New (free) tech paper

[JulianEdgar]

Originally Posted by aerohead

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.
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5) And observing those streamlines that have left the Porsche, had the roofline continued on, following the streamlined path, they'd be underneath more widely-spaced streamlines.
6) velocity would be lower there, by definition.
7) lower static pressure above the panel would be higher.
8) lift would be reduced.
9) if the body were extended to full verjungungsverhaltnis, the local streamlines would be at maximum- spacing, lowest velocity, highest pressure, lowest lift, lowest drag.

It's an interesting theory, but it's one completely different to the last one you put forward!

It's hard to keep up when the theory changes between posts.

[aerohead]

Originally Posted by JulianEdgar

It's an interesting theory, but it's one completely different to the last one you put forward!

It's hard to keep up when the theory changes between posts.

Okay, so how about we try and unpack that?

[JulianEdgar]

Originally Posted by aerohead

Okay, so how about we try and unpack that?

Um, someone who completely changes their theory, literally within a space of a few posts, hasn't much credibility? I am afraid that's how I would view it. After all, who knows what tomorrow's theory will be? Not really a good way to advance knowledge on a discussion group.

[aerohead]

Originally Posted by JulianEdgar

Um, someone who completely changes their theory, literally within a space of a few posts, hasn't much credibility? I am afraid that's how I would view it. After all, who knows what tomorrow's theory will be? Not really a good way to advance knowledge on a discussion group.

Please point out the discrepancy. I have no idea what your experience is with respect to what I've contributed, and how I can reconcile the situation.
Just a few key words could get me off dead center.
We're talking about the permalink a few spaces above, with my long explanation? # 31

[JulianEdgar]

Originally Posted by aerohead

Please point out the discrepancy. I have no idea what your experience is with respect to what I've contributed, and how I can reconcile the situation.
Just a few key words could get me off dead center.
We're talking about the permalink a few spaces above, with my long explanation? # 31

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.

[aerohead]

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.

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.

[JulianEdgar]

Originally Posted by aerohead

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. )
----------------------------------------------------------------------------------
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.

You seem to be implying that the pressures on the trailing edge of the roof will match the pressures found in the wake. That is not the case on modern cars; it may be the case on older cars with premature flow separation.

Current cars:

[aerohead]

Originally Posted by JulianEdgar

You seem to be implying that the pressures on the trailing edge of the roof will match the pressures found in the wake. That is not the case on modern cars; it may be the case on older cars with premature flow separation.

Current cars:

1) ' The pressure on the rear face of the body, in the separation region, is found to be approximately uniform and equal to the pressure just outside the boundary layer at separation.' P. W. Bearman, Dept. of Aeronautics, Imperial College, London, England, Re: ' Review- Bluff Body Flows Applicable to Vehicle Aerodynamics, Journal of Fluid Engineering, Sept. 1980, Volume 102, page- 265.
2) This pressure induces lift over the horizontal component of the panel. This low pressure contaminates the wake, affecting overall base pressure.
3) Porsche's truncation of the roofline IS the premature separation, on a modern car.
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4) ' The angle ( Greek symbol Theta ) between the shear layer at separation and the free stream direction is an important factor in determining base pressure.' Bearman.
Porsche's premature separation gives up this additional advantage of vectored flow into the wake, which would have occurred had the roof not been ' complexified.'

[JulianEdgar]

Originally Posted by aerohead

1) ' The pressure on the rear face of the body, in the separation region, is found to be approximately uniform and equal to the pressure just outside the boundary layer at separation.' P. W. Bearman, Dept. of Aeronautics, Imperial College, London, England, Re: ' Review- Bluff Body Flows Applicable to Vehicle Aerodynamics, Journal of Fluid Engineering, Sept. 1980, Volume 102, page- 265.

I don't think that you can state that wake base pressure reflects body pressures at the end of the vehicle.

There are numerous pressure plots available of vehicles - some show similar pressure in the base area and trailing parts of the body, but many do not. The coloured pressure plots I posted above don't, and nor do the following:






I also had a good look through my reference library yesterday and could find nothing that supports the idea.

[aerohead]

Originally Posted by JulianEdgar

I don't think that you can state that wake base pressure reflects body pressures at the end of the vehicle.

There are numerous pressure plots available of vehicles - some show similar pressure in the base area and trailing parts of the body, but many do not. The coloured pressure plots I posted above don't, and nor do the following:






I also had a good look through my reference library yesterday and could find nothing that supports the idea.

Customarily, the drag force is only the net, horizontal, axial force, generated by the pressure differential from front, to rear, expressed in terms of the projected frontal area. It's just dynamic pressure times CdA. ( 1/2 x rho x Cd x A x Velocity-squared )
Take the drag force, at any velocity, then divide by the frontal area, and you'll get the net force per unit area, as a function of delta-P.
Hucho felt that since bluff-body automotive drag is ruled by pressure drag, and pressure drag was a function of flow separation, then the rear base pressure would be of paramount concern as regarding drag.
I've never seen any text in which this concept wasn't promoted.
We would treat any vertical components separately under lift.