Want low drag? Don't follow a template.

[freebeard]

Further to #79 — Here's a simplistic explanation of node editing: wiki.blender.org/wiki/Source/Nodes/Modifier_Nodes

Geometry Primitives
New object types are introduced to handle new primitive types.

Point cloud
Hair
Volume (already in Blender)
Those object types, as well as the mesh objects can be passed around in the nodetree via the "Geometry" socket. It is possible for the system to handle implicit conversion, but there are explicit settings required in some cases.

'Hair' for our purposes are streamlines.

[AeroMcAeroFace]

Originally Posted by Vekke

That solar car is exellent example lets use it with the template:



Sad there is not clear picture the roof head cover at their pages, but with quick tilt of the picture is fills the template form pretty nicely to my eye.

Coincident? I dont think so.

I don't think anyone disagrees that perfectly following the template with a true half body of revolution on a plane is incorrect. That is what that drivers pod is, the perfect and correct use of the template and that shape.

What is incorrect is to say that the shape can be applied universally to non half bodies of revolution, or that a car that doesn't follow the shape is somehow wrong.

[aerohead]

Originally Posted by AeroMcAeroFace

But isn't that his point entirely? That low drag can be achieved without the/a template?

It is irrelevant whether it was for styling or drag, it achieves low drag, and doesn't follow the template.

"I have understood there can be attached flow with even steeper curves, but the the lowest drag is achieved if you follow the template." where is the evidence for this claim?

The template, I am pretty sure, without wheels, theoretically has a drag coefficient of 0.09.

Many solar cars with wheels, that are real, have drag coefficients around and lower than 0.09. But also none, of the many papers I have read about solar car design, say "we used an arbitrary template as the starting point".

I don't see what is relevant about being a mechanical engineer, mechanical engineering and aerodynamics are completely different things.

(I am not saying the template is useless, but it has many claims, and I have seen contrary evidence elsewhere in scientific research papers (not just in Julian's videos))

* In free air, the streamline body of revolution that the template is derived from is Cd 0. 03769.
* The half-body derived from the streamline body of revolution is Cd 0.07538.
* The data is from Sighard Hoerner's 1951 drag table, at Reynolds number = 10-million, reproduced in Hucho's 2nd-Edition, Figure 4.119, page 200.
* There's nothing theoretical about it, since 1923.

[aerohead]

Originally Posted by AeroMcAeroFace

"What I am saying that template leads to lowest drag coefficient"
it just doesn't though, low drag yes, lowest, no.

A solar car having drag coefficient of 0.08 is evidence that lower drag coefficients than the template are possible.

The development process is irrelevant if they get a lower drag coefficient.

There's no question as to lower drag coefficients.
Consideration of the template involved the context of a 4-to-5 passenger family car, with an overall height of, say, 55.5-inches, like an Optima, Sonata, Camry, Accord, Jetta class vehicle. Room for luggage. Conventional, upright seating position.
Some of the solar cars are already as large as a 1990s, Chevrolet Caprice Classic /Buick Roadmaster. To 'inflate' them, to large enough dimensions in order to facilitate what is now a full-recumbent occupant, into a conventional seating position would require dimensions as large as a UPS deliver truck.
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Delft Technical University's 2015 Ecorunner- V registered Cd 0.0512.
This remarkably low Cd must be understood within the context of a 'small', 'slow', sub-critical Reynolds number, laminar profile, with recumbent driver.
If also scaled to a height of 55.5-inches, the length would run to 259-inches, without any possibility of an 'extensible' tail section, due to it's particular architectural / geometric demands. Few garages could ingest it.
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Again, there's no dispute as to the ability to find lower drag forms, however, translating any particular form into a passenger vehicle opens up a large host of challenges, never seen on a closed-course SAE or SHELL event, or escorted, controlled-access World Solar Challenge-related, public thoroughfare.

[aerohead]

Originally Posted by JulianEdgar

No current professional car aerodynamicist uses the template for anything.

In fact one such aerodynamicist, who has worked for major car companies including leading car aero development (ie a real car aerodynamicist, not amateurs like us) told me how he very much enjoyed my video that debunks the template.

He didn't say: "Oh my gosh, don't you realise that the template is fundamental to all we do."

For those who think the template is so important, just read any current car aero textbook. "Optimal shapes" (all of them) are typically given only a few pages in the whole book - they're just not that important.

