The Fastback bloc

[aerohead]

Originally Posted by California98Civic

Aerohead, how might the distinction between Coefficient of Lift and the Coefficient of Pressure affect this discussion? It seems to me, aerohead, your observation about fastback bodies was about pressure, not lift. How are they different? I have been reading this really interesting paper that reports pressure and lift as part of an aerodynamic comparison between fastback and notchback body types:
https://www.researchgate.net/publica...DrivAer_Models

1) the first complication for us is,that neither model is a 'notchback' or 'fastback.'
2) they're both 'sportbacks.' One more aggressive than the other.
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3) the thing about lift is, that you have pressure acting over a surface of some area.
4) on a car, the lowest pressure is just head of the windshield header. That's your minimum pressure. All air around the car would like to go there. It's where the highest lift occurs.
5) on a 'streamline car, the negative lift at nose and tail cancels this lift.
6) on a non-streamline car there is separation. And typically, it's at the rear of the car where it goes off 'template.'
7) the closer the separation line is to the windshield the lower the pressure. ( it's faster and MUST be of lower pressure for conservation of energy).
8) many, if not most 'fastback' cars are 'pseudo-Jaray' forms. While they're 'swoopy' looking, they violate the ground rules of fluid mechanics, as Hucho refers to them.
9) if a fastback roofline descends too rapidly ( Taycan), the deceleration due to the expansion creates a pressure rise which the turbulent boundary layer cannot withstand.
10) the air adjacent to the body is at rest. It's at zero velocity. In the back of the car, the only reason the air doesn't flow forwards, is that momentum from the clean air above the boundary layer is 'shooting' kinetic energy of this momentum right down to the 'wall', the surface of the car, literally pinning it in place.
11) without a streamline contour, a too-rapid divergence creates a magnitude of pressure rise which exceeds the energy balance at that location, requiring the air to 'slow down', when it's already at zero velocity.
12) that's impossible, because it's already at rest. The result is that kinetic energy which formerly attacked the boundary layer, now just skips over it.
13) without the momentum interchange, the air slips, begins to move forwards towards the windshield, rolling up into eddies, then full-blown turbulence, with all kinetic energy now lost, other than heating the atmosphere.
11) Where this separation occurs, determines the local pressure which will be translated to the wake as base pressure/ pressure drag/ aerodynamic drag.
12) any flow, breaking away from the body before reaching the end of the car will be a t a lower pressure than achievable if streamlined.
13) and the entire area under this region of low pressure experiences lift, just like a wing. This is exactly what originally happened on the Porsche 911.
14) in the far back of Hucho's text, he comments that something like the duckbill spoiler added to the 911, reached up out the region submerged in turbulence, allowing inviscid ( outer clean flow) to capture a locked-vortex of which the inviscid flow would travel over as if it were solid, plus, block this region from the new wake, created by the spoiler, so as not to allow low pressure air access to the wake. The air would expand out only as far as the spoiler would let it, arriving as 'slower', higher pressure ( Bernoulli Theorem) air as it reached the spoiler/ wake.
15) simply lofting the back of the car would have cut drag and lift even more, but would violated the 'look' of the car; something that would take decades for Porsche to resolve, increment by increment.
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The original Mitsubishi Lancer would have been subject to extreme separation-induced drag/lift, partially mitigated by VGs and the rear wing of the 'Evo.'
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The Volkswagen XL1 would be a recent example of a streamline body. The main body is boat-tailed just like the greenhouse. With the speed limiter defeated, the car is capable of in excess of 100-mph. You don't see any splitters or spoilers on it, except for the diffuser. Smoke flow images reveal fully-attached flow all the way to the rear, then a clean Kamm truncation.It's dead-nuts on the 'template.' Just a coincidence.
So in summary, local pressure has everything to do with lift.And shape has everything to do with local pressure.
All wings have an angle-of-attack which produces zero lift. You can design cars that look like wing sections with zero lift. Pressures at nose and tail cancel any lift in between.

[California98Civic]

Wow. A lot there and more than I can digest. Let me redirect and try and be clearer on a narrower point. You had written that with a "proper fastback roof contour, the effective static pressure acting over the rear of the greenhouse is at the highest pressure of all roof profiles." I think Figure 7 in the article I linked supports that claim: at the rear of the roof, just ahead of the backlight, the coefficient of pressure is greater on the fastback than on the notchback. Nonetheless, the same article shows that the fastback produces a little more body lift (Fig. 5). For this specific comparison, both your claim and Julian's appear to be true. Isn't that right?

