fine mesh as grillblock?

[lunarhighway]

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.

[101mpg]

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.

[holypaulie]

Is that what are you talking about?
http://www.titantalk.com/forums/attachments/titan-pictures-media-post/16171-new-dunder-grille-installed-closeup-grill.jpg

[lunarhighway]

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?

[mustngr]

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.

[8307c4]

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.

[lunarhighway]

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

[cfg83]

8307c4 -

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?

CarloSW2

[katoranger]

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.

Allen

[lunarhighway]

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_Files/Roberts_Fume_Hood.pdf
here's the abstract:
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.

[mustngr]

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.

[cfg83]

lunarhighway -

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_Files/Roberts_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
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.

CarloSW2

[DarbyWalters]

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.

[RoadWarrior]

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.

[holypaulie]

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?
http://www.sureflowequipment.com/custom-engineered/perforated_metal_mesh.gif

[RoadWarrior]

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.

[Dynamically Aero]

Take a look (www.slotcars4u.net/darkside.html) 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.

[lunarhighway]

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.

[RoadWarrior]

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.