Another silly idea, ducted fans for a radiator! Options

[Dave_Darling]

If you've seen "tuft testing" on a 914, you'll see that air goes forward along the upper surface of the rear trunk and into the engine bay when the car is at speed. I don't think that really makes the rear window area very low-pressure.

The train tray is there on the four-cylinder cars, but there is an easy pathway for the air to get around it. You could cover up the two side grilles and the engine should still cool and run without any serious difficulties.

--DD

[Goge]

Sounds like somebody needs to do some testing with some ribbons in the airstream...

Tony and I talked about this in an e-mail conversation... whatever way you have the radiator setup, you'd like it to get some free air flowing through it... and you certainly wouldn't want your fans to fight the natural flow of air through it.

I think we're all agreeing that that the air would naturally flow from above to below (hi pressure above to low pressure beneath the car)... can somebody confirm that with a subaru setup?? Perhaps put a couple of ribbons of cassette-tape or something in there and watch what they do?? You could probably just do it without the engine cover on there so you could see what was going on better. Need a passenger probably to eyeball them, and oh yeah- a car that is actually running!

-TH

[TonyAKAVW]

QUOTE

If you've seen "tuft testing" on a 914, you'll see that air goes forward along the upper surface of the rear trunk and into the engine bay when the car is at speed. I don't think that really makes the rear window area very low-pressure.

So a couple of questions. Was that testing done wit the engine sucking air into the engine bay, both the air intake and the cooling impeller???

Also, relative to the bottom of the car is this low pressure or not? And how about with a front air dam? Does the bottom of the car become very low pressure? Perhaps it is better to pull air down through the radiator and then provide some equivalent of engine tin to seperate the hot air form the cold air... (Miles suggested the use of engine tin a while back)

-Tony

[jsteele22]

Hi Tony,

I think with air-handling equipment, the relationship between flow (CFM) and thrust is in general pretty complicated, and usually given (if at all) in terms of a graph. If only one parameter is specified, you can be sure it is only true under the most ideal (i.e., worthless) case. So CFM is usually specified for no restriction on airflow; try to blow the air thru a duct or restriction and the CFM drops rapidly. And thrust (or pressure difference) is specified with the outlet blocked off; let any air escape and the pressure differential drops. Morale : don't trust the specs unless you get a graph ! In the case of a ducted fan like you describe, I would imagine the manufacturer has a tad of honesty and specifies the thrust as the force per unit area at a specific distance from the exhaust end of the duct, assuming no restrictions on airflow. Clearly, if you're pushing the air through a duct or restriction the thrust is gonna suffer.


In reality, I think the way to approch a problem like this one is either : a) tons of careful engineering, (IMG:http://www.914world.com/bbs2/html/emoticons/cool.gif) trial and error, or c) both. Oh yeah, or d) copy what's already been done. I think the existing fans made for radiators are probably the surest route. You know they can handle years of the heat, vibration, dust, and grime of an engine bay, and they manage to get sufficient air flow in conditions not much different from yours (i.e., the car at rest or low speed). If you're worried about using the full area of the radiator, I'd think there's room to separate the fan from the radiator a little more and have the duct flare smoothly to the full radiator size.

[brer]

there are custom hot rods that get their cooling from a "cooling rail" of fans mounted directly to the radiator.
The trick is to have an efficient fan shroud. done right and you need no direct airflow, 100% fans.

check out some hot rod magazines.

[bondo]

QUOTE (jsteele22 @ Nov 10 2005, 05:43 PM)

And thrust (or pressure difference) is specified with the outlet blocked off; let any air escape and the pressure differential drops.

The way thrust is measured for model airplane powerplants is to mount the thrust producing device on a horizontal rail of some sort that allows it to slide smoothly along the axis of thrust. You then simply turn it on and measure the force it can pull against a spring scale.

It seems like there would be a way to convert that to CFM, based on air density. (thrust of less air faster = thrust of more air slower) But then that all gets thrown out the window if the effect of pushing against the surrounding air is significant, because then it would depend on the cross sectional area of moving air. This is where my understanding of physics starts getting foggy.

[jsteele22]

QUOTE (Dave_Darling @ Nov 10 2005, 05:33 PM)

If you've seen "tuft testing" on a 914, you'll see that air goes forward along the upper surface of the rear trunk and into the engine bay when the car is at speed.  I don't think that really makes the rear window area very low-pressure.

Hi Dave,

I think this is a case of mistaking cause and effect.

Q : Why would air rush forward along the trunk into that area behind the window?
A : Because the pressure there is ... low.

The powerful airflow passing over the roof and off the trailing edge of the rollbar wants to keep moving towards the rear, and the layer of air just below it is going to get dragged along with it. And the layer below that gets dragged along, etc. But these dragged layers have to get air from somewhere, so they pull it from the air behind the window. But as the pressure there continues to fall, pretty soon air starts getting pulled back in. Not from the fast-moving air coming off the roof, but from the slowest moving air available : the air back over the trunk where the flow lines have spread out to fill the increased volume. Spreading flow-lines indicate decreasing velocity. So the result is an eddy, or haystack, just like the water behind a large rock in a river. A small amount of air flowing over the car curves downwards towards the top of the trunk, turns forwards towards the rear window, and then turns upwards, where it gets pulled backwards again. Just like all the grocery bags, Big Mac wrappers, and possibly leaves that swirl in a circle on the downwind side of a large bulding on a windy day.

