[aerohead]

It was difficult for me to watch.
* It's intended for non-engineers. If the expert reviewers and proof readers considered that only lay people would be reading these materials, perhaps a certain 'latitude' was given. I don't know.
* Kind of a dog's breakfast.
* Contextual comments are made without the context, and passed along as if they were universal truths.
* At least one comment could never withstand scientific rigor, as there's counterfactual evidence available in the public domain.
* The data is never captured as in official investigations.
* The data isn't presented as in official investigations.
* The data is never normalized to reflect 'standard' conditions.
* There's a risk that the data is being misinterpreted.
* Teachable moments fall through the cracks.
* Mechanisms associated with the observed phenomena never mentioned.
* I experience over-simplification and confusion with respect to rather sophisticated fluid mechanics principles, which might not best serve the audience. And it involves the basics Dr. Hucho insisted engineers learn, one-third of the reason he published his 2nd-Edition.
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I'll do some more thinking on this between now and Thursday when I return. I've got some materials at home I'd like to revisit.
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Picking up where I left off:
I watched the video again for a number of times. And I really didn't experience any useful, actionable information, as was my initial reaction.
* While things were said, there was nothing offered which would offer insight into the mechanisms associated with 'pressures.'
* A speculation is that, Mr. Edgar and Vman455 haven't looked into Hermann Schlicting's 'famous' ( as described by Hucho ) Boundary-Layer Theory, all of which was verified by empirical tests as of the late 1970s.
* Nor did they respect Hucho's admonition about mastering Fluid Mechanics, something Hucho had Dr.-Ing. Dietrich Hummel write a complete chapter on for Hucho's second edition. A book Mr. Edgar has been in possession of.
* Both Hucho and Hummel studied under Professor Schlicting, and Hummel was one of the proof readers for the English version of Schlichting's 7th-Edition.
* ' Boundary-layer theory gives an answer to the very important question of what shape must a body be given in order to avoid this detrimental separation.' Schlicting, page-2, Introduction, 7th-Edition.
* Road vehicle aerodynamics is about reducing or eliminating flow separation at the rear of the vehicle.
* ' at separation, the increase of pressure over the rear part of the body does no longer, or only partly, occur. The dead space assumes a reduced or even a negative pressure.' Sighard Hoerner, page 19, Aerodynamic Drag, 1951.
* ' flow separation due to the adverse pressure gradient.' Adrian Gaylard, 2013.
* Aerodynamics is about the 'shape' of the vehicle, especially, all the car that's missing!
* It doesn't matter what the pressures are in between, nor their horizontal force vectors on inclined surfaces of the body.
* You, as the 'engineer' is tasked with doing everything you can to see that the vehicle's aft-body does not generate adverse pressure gradients which would be the trigger for flow separation and its attendant pressure drag.
* ' As a result of (a) bent roof, the flow is exposed to a deceleration that further increases the static pressure at the back... Simultaneously, the influence of the (base ) pressure (is) reduced by the downsized effective vertical area at the rear end. The result is a significantly lower air resistance force.' Mario Hirz & Severin Stadler, Graz University of Technology, SAE Technical Paper 2013-01-2414.
* ' A shape that exhibits no flow separation is called a 'streamlined' body.' Goro Tamai, page-8, The Leading Edge.
* The streamlined body provides the Attached-Flow Zone of Habitability.
* It's that simple.
* It's not a 'black art'.
* As far as really-low-drag, there will never be any new 'breakthroughs.'
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* The YouTube comment declaring about vehicle wakes: ' we'll never get those pressures positive,' is non-factual.
1) An Adler-Jaray sports car, investigated by Sawazki, VDI, Deutsche Kraftfahrtforshung Heft 45, 1941 ( Re. Fig. 9.4, page-160, Hoerner , 1951 ); reported positive pressure over the last 23% of the body, which if tested at the conditions of the Tesla Model 3 shown in the YouTube, would generate + 31.8-Pa, at the rear.
2) The 1961 Ferrari 250 GT, under the conditions of the Tesla Model 3, generates + 11.5 Pa in it's wake according to Professor Alberto Morelli and Sergio Pininfarina.