[JulianEdgar]

Originally Posted by aerohead

1) the first complication for us is,that neither model is a 'notchback' or 'fastback.'
2) they're both 'sportbacks.' One more aggressive than the other.
--------------------------------------------------------------------------------------
3) the thing about lift is, that you have pressure acting over a surface of some area.
4) on a car, the lowest pressure is just head of the windshield header. That's your minimum pressure. All air around the car would like to go there. It's where the highest lift occurs.
5) on a 'streamline car, the negative lift at nose and tail cancels this lift.
6) on a non-streamline car there is separation. And typically, it's at the rear of the car where it goes off 'template.'
7) the closer the separation line is to the windshield the lower the pressure. ( it's faster and MUST be of lower pressure for conservation of energy).
8) many, if not most 'fastback' cars are 'pseudo-Jaray' forms. While they're 'swoopy' looking, they violate the ground rules of fluid mechanics, as Hucho refers to them.
9) if a fastback roofline descends too rapidly ( Taycan), the deceleration due to the expansion creates a pressure rise which the turbulent boundary layer cannot withstand.
10) the air adjacent to the body is at rest. It's at zero velocity. In the back of the car, the only reason the air doesn't flow forwards, is that momentum from the clean air above the boundary layer is 'shooting' kinetic energy of this momentum right down to the 'wall', the surface of the car, literally pinning it in place.
11) without a streamline contour, a too-rapid divergence creates a magnitude of pressure rise which exceeds the energy balance at that location, requiring the air to 'slow down', when it's already at zero velocity.
12) that's impossible, because it's already at rest. The result is that kinetic energy which formerly attacked the boundary layer, now just skips over it.
13) without the momentum interchange, the air slips, begins to move forwards towards the windshield, rolling up into eddies, then full-blown turbulence, with all kinetic energy now lost, other than heating the atmosphere.
11) Where this separation occurs, determines the local pressure which will be translated to the wake as base pressure/ pressure drag/ aerodynamic drag.
12) any flow, breaking away from the body before reaching the end of the car will be a t a lower pressure than achievable if streamlined.
13) and the entire area under this region of low pressure experiences lift, just like a wing. This is exactly what originally happened on the Porsche 911.
14) in the far back of Hucho's text, he comments that something like the duckbill spoiler added to the 911, reached up out the region submerged in turbulence, allowing inviscid ( outer clean flow) to capture a locked-vortex of which the inviscid flow would travel over as if it were solid, plus, block this region from the new wake, created by the spoiler, so as not to allow low pressure air access to the wake. The air would expand out only as far as the spoiler would let it, arriving as 'slower', higher pressure ( Bernoulli Theorem) air as it reached the spoiler/ wake.
15) simply lofting the back of the car would have cut drag and lift even more, but would violated the 'look' of the car; something that would take decades for Porsche to resolve, increment by increment.
-------------------------------------------------------------------------------------
The original Mitsubishi Lancer would have been subject to extreme separation-induced drag/lift, partially mitigated by VGs and the rear wing of the 'Evo.'
------------------------------------------------------------------------------------
The Volkswagen XL1 would be a recent example of a streamline body. The main body is boat-tailed just like the greenhouse. With the speed limiter defeated, the car is capable of in excess of 100-mph. You don't see any splitters or spoilers on it, except for the diffuser. Smoke flow images reveal fully-attached flow all the way to the rear, then a clean Kamm truncation.It's dead-nuts on the 'template.' Just a coincidence.
So in summary, local pressure has everything to do with lift.And shape has everything to do with local pressure.
All wings have an angle-of-attack which produces zero lift. You can design cars that look like wing sections with zero lift. Pressures at nose and tail cancel any lift in between.

So much here that is wrong or confused. But let me ask for just one piece of evidence.

Aerohead wrote:

13) without the momentum interchange, the air slips, begins to move forwards towards the windshield, rolling up into eddies, then full-blown turbulence, with all kinetic energy now lost, other than heating the atmosphere.

Can you give me even one example of where that can be seen occurring (wool tufting, smoke testing) on any modern car?

