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Old 09-28-2023, 12:45 PM   #1 (permalink)
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'There is no single best aerodynamic shape for.....'

' There is no single best aerodynamic shape for a road car.'
Julian Edgar, from Vman455's thread, about R.H. Barnard and the template, recently found at page-29, here at the Aero. Forum.

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I'll take a stab at this, within the context of streamlining, and very low drag, as is my interest, and why the aerodynamic streamlining templates were offered as a tool.
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Caveat:
* this has nothing to do with corporate committees which decide the technical specifications for production road vehicles. It has only to do with off-the-shelf technology, and the feasibility of producing passenger cars with a drag coefficient in the neighborhood of Cd 0.07, to Cd 0.09, as per Hucho, December, 1986.
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1) There IS a single best shape for a road car of Cd 0.07, to Cd 0.09.
2) ' The perfect aerodynamic shape is the teardrop... The design of a perfectly aerodynamic car would need to head in this ( teardrop ) direction.'
Dr. Teddy Woll, Daimler-Benz, Mercedes-Benz.
3) ' The 'ideal' form for a road vehicle is... a cambered version ( of a half-teardrop )...' Dr. Richard H. Barnard
4) ' The teardrop has the shape that air can flow around with particularly low resistance- round at the front and tapered at the rear, so there's no 'wake' and no drag against the direction of travel.' Marcel Straub, Porsche.
5) ' A teardrop-shaped body offers minimal drag.' Rick Voegelin, MOTOR TREND.
6) ' The ideal teardrop shape, for example, has a long tail.' Rich Taylor, Popular Science.
7) 'A teardrop shape has a minimal wake-the streamlines come together at the end of the long tail.' Julian Edgar, p.127, 'Modifying.................
8) ' Aerodynamic experts say that a car with the best- streamlined shape will look like a teardrop.' HT Auto Desk
9) ' Optimum shapes are 'streamlined' bodies having a very slender rear part.' Dr._Ing, Dietrich Hummel.
10) ' the longer the tail, the less wind drag occurs.' Professor Herbert J. Carver.
11) ' The coupe-like roof form and tapering rear end are also designed for ideal aerodynamic performance.' Daniel Scharfscwerdt.
12) ' When vehicles developed for the lowest drag coefficient are evaluated for Cd vs length, the correlation is discerned and expected trend is in fact confirmed.' Hucho.
13) ' A long tapering rear is still good for a vehicle's aerodynamics.' Dr. Teddy Woll.
14) ' the drag reduction from an elongated tail varies almost linearly with the reduction in cross section area.' Dr. Jeff Howell et al.
15) ' High base pressure explains low drag.' Julian Edgar, p.80, 'Modifying...
16) ' Steep tapering ( of the rear contour ) is limited as the resulting flow separation increases air drag.' Hucho.
17) ' our data suggest that road vehicles do ( have optimum fineness ratio values ). William H. Bettes
18) ' For well-designed forms ( of bodies of revolution ) the drag coefficient may be as low as 0.04.' Richard Von Mises
19) The lowest friction and pressure drag sum for a streamline body of revolution, Cd 0.04, occurs with a free air fineness ratio of 2.5:1 ( re: Hoerner )
20) ' With refinements in aerodynamics progress is towards the ( streamline) body of revolution.' Hucho.
21) ' the optimum shape ( of a road vehicle ) in terms of drag is a ( streamline ) half-body, which forms a complete ( streamline ) body of revolution together with its mirror image- produced through reflection from the roadway.' Hucho.
22) '(T)he drag of the basic ( Cd 0.07 to Cd 0.09 ) body (of revolution-derived vehicle ) is achievable. To what extent this can be approached in the development of a production vehicle is therefore more a question of the balance of the requirements of the specification, than of technical feasibility.'
Hucho, December, 1986.

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Last edited by aerohead; 09-28-2023 at 12:49 PM.. Reason: add data
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Old 09-28-2023, 03:07 PM   #2 (permalink)
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'There is no single best use case for a road car.'

