Quote:
Originally Posted by JulianEdgar
I am not sure that is the best approach. In many cases, it appears more to be the shape of the C-pillars. I don't know of any comparative testing that has been published, but just by observation it's my guess (and only a guess) that C pillar designs that prevent airflow wrapping around the pillar onto the rear window reduce the strength of the vortices on fastback shapes. In other words, the more the airflow can be kept travelling longitudinally, the better.
If you look at the XL1, you can see this may well occur:
My tuft testing of the Tesla Model S shows this occurring:
..and the rear view shows the shape of the pillars well.
The Taycan is very similar:
Contrast that with my tufting of the XE Jaguar, for example, where you can clearly see airflow wrapping around the smoothly curved pillars:
...and look at how low the pressures are on the upper part of the rear pillars
(hotter colour = lower pressure).
(And if you look at the tuft and CFD images of the Jaguar, you can see - of course - that lift is created by the attached airflow... and not some weird theory that separation is responsible for lift on these shapes.) |
The tufts indicate that the airflow over the roof is moving faster, and is at a lower local pressure,as per Bernoulli's Theorem.
The higher pressure/ slower air along the sides is attracted to the low pressure above it.
On a 'streamlined' body, the sides of the vehicle are boat-tailed for approximately 66% of body length, and their velocity and pressure profiles mimic that over the roof very closely. There's no 'attractive' low pressure over the roof to induce transverse flow which would otherwise roll-up into vortices.
If you watch the 'Streamlined Toyota Pickup in the Wind Tunnel' video, I spent quite a bit of the session imaging the rear of the boat-tail with smoke. You'll notice completely-attached flow all around the perimeter of the tail, indicating completely attached flow, free of vorticity. You'll also see the thickening of smoke filaments as the air decelerates and gains pressure, on its way to the separation line on the tail.
I'll bet you a donut and coffee that the XL1 is vorticity-free.