perforated grill block?

[Frank Lee]

Sure about that?

[aerohead]

I'm in the,use solid material where your going to block,and leave open the areas your not going to block camp.The perforated metal shown,has no leading edge radii for the openings,and will result in what is known in fluid dynamics as high vena-contrata losses .Its virtually an aerodynamic abomination,a torture chamber,if you will, with regions of separated flow in either laminar or turbulent boundary layer conditions at each and every opening,and imparts the very drag your attempting to abate.Keep it simple.

[Frank Lee]

That's kind of the fancy way of saying what I said in #6.

[cfg83]

Frank -

Originally Posted by Frank Lee

Perforated screens are velocity sensitive; at low speeds they let in about what you'd expect- say, a 30% open screen will let in 30% of what an unobstructed vent would- and at higher speeds they "close up" to the oncoming flow and appear to the air to be more "solid".

At what speeds do you think the holes will "close up"?

CarloSW2

[Frank Lee]

I have an opinion, and we all know what that's worth!

It also depends on the screen. Some are much less restrictive than others.

But it's sort of an educated guess, and of course there is no "open/closed" behavior to this- it gradually goes from acting unrestricted to acting very restricted. To assign a speed value to when it acts "closed", then, we must decide when the most noticeable change of behavior is noted.

So, I really shouldn't even put a number out there, but for the off-road vehicles I used to design, I figured this occurred around 40 mph.

[brucepick]

Part of this discussion is based on the idea that less cooling air is needed at high speed.
Which is why the perforated panel holds some promise.

False premise.
Actually at highway speeds cars (my car, at least) need MORE cooling air.

I do appreciate the explanations of how/why the perforated panel tends to pass a lower percentage of air at higher speeds. Good stuff to know.

I built the grill cover panel you see in my avatar this past Sept. and installed it just before an early October road trip, 300 miles each way. With temps around 75 deg. F, it was fine in town but got a bit hot on the highway doing mostly 60-65. Frequently the temp gauge went up about 3/16" to 1/4" over the center point where it usually rests, and the elec rad fan came on to help cool things down. A few times, I ran the heater to cool it.

With cooler weather its been fine all winter. I'll be building a new one with a larger opening for summer.

[diesel_john]

variable grill block, i got two runners for a kitchen drawer and i just havn't gotten around to building yet. my plan is to have a NACA opening with a sliding cover that will start uncovering at the bottom first and gradually openup either by a choke cable and linkage or automatic by a temperature sensitive cylinder and linkage.