Idea for cars with un-aerodynamic windshields

[newguyintown]

So some cars have a gently sloping hood, and then an abrupt steep windshield, with a sharp edge at the top. This would accelerate airflow and then "launch" it upwards, right? That would cause separation of airflow, just like a vehicle would have on the back end. I was thinking you could put some kind of top on the car that starts at the slope of the windshield and then rounds off to keep airflow attached, then sloping backward toward the back of the vehicle, with possibly a kammback on the back end.

My question is, because the frontal area increases, is it still possible to have a net gain? I think so, because the total area affected by air could possibly be less, and the back end could be more aerodynamic.

Take a look at the thread "AeroTruck", on page 4 there is an example of airflow, where the frontal area increases, but the airflow remains attached, and is brought back downwards, reducing the wake.

I'm thinking this might possibly be a good modification for cars with a poorly designed windshield, that don't have problems with clearance.

[Frank Lee]

It is possible.

Were I to worry about it that much, I'd consider chopping the windshield down at the top and adding the curvature WITHOUT adding frontal area. If one is to go through all that work, they might as well shoot for a real gain.

[Cd]

If I understand your idea correctly, you are thinking of adding a small airfoil shape to the top of the roof of the car to direct the flow in the correct direction ?

We have discussed something similar here : Teardrop roof for boxy SUV?

If there is concern that you have flow separation at the top of the windshield, you could always just round off the edges. It doesn't take much of a rounded edge to keep flow attached.

You could then tuft test to see if there is indeed a problem still.

I remember reading a comment from one of the guys that worked on the EV-1 aerodynamics. He stated in a magazine article published in HotRod Magazine
( i think this is it here : Bonneville National Motorsport Event - Hot Rod )

He stated that one of the aero myths out there is that you need a steeply sloped windshield to get good aero. I think 45* degrees was quoted as the angle at which airflow becomes affected.

[newguyintown]

Thanks for the responses.
That thread is pretty much exactly what I was talking about thanks for the link.

[euromodder]

Originally Posted by newguyintown

So some cars have a gently sloping hood, and then an abrupt steep windshield, with a sharp edge at the top. This would accelerate airflow and then "launch" it upwards, right? That would cause separation of airflow, just like a vehicle would have on the back end.

On trucks, they try to get around this issue by putting deflectors / corner vanes on the windshield/roof transition as a damage-control measure.

These often double as sunvisors, and force the air to bend and follow the roof line.