Want low drag? Don't follow a template.

[JulianEdgar]

Originally Posted by freebeard

How do you get from measured pressures to significance and action? Panel orientation to direction of travel? How do you generalize that?

Car aerodynamic pressures develop forces that act at right angles to the panel. You can then use the 'triangle of forces' to work out the magnitude of the drag/thrust and lift/downforce.

To understand its significance, the force is then multiplied by the area over which that pressure is acting.

I've done lots of videos.

https://www.youtube.com/watch?v=ZPsSHgQx5FA

https://www.youtube.com/watch?v=DOUAixnqO9M

https://www.youtube.com/watch?v=PoEGHtZB8TA

https://www.youtube.com/watch?v=sB-FPrYWLJo

[AeroMcAeroFace]

Originally Posted by aerohead

* 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.

And that is with zero ground clearance, lifted off the ground it is higher than that. I think I remember it as 0.15.
If there is far lower drag coefficient shapes then why are we chasing non-optimal? (non optimal in the sense of not in free air and close to a ground plane, basically where cars are)

Even if the template can predict separation, it would be able to do so only exactly on the centreline, because that is the only place where the flow is parallel to the direction of travel.

I think chapter 4 in Aerodynamics of road vehicles says it best
"In spite of its comparatively low drag, the passenger car is closer to a
rectangular box in terms of fluid mechanics than it is to a body of
revolution, though with refinements in aerodynamics progress is towards
the body of revolution. The flow round a car body is characterized by
separation (Figs 1.1 and 1.2) and its drag is primarily pressure drag.
Attempts to relate drag to primary shape characteristics (see section
1.2.3) have been unsuccessful. The number of parameters describing the
geometry of a car is too large and the interaction of the individual flow
fields too complex."

[kach22i]

Originally Posted by Cd

Two symetrical areas that cleared down the sides, with the center section having snow remaining.

I got the same pattern even more so on the top cover behind the cab of my pickup truck.

[kach22i]

Originally Posted by AeroMcAeroFace

I think chapter 4 in Aerodynamics of road vehicles says it best
"In spite of its comparatively low drag, the passenger car is closer to a
rectangular box in terms of fluid mechanics than it is to a body of
revolution, though with refinements in aerodynamics progress is towards
the body of revolution. The flow round a car body is characterized by
separation (Figs 1.1 and 1.2) and its drag is primarily pressure drag.
Attempts to relate drag to primary shape characteristics (see section
1.2.3) have been unsuccessful. The number of parameters describing the
geometry of a car is too large and the interaction of the individual flow
fields too complex."

I might have to agree, the last set of colorful CFD images I posed were from a PDF focusing on rounding the rear corners of the roof slope in order to reduce the chance of vortex formation.

That is to say, make the rectangular box more like the "primary shape - Aerotemplate Part-C".

I don't recall if it is a student paper or a professional one, will check.

EDIT:

Link says SAE paper.

2011-04-12
The New Audi A6/A7 Family - Aerodynamic Development of Different Body Types on One Platform 2011-01-0175
https://www.sae.org/publications/tec.../2011-01-0175/

Originally Posted by freebeard

Detachment is always associated to vortexes, yes no, maybe?

A vortex is just self-sustaining ordered turbulence?

Good questions.

And what is buffeting?

I'll post some images that may lead to conclusions.

EDIT-2: Will need to specify near wake or far wake region apparently.

PDF download.
https://www.google.com/url?sa=i&url=...AAAAAdAAAAABAJ


More of the same here:

The Effect of Vehicle Body Shapes on the Near Wake Region and Drag Coefficient: A Numerical Study
Hayder Kareem Sakran
Published 2016
Physics
The Journal of Engineering
https://www.semanticscholar.org/pape...f846b5309b934a




Another PDF

https://www.google.com/url?sa=i&url=...AAAAAdAAAAABAY


The angled vector represents both lift and drag.

If this is true for the rear, then negative pressures on center-line of hood up front would mean lift again and "negative drag" also called thrust.

Yes?

Did anyone ever show vector arrows for the hood?

Just assuming they did somewhere at some time.

[JulianEdgar]

Originally Posted by aerohead

' To know and not tell makes cowards of men.' Abraham Lincoln
Okay, so what does it say specifically. Otherwise you're just name-dropping. I'm not impressed by book covers.
Perusing SAE International's website, and the PREVIEW feature for recent SAE Papers hasn't lead to any revelations in modern road vehicle aerodynamics that I can discern. Quite the contrary.

