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Remember also that the direction of rotating vortices depends on the overall lift/downforce of the body. That is, when viewed from behind, bodies that develop lift have clockwise vortices on left, and anticlockwise on right, whereas downforce bodies have vortices that rotate in the opposite directions.
It's easiest (for me at least) to think that this is so because, on a lifting body, there is a lower pressure on top, and so air passes from the sides to the top of the car. (Very clear on A pillars for example.) Now if there is this airflow direction, but the car is moving along, the 'flat' flow becomes a spiral ie a vortex.
On that basis, you'd expect vortex formation to be lowest on a body with zero lift/downforce, and I understand that this is the case. But then Rob Palin told me that in the wind tunnel, you can see vortices coming off every rear edge of the car, so as I say, it gets much more complex. When I told him I had borrowed the 16 channel high speed logger, he wrote:
Measuring pressures on-car is always interesting, and gets even more so (although increasingly obfuscating) as you start to dig into transients, both 'intrinsic', from periodic shedding & couplings around the car, and 'extrinsic', from ambient turbulence, and interactions/admittances/resonances between the two. Something I've definitely witnessed is the coherence & amplitude of such transient phenomena becoming much larger as the car's shape gets more slippery, and a lot of the 'white noise' from bluff body / chaotic turbulence fades away. It makes the analysis easier, but what shows up can raise the eyebrows!
Re the 5th edition, I find it a much harder book than earlier editions - there's just so much in it on every area of car aero.
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