Air Flow - Up and Over or Over and Under
 

Tried to get an answer on a thread about a front air dam, but thought a more direct post would be better.

Looking at the aero-civic, the front looks designed to push the air over the car.
Some cars, like the Aptera, are designed to have air flow over and under.

Say my front end came out to a splitter, forcing air either over or under (air won't have to be forced sideways). Would I have to make sure the under-body is designed to keep the air clean so that it flows through?

A discussion on this might be beneficial for a small SUV that has a high ground clearance. If this were designed correctly, might be able to avoid adding more material to the front, sides and rear of car to make it "lower". Actually lowering the car may be an option as well, but then you have a increased worry over road hazards.

Hoping to have a good discussion!

I directed most of the air over my car because of its limited ground clearance. I didn't want to redirect so much air under the car that it was compressed and restricted by the limited space under there and develop a high pressure zone under the car. In outline, my car is a catamaran with an air splitting "pod" running along each side enclosing the wheels and a smooth underbelly promoting laminar flow under the car and expanding (diffusing) at the rear as the underside of the boattail rises upward.

The ideal shape for an ultralow Cd vehicle would provide a high ground clearance with a smooth underside with enough space under the vehicle to avoid interference with the air flowing under the vehicle. In essence, a low flying aircraft fuselage, which is the approach they take with many ultra-streamlined solar powered racing vehicles.

With an SUV, you have more ground clearance than my car has, so you could direct more of the air under the vehicle than I can by using a higher stagnation point, but you would still need to direct much of the air over and around the sides of the vehicle. The thing you want to avoid is directing so much air under the vehicle that it piles up, loses its laminar flow, and develops upforce under the vehicle. And needless to say, you would need to have a smooth transition from the front splitter to the totally smooth underside to keep the air flowing smoothly and laminarly under the vehicle.

Let's try this one: We are familiar with the wake of air behind a truck, moving with it, na? An air dam produces a similar wake, reducing the air speed seen by the rough undercarriage. It is like having a kammback that has an extension on the bottom that has been moved forward. If it is low enough it also lets a low-pressure area extend under the car, which reduces induced drag and helps cancel lift. With a splitter you force the flow up or down, but it will still try to follow the pattern you'd get with a simple rounded area which develops a stagnation point. It is nice to get that point at the vertical part of your curve, but not essential. Ideally, air going under should have the same surface speed as air going over, and be on an equally smooth surface. Unfortunately, the wheels have to intrude on the bottom, and there may be cooling openings too.

but there is more than one good aerodynamic shape
 

if you do a little looking you will find quite a few aerodynamic shapes. all you have to do is find one that resembles your vehicle and "fill in the gaps".

Of course the air is also flowing around the sides of the vehicle. In fact, there is more air going around the sides of, say a minivan, than going over. Often overlooked, I think, because of all the profile images we see. Wind tunnel plan views seem to be exceedingly rare.

Have you seen any?