Out of interest, I had a look in this book to see its coverage of 'basic low drag shapes'.

There are just two pages on this topic - in a book of a little under 1300 pages!

As far as I can see, these shapes are never mentioned again - and certainly are not used for any of the following purposes:

- 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

In the real world of road cars, basic shapes like the template are pretty well irrelevant.

[aerohead]

Originally Posted by JulianEdgar

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.

Some things to consider:
* none of your experts may have ever been tasked with designing for really low drag. The automakers associated with your consultants have never produced a low-drag vehicle, excepting Volkswagen, of which Buchheim's 1981 VW 'Flow' body long-tail was, in part, an inspiration for the 'template.
* If you include Hucho in your mix of five, things become extremely problematic, as the 'template' is a derivation of Hucho's default checklist for future low-drag vehicle specifications, going as low as Cd 0.07.
* According to Hucho, something very much like the 'template' would be the only path one could take.
* Any of your consultants who've read Hucho's text would be familiar with the context of the 'template.'
* Any of the aeronautical engineers would realize that the progenitor of the 'template' has the lowest drag known for 3-D flow, at supercritical Reynolds number.
* And they'd also know that the half-body derived from same, would have the lowest drag as well.
-----------------------------------------------------------------------------------
All that said, there are considerations that would preclude the use of the 'template', however, those have nothing to do with aerodynamics. As has been clearly stated by Hucho and others.

[aerohead]

Originally Posted by JulianEdgar

The reality is that not only was a professional aerodynamicist (Dick Barnard) used as a technical consultant throughout the book's development (reviewing every paragraph), but the book was also reviewed in detail by another three experts as it was being written (ie I sent them each chapter in turn and they wrote extensive comments).

Since publication, the book has been reviewed by a further three professional aerodynamicists (you know, like the current head of Porsche aero) who have also given me extensive feedback.

None of the feedback from these professional car aerodynamicists would cause me to change the book sections you say are incorrect.

So I could believe what five top experts in car aerodynamics tell me - or I could believe Aerohead on Ecomodder. Hard choice, I know.

* Can Mr. Barnard explain why automakers have designed new products, every product cycle, since 1987, which are more and more like the 'template' ?
* Can Mr. Barnard explain why the most expensive automotive products on the market today, incorporate the 'template' roof contour ?
* Can Mr. Barnard explain why the most recent production car of lowest drag happens to incorporate the 'template' contour? ( Figure 2-45, page-48, ' Modifying the AERODYNAMICS of Your Road Car.'
* And why some future products do also ?

[aerohead]

Originally Posted by JulianEdgar

Seriously, you are now just making up stuff.

The paper says absolutely nothing of the kind about the template or any such 'optimal shape'. Nothing at all!

And I have never said that any of the things you are quoting me as saying are impossible.

Perhaps you can find some direct quotes to support all this?

* Which brings us back to the 'perspicacity' issue.
* The SAE Paper says exactly what I say, if one connects the dots.
* For over 6-months now, you've written tirelessly about my 'theoretical' impossibilities. B.S.. Misguiding. Making stuff up.
* So far, you've refused to learn fluid mechanics, especially boundary layer theory. And until you do, nothing I say is likely to make sense to you. However, that does not impact the existence of fluid mechanics.
* Reality is what lay outside our minds.
* Your book is written for non-engineers.
* I have a B.S. in Mechanical Engineering Technology. A Ph.D. library.
* For $ 75,000 you can pay for up to your 3rd year in the curriculum. That's where you'll be introduced to fluid mechanics.

[aerohead]

Originally Posted by JulianEdgar

Yes of course they all read both pages. They didn't seem to have any issue - but hey, what would they know?

* As an engineer, and 46-year student of road vehicle aerodynamics, it's very disturbing to see logic completely reversed within 19-pages within the same text. Especially when the specifics of the reversal were clearly laid out by Hucho.
* Which forces me to bring up the subject.

[aerohead]

Originally Posted by JulianEdgar

I've shown you the reference. Go and read it - and learn a great amount.

* Perhaps this will be better served if you attempt to regurgitate the information in your owns words. Otherwise, we'll never be able to pinpoint exactly where your understanding leaves the world of physics.
* I'm pretty current in my reading.
* There's nothing in the extant publications which have upended anything I've ever advocated.
* The burden is upon you to provide the specificity to your arguments.
* If you want to play prosecutor, present your brief.