Just like to add to the cursory discussions on simple concepts of lift, drag, pressure & velocity. Unfortunately our discussion images/even video flow analysis are usually done in 2D, but further evaluation really requires 3d visualization. Vehicle design and actual airflow dynamics are realized in three dimensions. If you only choose a 2d cut plane of any shape, the flow will actually give you different values if it is done in 3d analysis. Armchair aerodynamicists, ecomodders, do not have (yet) easily available 3d flow aerodynamic performance tools. Maybe soon.
http://ww1.prweb.com/prfiles/2009/01...s15cropped.png
Exa PowerVIZ