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Old 05-03-2011, 02:35 PM   #41 (permalink)
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From Wikipedia a flat plate perpendicular to the flow will have a Cd of 1.98-2.05. However if we extrapolate the dissertation to a mesh with 0% open area the Cd will be significantly higher. I think the two test situations are different enough that the results do not directly translate.

I think the Cd changes if air can flow around the obstruction rather than just being stopped by it.

With a stock grill the issue is the radiator shroud, the engine and the engine bay acting like a blockage. Where as the solid grill block directs the air around the blockage giving us our MPG gain.


Here, California98Civic says,

Quote:
Drag is important, and it is our normal obsession. But I would trade marginal drag increases in my grille blocks if it meant significantly reduced radiator-fan run times.
I think he is talking about the low speed benefit of mesh letting air flow through as opposed to his current solid grill block. So he would like something that has worse drag as long as when he was stuck in traffic he could draw more air and cool faster, which is what is being theorized about using mesh instead of a solid piece.

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Old 05-03-2011, 08:07 PM   #42 (permalink)
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A year or two ago I was arguing with somebody here about internal grill blocks. I said that engine bay resistance is equivalent to a mesh, because it partially restricts flow... So if the resistance is that great, it would have a similar effect to a hidden grill block.

Once the air is inside the engine bay, its speed drops drastically and its direction changes to go around the components. The only thing the outside flow of air knows is that it's going into a hole, and that hole has somewhere between 0% and 100% resistance to flow.

That would mean that a mesh mounted behind the grill wouldn't have any benefit. It would only add resistance to what already exists.

If X is engine bay resistance, Y is grill mesh resistance, and 100% is the resistance from a total block... You either have X, X + Y, or X - X + 100%.... None of which necessarily would reduce drag.

IMO, a mesh would have to be external and shaped in such a way that it smooths out airflow.
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Old 05-03-2011, 08:11 PM   #43 (permalink)
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Quote:
Originally Posted by jthistle View Post
Just so I understand the 34% coverage area is equivalent to a free hole area of 66%. So the lower end of the table I posted the 25% free hole area mesh will have a coverage of 75%. Am I correct in interpreting your response?



What coverage mesh did you use for your testing?

more or less your first response is correct, but they state its more likely to be 64% open area due to the manufacturing process.

I used the screen size we have been talking about, because that is what I had on hand
I ran it on my 99 ranger xlt for 5 full tanks and didnt change a thing in my driving habits.
I have it all written down somewhere, and have looked for it already with no luck, but will continue to look more later this evening.
I saw absolutely no benefit, no increase in FE.
my tuft test showed no improvement at highway speeds (although there was some at slower speeds of < 40mph)
all in all, my guess is if you were staying within town/city driving, then you might actually see some gains, but once you get on the hwy, those gains would be marginal, at the very best.
but once you start to introduce angle to the build, you blockage is going to increase incrementally with the angle.
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Old 05-04-2011, 02:27 PM   #44 (permalink)
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I got the meter today and did some testing just holding out the window of my car as I drove.

I made some observations about taking measurements with this device.
1)Traveling perpendicular to the ambient wind gave the best results.
2)Buffeting from other vehicles causes large swings in MPH readings
3)Rotating the meter head to simulate a non-vertical grill had a large impact on the MPH. I think that is because it decreases the area exposed to the air flow (circle to ellipse).
4)Keeping my arm steady at 60 MPH was not as difficult as I thought it would be
5)Within my reach I found resting my hand on the side mirror so the meter head was in front of the mirror gave the most consistent results.
6)People kept honking at me and looking at me like I was insane.
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Old 05-04-2011, 03:20 PM   #45 (permalink)
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I also received the mesh yesterday and I made some bad choices for the mesh. On Amazon I was only looking at the %open area and did not consider the mesh sizes. So I ended up with some mesh that has very thick wires an will be difficult to work with. Went to wikipedia and learned something about mesh Mesh (scale) - Wikipedia, the free encyclopedia and I have to order some more mesh that has thinner wires so it can be workable. I'll post what I have so you can see my mistake.

