Window screen grille block?
 

In the thread on perforated bellypans it was suggested that window screen makes "a perfect grille block on the highway" and allows "normal cooling in red light traffic." Is that true? Seems plausible. I would post this question in the original thread but it seems like a distraction from that thread's question about underbelly design. If true, I might redesign my full upper grille block. My fan starts up too often in warmer temps.

I saw that casual mention as well, and a brief internet search didn't turn up anything interesting. If there's some research out there, I'd love to see it. I'm not sure how to design the experiment to test it out, since it's a combination of drag and cooling effects.

I think in the case of a under body panel the mesh works fine because the airflow is parallel to the surface. However if the mesh is perpendicular to the air flow, like in a grill block, I would expect the air to go right through like a window screen. It might be possible with a fine enough mesh to create a pressure barrier that blocks flow at high speeds.

The effect of the mesh can be tested by putting an air flow meter at one end of a tube and the mesh at the other. Then mount it on the outside of your car. You could then determine what size mesh you would need and at what speed the blocking occurs if it does.

http://img37.picoodle.com/i54t/jthis..._234_u9tbf.jpg

^^^ this

although, a thicker screen material will block quite a bit of airflow, I work with the stuff and I can say with absolute truth that with the thickest screen material you can find you will block about 34% airflow.
whether that is worth it or not is up to you.
personally, Im going to build my grill blocks with perforated sheet aluminum that allows 43% airflow. this should give me the best of both worlds.

The way I picture it in my mind, the shallower the angle relative to airflow, the more gets blocked. So with a nicely angled grill, it would help more than a perpendicular one.

absolutely, the more angle the more frontal area is being covered.
so the application is definitely going to play a huge role on how well it works or not.
neither my old ranger nor the Xterra I have now would benefit at all with screen blocks, which is why Im going to use something of more substance.

Has anybody tried thin cloth? Spandex?

I think using screening as an air block material that is invisible to airflow at low speed and opaque to air flow at high speed is brilliant.

It is common knowledge in ventilation system design that the higher the volume of air trying to pass through a restriction the higher the resistance. It isn't possible to say that a screen would only block 34% of air, period. It depends on the flow rate of the air trying to get through. It is likely true that a screen in a window in a light breeze would block about 1/3 of the air flow, but the same screen in the same window in a hurricane would block a much higher percentage. Used as a grill block, I think most of the air would be blocked at 60 mph. However, if the screen does pass too much air a simple way to solve that problem would be to use two layers of screening, which would halve the open area. You could even use several layers applied in different areas to fine tune your grill block to let more or less air through at various places.

I can also envision the airflow deflected around the screen being more laminar and less turbulent than a solid grill block because the air flow would be changed less abruptly than with a solid piece. This would be more pronounced with a vertical grill rather than with a sloped grill.

what you say sounds good, but its false.
I can promise you it doesnt make much of a difference regarding wind speed, what makes the most difference is angle.
if you had a nicely angled surface (say, 45*) then yes, you would see more air flow blockage, but full frontal isnt going to be any more than what its rated for.
screen is simply not very realistic material to work with for extreme blockage.

it was mentioned on here in another thread about using screen for a belly pan, which might actually work, if you can keep it rigid, not a simple task.
to keep screening rigid at highway speeds (we'll say 60mph) would require a heavy duty frame work, and then would require some way of improving the "grip" of the screen spline that's holding the screen to the frame, most likely silicone.
the screen would deteriorate fairly quickly as well, so plan on replacing it probably once a year.
and if it received even a single rip, your aero would actually be worse than before the belly pan since it would effectively act as a parachute.

sorry for digressing, just thought it would need putting out there before its mentioned :)

Δ

"Rated for"?

You sound very confident. However, this is opposed to everything I was taught in grad school about pressure, flow, velocity and how they relate to resistance to airflow. The velocity of the air makes a huge difference in flow resistance. In fact, the resistance to flow across any restriction increases as the square of the velocity.

The formula is V=4005*√VP, where VP is the velocity pressure in inches of water (measured as the change in pressure, ΔP, across the screen). The formula indicates that as the air velocity increases linearly the resistance to flow by the screen increases exponentially. (4005 is just a constant and can be ignored here).

So I think I will stick to the engineering formulas that say as the vehicle's speed increases, the screen (vertical, slanted, whatever) will block a greater and greater proportion of the incoming air. How much screen, how many layers, or how small a mesh size are just engineering details to be worked out. But the idea of using mesh as a variable air block material is a good one.