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Old 02-19-2008, 06:57 PM   #11
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I wasn't suggesting that a "closing up effect" would occur, per se.

The behavior of the air after it passes through the holes aside for now:

Let's say we have a grille opening of 12 square inches.

A. We can block half of it solidly for a reduction of half the surface area, which will direct ALL the (cold) air to one side or the other

B. We can block it off and drill six holes evenly spaed comprising one square inch each, same reduction of surface area, less volume entering engine compartment but with good velocity in all the right places.

C. (and the part I'm not as certain about) we can drill 5,626 holes comprising 6 square inches for the same reduction (50%) in surfacearea, but I'm betting even more surplus air going harmlessly downstream over the car rather than argue with other molecules over a little hole.

Optimize it and give the spent air somewhere to go and you get your grille block + adequate cooling.

Make sense? Feel free to shoot "holes" in my theory.
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Old 02-19-2008, 06:59 PM   #12
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lunarhighway -

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Originally Posted by lunarhighway View Post
cfg83

that's rather brilliant! transparent plastic drinking cup with the buttom cut out and taped to the rearviewmirror would make a great mini wind tunnel!!!

i was just googeling a bit on the subject and i came across this
http://www.americanmesh.com/PDF_File..._Fume_Hood.pdf
here's the abstract:


no real "closeing up effect" (although that was never said to exist for mesh... only for a preforated sheet) but some interesting info i have to look at more in detail when i'm a little more awake.
The dude with the Red Cavalier put mesh under his car and was going to paint the mesh to form an impermeable layer :

underside air flow
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Originally Posted by Hateful View Post
I've finished covering the entire underside of my car with window screen. This is touching the exhaust components in several places and has not burned nor melted. I plan now to paint the screen in some places to reduce friction more,leaving other parts near the exhaust unpainted for exhaust cooling. I know I'll have to cut a small hole when I change the oil ( filter is reachable from the top if I remove an electrical component from the fire wall and replace). Just drove 20miles from a cold start and managed 40mpg on 45/55mph zones.
If the holes are small enough, you can paint your mesh to simulate any kind of perforated sheet. This is good because you can pick the density of the openings after doing some optimization tests.

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Old 02-19-2008, 09:16 PM   #13
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When I was racing Miatas for Mazda...we tried a fine mesh and actually overheated a bit...we went with a much more corse mesh and temps dropped back down. We did it to keep trash out over the course of a 12 or 24 hour enduro. Of course we were at WOT or FOB (Full on Brakes) most of the time.
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Old 02-26-2008, 07:38 AM   #14
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As a rule of thumb, I calculated a while back that at our typical highway speeds with typical car dimensions, that surface features under about 1/4" in size, cease to be very aerodynamically significant, due to the boundary layer being about that size at those speeds. Therefore I'd say that meshes with smaller than 1/4" holes will begin to block airflow significantly at 50mph+. However, the effect would be further enhanced if the mesh was not vertical to the airflow. Hence I would suggest that if you have a protruding bumper, you might like to "tent" the front of the vehicle with a mesh, from the rad support/front crossmember down to the bumper or something, to give it some angle.
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Old 02-27-2008, 10:33 AM   #15
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Which mesh is the best?

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Originally Posted by RoadWarrior View Post
As a rule of thumb, I calculated a while back that at our typical highway speeds with typical car dimensions, that surface features under about 1/4" in size, cease to be very aerodynamically significant, due to the boundary layer being about that size at those speeds. Therefore I'd say that meshes with smaller than 1/4" holes will begin to block airflow significantly at 50mph+. However, the effect would be further enhanced if the mesh was not vertical to the airflow. Hence I would suggest that if you have a protruding bumper, you might like to "tent" the front of the vehicle with a mesh, from the rad support/front crossmember down to the bumper or something, to give it some angle.
Could you tell me which mesh would you choose?
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Old 02-28-2008, 06:45 AM   #16
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I can't say what will be best for your application really, it's dependent on how much air you can get away with blocking without overheating. I would guess that somewhere between the 1/32 and 3/16 (inclusive) mesh/holes is probably good. Too fine a mesh will block rapidly with stuff like thistle down. It may also hold water from summer thunderstorms and overheat you while standing because of that. My window screen in the window where I have a fan in the summer blocks up from downy stuff every week and I need to clean it. Personally, I'd probably plump for a heavy duty window screen if it was good in an unsupported area, or could be stretched to a frame. I think it's a bit wider than standard window screen. If it needed to be self supporting I'd probably go with the 1/8 diameter hole perforated stuff. I'm thinking that might be a reasonable compromise for parts of belly pans too, but it could still whistle like hell.

Just noticed new pontiacs like the G6 have a mesh grille, would be interesting to see what they use on those.

By the way, boundary layer is compressed at the leading edge of an object, so you can't really use mesh as a nose cone per se, the flow has to be established. So if your bumper isn't really pushing air up over the grille already, then the mesh isn't going to block much more than it's closed area. Basically the mesh isn't going to block much when there's a great pressure differential, so some effort must be taken to lower pressure over an area where you'll use it. For example, don't stick hood vents far back on the hood, because near the base of the windshield is typically a higher pressure area... they'll tend to pull air in and under the car. (Principle of a reverse ram intake)

If you are thinking of tenting a mesh between the rad support and the bumper as I suggested, a short area of solid surface to help establish flow may be desireable, just an inch or two strip of material at the same angle as the mesh.

Hmmm trouble with figuring out fluid dynamics rules of thumb is that they have all sorts of caveats and apply to narrow ranges of situations.
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Old 02-28-2008, 09:34 AM   #17
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Some Photos

Take a look at what was done to the the Dodge Charger 500 and Ford Torino (standard and Talladega) models that ran in NASCAR in 1969. Photo quality may not be very high, but it's good enough to get across the point.

The pole speed at Daytona in 1969 was 188.901 mph, so aero was very important.
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Old 02-28-2008, 10:56 AM   #18
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Quote:
don't stick hood vents far back on the hood, because near the base of the windshield is typically a higher pressure area...
on most cars that where the interior cooling air intakes are located for that reason...

so basically from what you're saying and what i'd gathered in the article the mesh may not be the "magical" grillblock i'd hoped it would be.
on the other hand it might be a good thing to have a wide area of the radiator exposed to the airflow without it being a hoge gaping hole...

what i mean is that it's proabably better for cooling to expose the whole surface of the radiator to some airflow than a small patch to a bigh stream.

thus perhaps the total area that can be blocked before overheating occurs is greater with a perforated sheet than with a small hole in a solid grillblock.
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Old 02-28-2008, 11:05 AM   #19
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Yes, maybe. Also when the cooling fan kicks in, it will create slight negative pressure on the inside of a mesh, causing the pressure differential to alter, and more air to enter. It doesn't suck hard enough to create enough negative pressure to suck it through a single restricted opening much faster though, but a lesser pressure difference over a larger area of mesh should make a difference.
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