Grill Block question?

[winkosmosis]

Frank, you are misunderstanding what I'm saying. I'm not saying that the flow inside the engine compartment has no effect, but that the only effect it has on aerodynamics is the resistance to air going into the grill. That resistance could be equivalent to a 50% closed mesh, or a 75% closed mesh, but a rear block is equivalent to a 100% blockage of flow.

Your links don't refute that at all. I also stand by my statement that we can't know for sure whether air exiting the bottom of the car is good or bad for aerdynamics. It could very well be pulled out by the slipstream if you have a flat bottom. How about putting a cowl flap on your car?

Can you visualize a fan with enough power to suck through ALL the air that can possibly enter the grill? That's at one end of the continuum, and it would create forward force.

A car with an empty engine compartment that allows free airflow is down the continuum-- air can flow freely into it and exit underneath the car or through cowl vents.

A regular car is in the middle and there is more drag from the resistance to flow that is manifest at the grill.

A rear block is at the other end, with zero flow through the grill.


If you really think the resistance created by having stuff in the engine compartment is so high, then it's equivalent to a rear grill block anyway.

[shovel]

So.. cut a slot in your hood and "plumb" the air that passed through your radiator, smoothly - up into the air stream over top of your car. Then put a full pan under yer belly since you no longer have to dump engine heat down there, and you won't have to worry about the aero of your engine bay 'cause it will be isolated, right?

[winkosmosis]

Originally Posted by shovel

So.. cut a slot in your hood and "plumb" the air that passed through your radiator, smoothly - up into the air stream over top of your car. Then put a full pan under yer belly since you no longer have to dump engine heat down there, and you won't have to worry about the aero of your engine bay 'cause it will be isolated, right?

Discussion of hood vents
http://ecomodder.com/forum/showthrea...vent-6543.html


I have no idea if the top vents would be good or bad for aerodynamics. I read somewhere yesterday that race cars reduce drag by using the top of the hood for air outlets, but I'm not sure if that is true.

[wyatt]

Originally Posted by 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.

If I understand parachutes correctly, they work by increasing the surface area. The shape of the parachute is secondary and works twofold. The cup shape is better at catching air, but that takes me to the second point, it has to be cup shaped to stay inflated, i.e. it has to be inflated to maintain the surface area... If you could make a parachute that was flat and rigid of equal area, (and weight and was aerodynamically stable) you would fall faster, but it would still slow you down almost as well. I will argue that the surface area is king here. So I will agree, the front grill of a car with the grill blocked from behind is parachute shaped, but not of a greater area.

[donee]

Hi Wyatt,

Ok. Well, I do not think its only surface area. Ther aero guys seperate the two - one called form drag, the other parasitic (surface? ) drag.

To see the difference. Make a rigid hemispherical shell, and tie strings to it with cup up and down. Then time the drop with the same weight and see if you get the same fall times.

I am pretty sure the cup down orietation will fall more slowly. It has greater form drag. A simple model is that the air goes in, and them comes out. So, that in effect doubles the surface area. But in reality a buble forms, and some air is trapped going around in a circle inside the the shell. The ratio between the circular air flowing volume, and the overflow volume does not neccassarily need to be 2. But its greater than 1, and that is why the parashell falls more slowly cup down.

The reason the difference is so great in a parashell is that all the drag is associated with the parashell, and nothing else. In a car there are other issues.

Which brings up the idea of another experiment. Get a half cylinder shell (PVC tubing cut in half) the same height as the the grill work. And do a coast down test with the cylinder open end forward, versus rearward over the grill work.

[winkosmosis]

Originally Posted by wyatt

If I understand parachutes correctly, they work by increasing the surface area. The shape of the parachute is secondary and works twofold. The cup shape is better at catching air, but that takes me to the second point, it has to be cup shaped to stay inflated, i.e. it has to be inflated to maintain the surface area... If you could make a parachute that was flat and rigid of equal area, (and weight and was aerodynamically stable) you would fall faster, but it would still slow you down almost as well. I will argue that the surface area is king here. So I will agree, the front grill of a car with the grill blocked from behind is parachute shaped, but not of a greater area.

A parachute has more air resistance than a flat sheet with the same frontal area.

[gascort]

I think the correct thing to mention here is the possibility of air entering and then exiting, in the instance of the grill not being perfectly vertical and forming a pelton-wheel type of impulse.
Pelton wheel - Wikipedia, the free encyclopedia

[donee]

Hi Winkosmosis,

You mean surface area. Because a parachute actually has less frontal area than that of a flat sheet of the same surface area, and less drag too.

A half sphere has 2*pi*r^2 area. Its frontal area is that of a circle, or pi*r^2. So, it has twice the surface areas as frontal area.

Cd of a domed parachute is 1.5. Cd of a rigid square plate is 1.0. For the same frontal area, the flat plate has 1/1.5 = 2/3s the drag. But for the same surface area the rigid square has 2/1.5 , or 1 1/3 times the drag of the domed parachute. This is probably why DaVinci tried to make a stretched square fabric parachute originally. Fabric was probably pretty expensive back then.

Which jives with my previous email, and causes a discrepancy to the parachutes work by surface area theory. Identical surface area devices have different drags. Parachues work by form drag. Identical frontal areas devices have different drags too. The form is important to the true drag.


Check out:

Parachute Design



Wyatt,

By saying that its only surface area, your in effect telling motorcyle guys that they would get the same drag reduction on their bikes if they mount their shells backwards (driver side towards the foward direction).


Now, given, this drag difference is small compared to that of an overall car. But mounting the grill block behind the grill, will have more drag. One can always paint the coroplass black, and on the road nobody will notice the difference. And if you start to overheat, its much easier to remove! And, if there is any defect in mounting the coroplas, it does not get blown back into the thin radiator fins, but just falls sideways.

[winkosmosis]

No, I mean frontal area. I saw a chart somewhere on the internet a few weeks ago, and the bowl with the concave side facing into airflow had a higher cD than a flat disc.

[winkosmosis]

Parachute Descent Calculations

The cD of a dome shaped parachute is about 1.5