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Old 02-18-2008, 03:45 AM   #1
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fine mesh as grillblock?

what would the effect of a fine mesh, like the kind used in flydoors etc, on the airflow and drag.
A while ago i came accross an article on ecomodder where someone sugested useing a perforated sheed as a grillblock where the perforations made up roughtly 30% of the total area. Most people sugested useing a solid sheet and leave 30% uncovered if that was needed. but one guy said there was a difference, as at higher speed small holes would "close up" and the whole thing would appear as a solid sheet to the oncomming airflow. This actually seems rather possible... toyota actually used perforated plates as a grill on their previous models of the corolla and the yaris... whatever the reason for that choise, it proves it's possible.

This combined with the fact that a lot of bugs find their final restingplace spached in the radiator (wich i imagine might be a slightly bad thing for the airflow trough it) got me thinking what the effect of a flyscreen on the overall airflow would be... Would this pervent some air from getting trought the radiator and direct it over the car, or would it add aditional drag? Fly doors can be blown open by the wind so they probably represent more resistance than their appearance would sugest.

i'm mainly interested in this as it seems to offer the best of both worlds, a fairly open grill at low speed and slow traffic where full grillblocks might become a problem and a solid front at high speeds where a small fast airflow allows for adequate cooling.
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Old 02-18-2008, 04:35 AM   #2
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This sounds like it has a good basis for testing. If you had a removable frame you could remove and clean the screen as well. Likely only a certain flow of air can be achieved total, meaning some air would get in, enough say to keep the radiator cool enough, but less than normal. I'd love to see some A/B/A testing on this one.
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Old 02-18-2008, 05:59 AM   #3
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Is that what are you talking about?
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Old 02-18-2008, 07:17 AM   #4
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what you show above looks more or less like the toyota grills i'm talking about...

what i'd like to use however is something with even finer holes...

Its hard for me to test anything like this Fe wise because all i have is tank to tank fillups wich leaves to much room for variaton

but i do recall autospeed had an article where they used little else than some tube and a waterbottle to measure air pressure difference to seach for the best place to instal a CAI... i might give that a try if a can find the time.

does someone have more insight in how air would behave as it encounters, lets say a plate with a small hole in it, and how that behavior changes as the speed increases?
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Old 02-18-2008, 09:46 AM   #5
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I think you're absolutely correct.

At speed, I think the mesh would effectively become a solid barrier, yet allow cooling air to be dispersed in a manner closer to O.E.M.

In other words, if you did a 50% solid grille blockoff (top or bottom, right-to-left, whatever) you would likely do so at the risk of cooling air not being suppied to things like the valve covers, alternator, etc. as everything will be redirected.

Your way gives the same reduction in surface area but eliminates a couple of downsides.

I'm ultimately biased toward saving my motor, so I used a digital thermometer rather than a manometer to measure grille block results. I put a probe on the firewall behind the motor and measured my car (with NO modifications) on 100 degree summer day. (Where I live we have about 100 such days a year, so I decided that I could live with a 30-40 degree increase in underhood temp with the car under these conditions. Beyond that I don't have much confidence in the cooling system's ability to recover quickly enough to prevent engine fry).

In my case my baseline was 150 F. with the car idling, after a 10 mile loop. After 3 fan cycles the fan was able to keep the temps at around 150 F.

Blocking just one of the lower grilles (the Metro has 2 horizontal lower grilles) got me to 190 under the same conditions. The fan cycled constantly, but it did at east eventually turn off. Underhood temps never got lower than 180.

At any point beyond blocking the lower grill the underhood temps never came down. In fact they continued to rise to over 230 F. (at idle) before I pulled them off. At this point your cooling system is beyond saturation, and the fan can't begin to keep up with the demand.

This was on a car with NO undertray. I had to remove my grille block once I installed my front undertray just to keep the temps liveable.

I think using mesh would promote a more "over, under, around and through" circulation pattern and help limit the heat soak.

And it kills two bugs with one stone.
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Old 02-19-2008, 02:21 AM   #6
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Just from a personal standpoint I endorse this for testing, it might not do anything but I don't see it hurting, be worth a try.
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Old 02-19-2008, 03:44 AM   #7
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here's an autospeed article that has some indication of the pressure drop over a mech screen
http://autospeed.com/cms/A_0652/article.html
look at the first section with the airflowmeter
althought they where actually looking to remove restrictions. it does however shows that as the air speed increases the mesh becomes more restrictive.
in fact
of course the situation for a grillblock is different as the air in a tube has no choise but to go trough whatever restriction is throwt at it, while the air in front of the car can go over, under and around it.

i'll start looking for some mesh
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Old 02-19-2008, 02:28 PM   #8
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8307c4 -

Quote:
Originally Posted by 8307c4 View Post
Just from a personal standpoint I endorse this for testing, it might not do anything but I don't see it hurting, be worth a try.
Is there a simple way to test different meshes? For instance, let's say I mounted a C-thru plastic pipe/cylinder/whatever with mesh screen "X" on my hood. Then I would mount some light string (X-mas icycles?) dead-center in the tube. The idea is, at low speeds, the string should be flying like a little flag. At higher speeds the mesh should act like a barrier, and the string should go limp. Probably don't even need to mount it. Just have a friend hold it outside the window.

Does that make sense?

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Old 02-19-2008, 02:52 PM   #9
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This sounds like a reasonable thing to try on my Jeep.

I am getting about 16-17 on my commute and I have done as good as 20 if I really drive nice.

I think a piece of window screen is where he is going with this and I may be willing to give it a try.

Keeping the bugs out of the radiator is another plus.

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Old 02-19-2008, 03:53 PM   #10
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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:
Quote:
It was known that a mesh screen placed across an airflow will have an evening effect,
distributing both the velocity and pressure across the screen, however this effect had not been
quantified and the effect of different mesh geometries was unknown. It was desired to
understand the relationship between airflow velocity, the pressure behind the screen and the
free hole area of the screen.
It was concluded that the developed pressure is proportional to the velocity for a given
free hole area, and inversely proportional to free hole area for a given velocity. Screens with
less free hole area also maintain laminar flow on exit for a greater distance.
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.
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