Passengers, mobile dwelling, ambulance; possibly some others.
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Old 09-28-2023, 08:13 PM   #3 (permalink)
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Yes, it very much depends on use case. Crumple zones? Cargo capacity/access? Headroom? Legroom? Curb weight? Weight balance? Underbody design and size? Winter or summer usage? Mid, front, or rear engine? Etc.

I’d say to look at nature. In my view, most fish are similar in shape, but different. The basic ideas of a round nose and a streamlined tail are present. What is employed to reach that shape differs depending on different factors.

However, I’m still an amateur.

I think Luigi Colani is a really good person to study in this regard. He designs cars that go against a lot of low drag ideologies and they’re all slippery.
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Old 09-28-2023, 09:18 PM   #4 (permalink)
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Luigi Colani

Cut his teeth on fiberglass Beetle bodies. www.bubblemania.fr/en/inventaire-colani/


https://i.pinimg.com/originals/e6/b6...207d33b628.jpg
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Old 09-28-2023, 10:36 PM   #5 (permalink)
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A somewhat on topic question for aerohead is:

If you had full ability (money, time, energy, creative reign) to redesign your T-100 from the ground up, what would it look like?

I would love to see that, even if a separate thread.
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Old 10-01-2023, 02:51 PM   #6 (permalink)
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Quote:
If you had full ability (money, time, energy, creative reign) to redesign your T-100 from the ground up, what would it look like?


Speaking of Julian Edgar:

I think he does a good job on the counterintuitive aspects of shape on pressure.
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Old 10-02-2023, 11:58 AM   #7 (permalink)
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'separate thread'

Quote:
Originally Posted by JacobLeSann View Post
A somewhat on topic question for aerohead is:

If you had full ability (money, time, energy, creative reign) to redesign your T-100 from the ground up, what would it look like?

I would love to see that, even if a separate thread.
We could kick that around if you all want to, but, so as not to adulterate this thread, and confuse members and guests, I recommend that we parse it out into it's own thread.
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Old 10-02-2023, 12:44 PM   #8 (permalink)
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'YouTube'

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.
------------------------------------------------------------------------------------
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.
-------------------------------------------------------------------------------------
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.
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Last edited by aerohead; 10-12-2023 at 12:35 PM.. Reason: typos
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Old 10-02-2023, 02:32 PM   #9 (permalink)
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Quote:
It was difficult for me to watch.
Like reading a newspaper story about something you were involved in, no doubt.
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Old 10-04-2023, 04:16 PM   #10 (permalink)
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Julianís video is a great example of testing: on a real car, in the real world, with attainable equipment (the pressure scanner is on the expensive side, but you can make measurements like this with very cheap equipment).

I know for a fact there were multiple back-and-forth discussions between the expert reviewers, as they argued over points and concepts and even word choices. Some of these even made it into the final version of the book, where two contrasting opinions are each given a sidebar so the reader can see the discussion and differing views (and Iím happy to have contributed one of those, contrasted with Rob Palinís view on the next page). Aerodynamics isnít a settled science. There is lots to investigate and research and learn still. The expert reviewers wouldnít have allowed their names to be attached to this book if they didnít stand behind everything in it.

That said, there is nothing untrue in any of Julian's points:
-Pressure drag accounts for 80-90% of the drag on a typical road car
-Pressures vary because of variations in local flow velocity (where flow is attached) and separation
-We donít have to assume or imagine pressures on a car because we can measure them, on real cars on the road
-Air behaves unintuitively and sometimes our mental models are incorrect
-To reduce pressure drag, we can reduce pressure on front-facing surfaces or increase pressure on rear-facing surfaces
-This also includes oblique surfaces where a pressure vector has a horizontal component because of the surfaceís inclination
-Rear spoilers, if shaped correctly, can not only reduce lift but also drag
-Tail extensions can reduce drag
-Reducing the base area exposed to the wake can reduce drag
-Low pressure on front-facing surfaces can contribute thrust and reduce drag
-YOU can measure pressure changes on the road and SEE if changes you make are reducing pressure drag

Thatís all I have time for, for now. Say my name three times and maybe Iíll appear again in future.

Quote:
Originally Posted by aerohead View Post
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.
--------------------------------------------------------------------------------------
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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