* A curious statement.
* Technical drawings ( blueprints) of contemporary passenger cars are available online to anyone. They offer multi-view images in 'true-length.'
* Comparisons to the template reveal current vehicles with template contours offered by: Porsche, Hyundai, Subaru, Chevrolet, Honda, Nissan, BMW, Buick, Mercedes-Benz, Mitsubishi, Toyota, Renault, Volkswagen, Aston Martin, Ford, Jaguar, OPEL, FIAT, SMART, Bugatti, Ferrari, Audi, Lightyear, Tesla,..........
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If you'd like to read about why the 'template' would not be chosen, a look at United States Patent Number 4,533,168, August 6, 1985 would be insightful. It's clearly spelled out.
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If you're interested in a specific contemporary aerodynamicist turning to the 'template' for a solution, I invite you to inspect Julian Edgar's handling of Audi's 2010, A7 Sportback, in SAE Paper Number 2011-01-0175, on page 186 of ' Modifying the AERODYNAMICS of Your Road Car, then the 180-degree logic reversal on page 195. Extremely curious!

[aerohead]

Originally Posted by JulianEdgar

No it is not - perhaps you need to do more than just watch from the sidelines.

There are (at least) four uses to which the template is often put here where the template simply does not 'work as it sits'.

It cannot be used to decide any of the following with any more accuracy than basically closing your eyes and guessing:

- Show where there is separated and attached flow on existing cars

- Guide the shape of rear extensions

- Show how rear spoilers on sedans should be positioned and shaped

- Allow the assessment of the ‘aerodynamic purity’ of cars

Therefore, when it is used in just these ways, it is massively deceptive and misleading.

I recommend we stay away from generalizations. Oversimplification.
I can, have, offered counterfactual evidence to all four of your assertions.
Some are within the borders of own your book.
On one page you appear to follow an SAE Paper script, and everything you write agrees with all my ' misconceptions.'
Nineteen pages later, your logic completely reverses.
There appears to be no continuity to your thoughts when you're off the teleprompter. Very confusing.

[aerohead]

Originally Posted by JulianEdgar

I am sure all that means something to you. I reiterate my point, which I would have thought so obvious as to not need to even be stated in the first place:

On a discussion forum I would have thought it normal behaviour to speak out when you know that there is misinformation being put forward in a post.

Excepting the times when you're on the receiving end, and we're attempting to correct your misinformation with vetted sources, meeting first with silence, then vitriol. No sign that you made even a cursory examination of data that others will spend $ 100,000 in tuition to gain access to.
Some, interested in science, would consider the evidence, admit fault, and move forward to a more productive use of resources.

[aerohead]

Originally Posted by JulianEdgar

Sure. But I think it's very different when a newcomer, asking for aerodynamic modification advice, is given a bunch of crap based around a mythical template - and people who actually know better, say nothing.

Maybe I am the only one those newcomers write to, saying how they cannot understand how their measurements and observations don't match what everyone else (apparently) endorses?

To use an old fashioned phrase, it makes my blood boil to see people being misled in this way.

* Your book is written for the car enthusiast and non-engineer.
* Those of us here, with an engineering background, will find certain aspects of your investigations and conclusions falling outside the bounds of scientific rigor.
* Three times already, I've attempted to direct you towards science which would provide a much deeper insight into your experiences and interpretations. And it looks like three times it was all for naught. None of your subsequent posting reveal any hint that you spent even a pico-second with the material. Had you processed it, it would be impossible for you to cling to your folk knowledge.
* I'm in doubt that your international panel of experts actually signed off on as much of your book as you claim. I'd very much like to see what your exchanges actually were, and did some things get lost in translation between an engineer-aeronautical engineer- aerodynamicist, and a sociologist-geographer.

[JulianEdgar]

Originally Posted by aerohead

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If you're interested in a specific contemporary aerodynamicist turning to the 'template' for a solution, I invite you to inspect Julian Edgar's handling of Audi's 2010, A7 Sportback, in SAE Paper Number 2011-01-0175, on page 186 of ' Modifying the AERODYNAMICS of Your Road Car, then the 180-degree logic reversal on page 195. Extremely curious!

I am sorry, but now you are just making things up. There is no reference whatsoever to a template in SAE Paper Number 2011-01-0175, the development of the Audi A7 Sportback.

[JulianEdgar]

Originally Posted by aerohead

I recommend we stay away from generalizations. Oversimplification.
I can, have, offered counterfactual evidence to all four of your assertions.
Some are within the borders of own your book.
On one page you appear to follow an SAE Paper script, and everything you write agrees with all my ' misconceptions.'
Nineteen pages later, your logic completely reverses.
There appears to be no continuity to your thoughts when you're off the teleprompter. Very confusing.

Isn't it extraordinary, then, that none of the professional aerodynamicist reviewers of my book agree with you?

I could believe five world-leading aerodynamicists - or I could believe Aerohead. It's not a hard decision.