My disappointment was that the digram you showed and ask us to imagine 3D showed the flow over and under the car but it was all about side pressures.
There is a silent minority at Ecomodder who hold that the air is standing still and it's the vehicle that is moving.
Which is to my point. If you have contact with Dr. A. Gaylard ask him this: All talk about boundary layer references free air. The underbody is a plenum with four open sides and two [relatively] moving walls. I know that when testing motor oil (analogizing from engine theory, I know) what they use is two concentric cylinders with an oil film between. The shear forces can tear oil molecules apart.
So the condition in the underbody should be shear forces instead of boundary layer. Or, I'm wrong. Whichever.
I have an idea for easy 3D diagrams, maybe I can prepare an example.