Quote:
Originally Posted by freebeard
It's hard for me to accept that. Lift/drag is a summary action on the center of pressure vs center of mass.
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It's fairly easy to justify, actually. Let's imagine a typical car with a lot of lift. That lift comes from having a lower pressure on the upper surfaces compared with the underneath surfaces.
If there is a higher pressure underneath than on top (the other way of putting it) then you'd expect some of the airflow to be moving from underneath towards the top surfaces. As the car moves forward, that airflow then develops a corkscrew pattern, leading to the development of a trailing vortex. Now apply that idea to anywhere lift forces are occurring, eg as airflow wraps around rear angled pillars (eg C pillars in a sedan - airflow moving from side of car to lower pressure on trunk lid). You can see that as lift increases, so would trailing vortex intensity. That was Dick's basic point (I hope I have paraphrased him correctly) ie all lifting bodies develop trailing vortices. That is, overall lift (or downforce of course) causes the vortices.
Therefore, one would seek to reduce lift to zero in order to minimise vortex development.
Adrian Gaylard (Jaguar Land Rover) and Rob Palin (ex Tesla) said to me that is much too simple. They say that with cars, there is no direct correlation between the amount of lift and the strength of vortices. They say that to suggest there is, is a carryover from aeronautical aerodynamics (eg as with wing tip vortices, that do develop roughly as outlined above). Rob goes on to say that any surface on the car developing lift or downforce sheds vortices.
Therefore, one would seek to reduce any surfaces developing lift or downforce, (of course without in turn then doing anything that creates drag).
Thomas Wolf (Porsche) said to me that he thinks not in terms of vortices, but in terms of separation. Any separation produces vortices, so therefore,
to minimise vortex development, attempt to minimise separation. You can see how Thomas's point would apply to things like A pillars, around which there is often a small amount of separation. His point would also apply to separation at the rear end of the undertray under the car, and at the trailing edge of the trunk lid on top of the car (etc).