of 'widely accepted' until you verify it for yourself.
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Old 11-24-2014, 09:39 AM   #18 (permalink)
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It's widely accepted because it's widely true. The main question is whether it's true for any particular car, and how much of an advantage there is. The answer may be a blanket yes (and very effective), or it may depend on your design.

Isn't this fun?
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Automatic .........................86%

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Old 11-24-2014, 01:38 PM   #19 (permalink)
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It's widely accepted because it's widely true. The main question is whether it's true for any particular car, and how much of an advantage there is. The answer may be a blanket yes (and very effective), or it may depend on your design.

Isn't this fun?
Yes, fun

I wasn't expecting an answer specific to my car or even a general one for the model, just wondering if anyone had looked into the alternatives and what is worth trying. Looks like everyone takes the cooling air in through the grill though and have never tried anything else!
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Old 11-24-2014, 05:05 PM   #20 (permalink)
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I think the bottom breather setup, simmar to the corvette, is about as different as you're going to find with a radiator in the typical placement. Many race cars will use that location and duct air out the hood. Off-road race vehicles will mount their radiators high in the rear / in the bed of the truck. Some mid / rear engined vehicles will have ducts on the sides of the car after the doors where the radiators are. Boxters have 2radiators in the front corners
All of those hinge on radiator placement. I'm sure with powerful enough fans, lots of ducting, and 7 alternators, it would be possible to draw air in from anywhere around the vehicle, and exhaust it where ever you see fit. You won't come out ahead running your radiator fan vs. a properly sized grill opening. After the losses from the fan converting elcrical power into air flow for the radiator, the alternator turning mechanical energy into electrical power for the fan, and the engine turning fuel into mechanical energy for the alternator. You're better off taking the more direct hit, using the engines power to create the air flow for the radiator

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