Quote:
Originally Posted by JulianEdgar
The theory moved from:
Low pressure, existing over any horizontal surface will impart lift.
Porsche's Macan would be an example. Mitsubishi's Mirage 'G' model would not.
And the separation is implicated in the Macan's Cd 0.37, vs the Mirage's Cd 0.27. Same basic roofline. One with separation, one without.
to
1) the flow separates where the truncated roof ends. You can see the streamlines continue on and the void left underneath.
2) the pressure in that void has the pressure imparted by the streamlines directly above the TBL at the point of separation.
3) since this low pressure region occurs 'over' the rear of the body below the void , it's imparting lift. Just like Taycan, Panamera, Cayman, 911,.........
4) this low pressure is also connected to the wake, lowering base pressure, increasing pressure drag, increasing overall drag.'
One theory was about separation over the roofline, the other about separation at the end of the roof. The first theory was hastily changed when the wind tunnel pic showed, of course, no such thing occurring.
Incidentally, I don't think either theory is correct.
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Great! Thanks!
1) the lowest pressure is near the windshield header, yes? ( suction peak )
2) the closer to the windshield header, the lower the pressure, yes?
3) closely-spaced streamlines indicate high velocity, and consequently low static pressure, yes? ( Hucho, page-2 )
4) widely-spaced streamlines indicate low velocity, and consequently high static pressure, yes? ( deduced directly from Hucho, page-2 )
5) if separation occurs, the pressure of the attendant turbulence will take on that of the streamline directly above the turbulent boundary layer at the line of separation ( Bearman et al. )
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6) Porsche had every opportunity to make the roof 'full-length' ( like the Mirage ) but chose to cut it short.
7) The separation line, which might have been where the streamlines were more diverged at the very back, is now moved forwards, closer to the suction peak.
8) A void cannot support flow unless there is a perimeter framing the area to create a 'pool' of stagnant air which the active flow can skip over ( 1980 Mustang GT ).
9) Porsche has no such pool. It's separation line is closer to the lowest pressure on the car. All the turbulence created by chopping the roof is of low pressure, on 'top' of the remaining length of body below the void, defined by the absence of smoke.
10) that low pressure void is creating lift, whereas, the Mirage would have none of that kind. It's separation line occurs where the streamlines are furthest apart. Like the VW XL1. McLAREN Speedtail.