SteveP, I do agree with what you said. I just think everything we develop here needs to be understood under the pretext of "it depends." I like these types of threads...they make you think.
I think we'll best understand variances in drag due to ride height by making some simplifications. If we assume a catamaran configuration with completely smooth sides, we have essentially created a wind tunnel, which have been studied immensely.
Smooth Bodied Catamaran
I can calculate transistion points and total drag forces on flat plates easily, but I've never solved for ducts. I will try to determine how, but others studying aerodynamics have probably had to solve such a problem in at least one class.
If we vary the height of the duct, we can see when the flow starts to break down. Once we understand the "ideal" state, we can better infer how normal vehicles might fare. This is the most logical approach in my mind.
- LostCause