Now how about the difference between pressure above vs below the car ? It really should be higher beneath the car and lower above. So if a car had no engine tin, no rain tray, (and it still worked), the air in the engine bay really ought to flow up. The reason is Bernoulli's Principle. Imagine two litle blobs of air very close together when an airplane wing comes along and slices right between them. One blob goes over, one under. In the wing's reference frame, the two blobs will pass along the surface and re-join at the trailing edge. (If they don't meet up, it means that air is building up somewhere, a condition that can't go on for long...) So lets assume they meet. The air above the wing has a longer path, due to the wing's shape. It travels a longer path in the same time as the lower blob, so its velocity is higher. Bernoulli says that higher velocity means lower pressure. So if the presure above is lower and the pressure below is higher, then there is a net force : lift. (At high enough speeds cars, like airplane wings, will take off, unless something "spoils" the lift. Hmmm... what are those things called...)

[jsteele22]

Oops, hit "add" instead of "preview". Anyway, my only real reservation about comparing a 914 to an airplane wing is the presence of the ground, and the copious amounts of turbulence, that Bernoulli assumes are not present. But, still, I think the basic principle applies.

Anyway, apologies to all who hate long and geekly treatises. Hope someone enjoyed it, and more importantly, I hope its right.

Cheers, Jeff

[Rand]

I think the presence of the ground is everything here. There's no way more air goes under the car than over. Since more air goes over the car, the pressure must be lower under the car.

[jd74914]

With that reasoning, even less air gets sucked into the engine via grill.

The only way I can see air getting sucked in is if it is like a cowl induction hood, but they are always in the direct path of the airstream, so this idea seems highly doubtful (to me atleast). Personally, I think that jsteele is probably the most correct.

[Dave_Darling]

Yeah, I think he is. Something was telling me that I might have had my interpretation backwards when I was posting, but I was in a hurry and didn't stop to think it through. (IMG:http://www.914world.com/bbs2/html/emoticons/sad.gif)

I do believe that the rear window is a (relatively) low-pressure area, but I think the area below the car is also low-pressure. This is most especially true if you have a big front air dam (see Trekkor's "snow plow" for one example). I would not be confident that the pressure below the car was significantly higher than that at the rear window.

The "tuft testing" I mentioned earlier was done on a fully-operational 914-6 race car. It did have complete engine tin.

--DD

[brer]

I think the Bernoulli effect could be useful. Like ridge vents that are used to ventilate an attic space.

Edit: OH! haha, I'm thinking attic venting and quote the principle of lift. Thats what you were talking about.

duh.

[lapuwali]

The pressure under the car is most certainly not lower, or we'd have net downforce. As odd as it seems, most road cars have some net lift, usually no more than 75-100lbs at 70-80mph, so all of those cars MUST have higher pressure below than above. The 914 is no different in this regard. A big front airdam is placed there to prevent airflow under the car, to reduce this lift.

The spoilers along the trailing edge of the firewall on the 914 are there to create a low-pressure area under the engine, so the fan has an easier time pushing cooling air through the engine. Without those, the pressure is high enough that the fan has to work fairly hard. This "low pressure" is still almost certainly higher than the pressure above the engine lid, but it's lower than it would be without the spoilers.

The reverse flow along the top of the trunk lid is there to fill the low-pressure area created by the targa bar and rear window, which is a pretty classic drag inducing shape. My money would be on the lack of engine tin or a cooling fan having almost no real effect on this flow pattern. Some air would flow up into the engine bay with no tin (and removing the underbody spoilers would help that considerably), so the engine bay would run cooler, and there may even be slightly less drag than in an air-cooled setup.

[Goge]

OK, how about this: with a type IV in our 914's, we are trying our darndest to keep the cool air above the motor from being mixed with warm air below the motor. At least we should be. That's why we keep the engine tin on there, it's important for cooling. Without our TIV cooling fan spinning, there would really be no net movement of air from above to below, or vice versa. At least pretty much nothing.

So any tuft testing or data previously collected is all for naught, at least with regards to what air would do around a Soob engine. Because cooling tin is now gone, and there is now a clear path for air to make it's way from above to below (or vice versa).

That's why I think we need Mr. Thacher or somebody to do some real-life tests to find out which way the air normally goes. Heck, I suppose it's possible that maybe it could change direction at some speed. Maybe it's all turbulent and we really can't predict it? I dunno. Gas dynamics are a little tricky in real life...

-Todd

[brer]

tie some strings to your engine lid scott.

(IMG:http://www.914world.com/bbs2/html/emoticons/beer.gif)