Grill Block question?

[winkosmosis]

Originally Posted by Frank Lee

Credentials? Backing data?

It's just the physics of aerodynamics itself. Drag is the force of the vehicle acting on the air that it's travelling through. The resistance of the grill is presented at the grill. Think about a small grill vs a big grill on the same car. Which grill creates less drag? The small one, even though the radiator behind it is identical, because smaller volume of air can flow more easily into it. Blocking the radiator behind that small grill can't physically reduce the grill's drag, because it makes it 100% opaque to flow instead of 50% opaque or whatever.



Metro, you're right that the air exiting the vehicle underneath affects drag, but I don't think we have any reason to believe it's negative. It could be positive if it pushes the airstream out from the rough stuff.

[Frank Lee]

Show me the physics then.

[winkosmosis]

Originally Posted by Frank Lee

Show me the physics then.

Show me the physics that tell you the drag in an engine compartment presents a worse source of resistance at the grill than an inside block.

[Frank Lee]

Motor Vehicle Dynamics: Modeling and ... - Google Book Search

Pgs 104-105: "...internal flow in the engine compartment can account for about 13% to 20% of the total drag..."

Incidentally... Pg 110: "Shape drag is what remains of the drag if the contributions due to friction drag and induced drag are removed and, in the case of road vehicles, it is mainly due to the wake."

OK instead of me transcribing a whole book for you, why don't you just read from my link in particular pgs 89-132.

Leading Edge #21: Remember Your Cowl Flaps "...All that air flowing through the engine compartment, though, also creates a tremendous amount of drag, which will rob you of cruise airspeed."

Compatibility of Automobile Interior Space With Low Aerodynamic Drag Body "...Since engine cooling flow is one of the dominant factors among aerodynamic characteristics, resistance due to internal flow in the engine compartment are also explained."

http://www.dd.chalmers.se/~lelo/rvad...on-to-drag.pdf No Adobe on this computer... what does it say?

As far as the grille: Image of a Fluid Flow

As shown in Fig. 11.87 there is a stagnation zone around the front of blunt objects. Whether the grille is blocked behind or in front, the effect is largely the same, as aerohead noted in post 4.

I have also read that headlight buckets in older style vehicles didn't really hurt aero because of the stagnant bubbles in front of them. Sorry could not find web links for that.

Now it's your turn.

[donee]

Hi All,

I think we can all agree that parachutes work. And parachutes do not have air flowing through them.

The front grill of a car , with the grill blocked from behind is a parachute. But, in the overall drag of the car, its not that big a percentage. It takes energy to form the air bubble. Otherwise, skydivers would die.

Letting air flow through the non-aerodynamic spaces of the engine comparment is going to have allot of drag too.

[diesel_john]

Frank, that .pdf file is titled, Contribution of different devices to the total drag. An eleven page paper with figures, pictures, conclusion is a little weak but basicly says people aren't ready to accept a truely efficient car shape. later gator

[Frank Lee]

Thanks John! (I'm not at my own computer)

As for the rest of it, I have come to the conclusion that this job doesn't pay enough. I have read and re-read several texts on vehicle aerodynamics. I have worked in vehicle design in industry. I have gotten all the aero training pilots get too. I have been to an SAE aero seminar and still have the coursework. I researched and composed an internal paper on aero for my former employer. I have toured several wind tunnels and spoke with the operators. I am not a professional aerodynamicist but I think I know more about it than the average bear. Not saying I couldn't ever be wrong, but...

Bottom line is, put forth the effort to find an authorative text and try to understand it. It's a bit much to cover piecemeal in a forum. THEN discussions about aero are less likely to be like discussions on religion and politics- more facts (or at least proven theories), less opinions.

I Googled but did not find online backup for my assertion that a behind-the-grille block is virtually no less aero than a before-the-grille block. As aerohead alluded to in post 4, a stagnant bubble forms then blah blah blah.

Same goes for parachute aero. If a parachute had a giant automobile shape behind it, well let's just say I would not want to be dangling from it. It is the aftbody influence...

Peace out.

[MetroMPG]

Thanks for the heroic effort anyway, Frank. I gave up on this thread (and another, with the usual suspects) quietly, much earlier than you did.

[Frank Lee]

Sometimes I don't know when to quit!

[some_other_dave]

Just as a note:

I have the radiator in my (US-spec) CRX completely blocked with a sheet of coroplast. I have no idea if it has helped the aero at all, but it sure as heck has helped the fuel economy! I suspect that is primarily through helping the engine warm up faster, particularly when I am doing EOC. But there could be aero benefits as well.

I have only had one occasion when temps rose to the point of being a concern, and that was on an unseasonably warm afternoon with a long-ish freeway trip. It took less than 5 minutes to remove the block (I have to unbolt the radiator brackets) and I was on my way again.

-soD