Quote:
Originally Posted by JulianEdgar
What Aerohead said on 12/5/2020:
You can be confident that there is no separation at all on the roof of the Cayenne - this is just two versions of Aerohead's weird theories of lift on squarebacks. (Or, if you want to overlook the inconsistencies - no lift on squarebacks.)
Or is Aerohead actually trying to say that lift is caused by the wake pressure acting on the angled rear glass? (It's so hard to understand when he writes in such an obscure style - what is a transom? I didn't know the Cayenne was a boat.)
Either way, the theories are wrong, as I explained in detail in the original post.
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1) the separation occurs right where the raked truncation is.
2) there are some wind tunnel images of smoke flow testing of other SUVs in which you can clearly see the boundary layer leave the roof, exposing a void underneath, which is of low pressure. Turbulence takes on the pressure of the separation line which created it.
3) since it's impossible for the flow to reattach, the entire wake is subjected to this low pressure over the backlight. If you draw the force vector at the backlight, you'll see the vertical component of lift acting on that incline, acting over the rear overhang.
4) this low pressure decreases the the base pressure, which in turn, raises the pressure drag.
5) since Hucho told us that the whole point of streamlining is to reduce, or eliminate separation and its attendant pressure drag, Porsche designers have succeeded in purposely raising the Cayenne's drag. It's not as bad as the Macan ( Cd 0.37 ), but it's pretty nasty.
6) 'transom,' we can use it. We're talking fluids.
7) Both flavors of Mercedes-Benz GLC are Cd 0.31 by comparison. The RAV4 is Cd 0.30.
8) See if you can find lift data for the Nissan Cube ,or Kia Soul. They would be a good examples of contemporary squarebacks.
9) as to 'theories', get ahold of Wolf Hucho, he studied under Hermann Schlichting, author of BOUNDARY LAYER THEORY, every aerodynamicist must study Schlichting's work.