2) The streamline body of revolution from which the ASTs are derived ,was analyzed by Georg Fuhrmann 101-years ago, and demonstrates positive pressure over the last 24% of its body.
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While we're to consider 'slower airflow speed and higher pressure', and ' faster airflow and lower pressure,' there's never any mention of the streamline in the background of the 2016 M-B IAA wind tunnel photograph, which are 'explaining' those pressures:
* closely-spaced streamlines = high velocity, low pressure, and vice versa.
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1) We're shown a modified Honda Insight, Gen-I, with a ducktail spoiler, and informed that the spoiler increased pressure on top of the car.
2) And the presumption is that the airflow was fully-attached down the hatch, to the OEM spoiler.
3) Had that been the case, the the streamlines would have already been fully- 'widely-spaced' as far as the body goes at that length.
4) And consequently, the airflow would already be at its minimum velocity, and highest static pressure acting against the body( Bernoulli Theorem ( also never mentioned ), and reflected in the OEM-configuration pressure tests.
5) While the spoiler isn't 'higher', it's extending rearwards, encroaching on, and 're-converging' the streamlines to a higher velocity and lower pressure as it departs the new 'tearing edge' of the car.
6) And it's increasing the size of the wake as far as the streamlines go.
7) It's also 'ruining' the 'angle-of-flow' towards the wake.
8) Under these circumstances, logic suggests that the OEM flow 'wasn't' attached, and that the spoiler provided the same 'negative pressure gradient' as was provided with the 1972 Porsche 911 Carrera RS 2.7 ducktail, and 2011 Audi A7 pop-up spoiler, mitigating separation-induced 'downwash'.
9) The tuft-testing was not capable of revealing the hidden vortices.
* ' It is very hard to see trailing vortices.' Julian Edgar
* 'C-pillar vortices are linked to separation.' Julian Edgar
* 'A way in which a spoiler can reduce lift and drag is by promoting flow reattachment of separated flow.' Julian Edgar
* ' if you reduce lift without increasing drag, you've likely reduced vortice strength.' Julian Edgar
* ' lift was caused by separation.' Julian Edgar, #34 ( permalink ), ' A book to buy and read,'
* ' The strength and location of the generated vortices is heavily dependent on the rear shape of your car,' Julian Edgar
* Separated flow does not follow the contour of the car's body (corollary to Julian Edgar.
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I'll continue the Monday.
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Picking up from last week:
* Any expectation of realizing significant drag reduction from a rear spoiler would require evidence which has never existed, and likely never will.
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* Tail extensions are a definite 'YES', as long as they present a streamlined contour, incapable of super-deceleration, and the adverse pressure that would generate, leading to flow separation, giving you a 'worthless' tail extension.
And it may come as a surprise to the author of the YouTube that, the tail extension of the 2016, Mercedes-Benz IAA, shown in the video, reaches it's tearing edge out to the contour of the aerodynamic streamlining template-III. As does the Mercedes-Benz EQXX, and Lightyear Zero. Think Flight's, version-2, configuration #6, estimated Cd 0.1764, aero-modded Subaru Impreza boat-tail also appears to live in the neighborhood of the template.
And 'expert' consultant Rob Palin's Tesla Model X, 3-position pop-up rear spoiler, and Tesla Roadster Gen-II rear upper trailing edge, both fit the 'template-III'.
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As to a 'smaller base area,' again, this requires context:
* If the smaller base ( wake) area is not accompanied by the pressure recovery of a streamlined contour during it's creation, it's of absolutely no value to an ecomodder.
( it pays to perform dimensional analysis ).
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Finally, anyone interested in conducting a coefficient of pressure survey on their vehicle would be encouraged to contact Lightyear, and see if they'd share their data on any of their iterations ( all their roofline contours are identical ). Whatever pressure profile they report, is exactly what you'd be looking for as your 'solution.'
If you were to recreate the development of the 1972 Porsche 911 Carrera RS 2.7 'duckbill' spoiler, the same way as it's creators did, and in a modern, rolling-road wind tunnel, you'd be looking at $ 96,000 just for the wind tunnel time. I'm sticking with the 'templates.'
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That's all I've got for now. If my single synapse happens to fire off some other 'parade of the horribles', I'll add them as they excrete from my cranium.