I don't think you will be able to find even one example because this is just your weird theory of how lift occurs. Last time I asked, you told me that the wool tufts were not really showing what was happening - and doubtless you'll now say that about smoke streamlines!

in the far back of Hucho's text, he comments that something like the duckbill spoiler added to the 911, reached up out the region submerged in turbulence, allowing inviscid ( outer clean flow) to capture a locked-vortex of which the inviscid flow would travel over as if it were solid, plus, block this region from the new wake, created by the spoiler, so as not to allow low pressure air access to the wake. The air would expand out only as far as the spoiler would let it, arriving as 'slower', higher pressure ( Bernoulli Theorem) air as it reached the spoiler/ wake.

Are you sure that you're not just making this attribution up? I don't remember any such text in Hucho, but I am happy for you to give me the page number so I can look.

[JulianEdgar]

Originally Posted by California98Civic

Wow. A lot there and more than I can digest. Let me redirect and try and be clearer on a narrower point. You had written that with a "proper fastback roof contour, the effective static pressure acting over the rear of the greenhouse is at the highest pressure of all roof profiles." I think Figure 7 in the article I linked supports that claim: at the rear of the roof, just ahead of the backlight, the coefficient of pressure is greater on the fastback than on the notchback. Nonetheless, the same article shows that the fastback produces a little more body lift (Fig. 5). For this specific comparison, both your claim and Julian's appear to be true. Isn't that right?

It's an interesting paper - thanks for linking to it.

For my money, this is the key graph:



It's what we're interested in in terms of lift (ie it integrates the effect of all the pressures), and it shows greater lift for the fastback compared to the notchback at all angles of yaw. What a surprise (not).

Note though the graph is a bit confusing, showing negative coefficients. (The paper says: "Hence, the maximum lift force is claimed at a yaw angle of β=10°" and yet a negative CL would normally indicate downforce.) So higher up graph = greater lift.

Anyway, so much for Aerohead's oft-repeated point that fastbacks have low lift!

[California98Civic]

Lift is one of the things we're interested in here. However, it is not THE thing. For modding purposes and general learning about about aero, I am curious about the pressure coefficient and how it relates to lift.

Here is the Figure 5. What, aerohead or anyone, do I misunderstand when I read this as demonstrating that you were right to say a fastback shape would have a greater pressure coefficient at the rear of the roof?



Also, as an aside, I am still persuaded that a properly streamlined body with a fastback would produce less life than the notchback. But that will never look as awesome as a late 60s Baracuda or Torino.

[JulianEdgar]

Originally Posted by California98Civic

Also, as an aside, I am still persuaded that a properly streamlined body with a fastback would produce less life than the notchback. But that will never look as awesome as a late 60s Baracuda or Torino.

Well, all the evidence is against you, including in the reference you posted!
(I assume life = lift)

I am not sure what it therefore takes to convince people - maybe, as I keep saying, do some measurements for yourself?

[freebeard]

Originally Posted by aerohead

simply lofting the back of the car ... would [have] violated the 'look' of the car; something that would take decades for Porsche to resolve, increment by increment.
...
The Volkswagen XL1 would be a recent example of a streamline body.

VW was able to let go of the air-cooled Beetle body, but Porsche can't.

[California98Civic]

Originally Posted by JulianEdgar

Well, all the evidence is against you, including in the reference you posted! ...

ALL the evidence is not against ME, Julian. You don't know all the evidence, and this is not personal for me. There is a point about the body of evidence on the aerodynamic qualities of fully streamlined bodies you have never been willing to discuss seriously. I haven't given up, necessarily, but I'm not interested right now in repeating what's been shared before.

[JulianEdgar]

Originally Posted by California98Civic

ALL the evidence is not against ME, Julian. You don't know all the evidence, and this is not personal for me. There is a point about the body of evidence on the aerodynamic qualities of fully streamlined bodies you have never been willing to discuss seriously. I haven't given up, necessarily, but I'm not interested right now in repeating what's been shared before.

Where is the evidence that a fastback shape has low lift? The evidence you have presented in the cited paper says the opposite - no surprise.

I am quite willing to discuss the subject seriously, as I have shown with the evidence that I have presented in this thread. Fastback shapes in fact have high lift - just as you'd expect by looking at them and comparing their shape to other high lift shapes (aircraft wings come to mind).

But if you can find a paper, or other credible reference, that shows that car fastback shapes have low lift, I am certainly very interested. Very interested indeed!

[California98Civic]

Originally Posted by JulianEdgar

Where is the evidence that a fastback shape has low lift? The evidence you have presented in the cited paper says the opposite - no surprise. ...

Again you miss the point. I am not your research assistant, and I have nothing invested in convincing you of anything.