d0sitmatr can you please give some suggestions about mesh sizes for ease of workability. The thinnest mesh I purchased was a 20x20 with a 0.023" wire diameter. Pages 392 and 393 of the McMaster-Carr catalog have a large selection of stainless steel meshes. McMaster-Carr
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Old 05-04-2011, 05:06 PM   #46 (permalink)
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screen

Beg,borrow,or s---- a couple of matching tennis racquets.Cover one with window screen.Then get a buddy to drive you down the 101,405, or the like at speeds you drive,hold both out the passenger window and see what you think.
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Old 05-04-2011, 05:48 PM   #47 (permalink)
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Quote:
Originally Posted by jthistle View Post
I also received the mesh yesterday and I made some bad choices for the mesh. On Amazon I was only looking at the %open area and did not consider the mesh sizes. So I ended up with some mesh that has very thick wires an will be difficult to work with. Went to wikipedia and learned something about mesh Mesh (scale) - Wikipedia, the free encyclopedia and I have to order some more mesh that has thinner wires so it can be workable. I'll post what I have so you can see my mistake.
nice article to find there, gave it quick peruse but will give it more of a look later.
Quote:
d0sitmatr can you please give some suggestions about mesh sizes for ease of workability. The thinnest mesh I purchased was a 20x20 with a 0.023" wire diameter. Pages 392 and 393 of the McMaster-Carr catalog have a large selection of stainless steel meshes. McMaster-Carr
if your going to get something from there, just go with perforated aluminum. Im trying to decide what I want to work with, aluminum, steel or just get a PVC application.
Im leaning towards this:
PVC perforated
if your set on using mesh of some form or another, I wouldnt bother ordering something like that, Id just go to a screen shop, we tend to throw away enough screen to use, depending on the application
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Old 05-04-2011, 06:03 PM   #48 (permalink)
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Those PVC sheets do look good but at >$50 a piece it would be too expensive to get multiple to do a complete test. However if I can find a %open hole area that works the PVC would be the best solution for an actual grill block because the stainless mesh gets expensive in larger sizes and the pliable meshes would need reinforcement to be used for a block.
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Old 05-04-2011, 06:58 PM   #49 (permalink)
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Wikipedia

Quote:
Originally Posted by jthistle View Post
From Wikipedia a flat plate perpendicular to the flow will have a Cd of 1.98-2.05. However if we extrapolate the dissertation to a mesh with 0% open area the Cd will be significantly higher. I think the two test situations are different enough that the results do not directly translate.

I think the Cd changes if air can flow around the obstruction rather than just being stopped by it.

With a stock grill the issue is the radiator shroud, the engine and the engine bay acting like a blockage. Where as the solid grill block directs the air around the blockage giving us our MPG gain.


Here, California98Civic says,



I think he is talking about the low speed benefit of mesh letting air flow through as opposed to his current solid grill block. So he would like something that has worse drag as long as when he was stuck in traffic he could draw more air and cool faster, which is what is being theorized about using mesh instead of a solid piece.
I think Wikipedia ought to go back over there numbers.I believe some special military parachute designs got above Cd 1.35,but not by much.
Cd 2.05 is not in the realm of planet Earth.
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Old 05-04-2011, 07:37 PM   #50 (permalink)
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Quote:
Originally Posted by aerohead View Post
Beg,borrow,or s---- a couple of matching tennis racquets.Cover one with window screen.Then get a buddy to drive you down the 101,405, or the like at speeds you drive,hold both out the passenger window and see what you think.
Not quite sure what this proves... of course the screen in the breeze will have more drag... but if that screen in the breeze is at the entrance of an aero-dirty engine compartment, and it induces more of the flow to go around rather than through said compartment, then overall drag is reduced, like some point between no grille block and total grille block.

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