[ChazInMT]

Lucky me, I was driving back from work in the rain the other day and was right behind a 2005ish Beetle and noticed the air patterns highlighted in the mist. My main observation was the 2 large trash can sized counter rotating vortex's spinning behind the rear fenders. I mean it was very noticeable. They were quite energetic and rotating fast with the insides of each vortex going downwards. To clarify, the shapes were like trash cans horizontal to the road with the open end of them pointing at the back of the car. There only appeared to be a foot in between the 2. Looking at other vehicles, I saw nothing of the sort, or very subdued versions of the horizontal tornadoes.

This has me thinking that a lot of the low Cd problem with the car is the energy required to generate these swirls of air. I think it would be interesting to make 2 models which were identical in the front, with the same frontal area, but design the backs so one creates these large vortices, and the other it's just "dirty air" and see the difference.

Something else was apparent, in order for these vortexes to be so close to the rear end of the car, the flow on the back centerline must be remaining attached quite far down the back enhancing the strength of vortices.