2010 Audi A7/ SAE Paper/ 'Want Low Drag?'

[aerohead]

Rather than let this get buried in the 'Want Low Drag? Don't Follow the Template,' thread, I thought it deserved its own thread.
If you have ' Modifying the AERODYNAMICS of Your Road Car,' by Julian Edgar, you'll find the SAE Paper# 2011-01-0175,and it's discussion, starting, page 186, Chapter 7.
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I obtained a blueprint for the Audi online, and photo-enlarged to match the size of the eight-'templates' I've used for dimensional analysis.
I have a physician's, internally-illuminated X-Ray viewer that I use as a light-table, with which to do the comparisons, like transparency overlays.
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* The Audi best fit the AST-II template.
* With drafting instruments, the car's body was extrapolated out to its 'long-tail' ( lang-heck ) termination, where other investigators always began their analysis.
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* According to Audi, the A7 suffers flow separation, which includes all of the backlight.
* The template identifies a reverse-step at the upper backlight body molding which may have been the 'trigger' for the adverse positive pressure spike, responsible for separation.
* As the body falls away from this location, according to the streamline contour of the template, there would be nothing aft of the separation line which could effect flow reattachment. The flow is 'lost.'
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* Audi claims that the A7 generates a separation-induced, attached, longitudinal counter-rotating vortex pair.
* The vortex pair induces a downwash.
* A small wake of low pressure is created.
* Low pressure above the separation line telegraphs low pressure over the aft-body for rear lift.
* Low pressure telegraphed into the wake lowers base pressure, increases pressure drag, higher total drag.
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* The addition of an active pocket spoiler, which rises up approx. 72.5 mm and to the rear, in a 'level'- top position, just reaches the 'template' contour line, essentially killing the separation-induced lift.
* Vorticity is weakened.
* Downwash velocity is weakened with concomitant pressure rise.
* Local streamlines are re- converged, with concomitant velocity increase.
* Local pressure above the restored turbulent boundary layers is lowered
below the threshold for the adverse high-pressure gradient separation 'trigger'.
* Any low-pressure of a captured -vortex, created by the spoiler is partially sequestered away from the wake, preventing low pressure telegraphing into the wake, with concomitant increased wake pressure, lower pressure drag, lower total drag.
* For operation up to 130-mph ( approx. 210 km/h ), the rear spoiler raises its trailing edge another 30mm into the 'free-stream,' while simultaneously angling the leading edge downwards to create ' direct downforce' and high-speed stability.
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* This is an instance in which the AST-II essentially predicted the separation line over the aft-body.
* From the dimensional analysis, the 'template' also predicted the rear spoiler ' solution.'
* Had Audi measured the A7's drag from a segmented 'long-tail', as Walter Lay, Kamm- Koenig, Porsche's Le Mans specials, W. A.Mair, or Dr. Liebold, with his M-B, C-111-III, we'd also know the difference the A7's truncation made to drag, as a function of aft-body percentage ( Verjungungsverhaltnis ).
* Had Audi borrowed the roofline from the 2009 Volkswagen L1, or 2010 Lamborghini Sesto Elemento, there'd be no need of a spoiler, except for direct downforce. No separation-induced rear lift would be possible.

[Gasoline Fumes]

This is it?

[aerohead]

Exactly?

[JulianEdgar]

Just to be clear, there is no mention whatsoever in the SAE paper about a template, standardised shape, etc. There isn't in any SAE paper that I have ever seen on the aerodynamic development of a road car.

And, as is so common with what Aerohead writes here, he correctly quotes, then misquotes, makes up stuff, and applies his own interpretations to what was actually written. Some examples:

• in the SAE paper (and my book) the spoiler is described as having one position, not two.
  
• the way the spoiler works, as described in the SAE paper, is substantially different to Aerohead's theory
  
• the change in drag is not related to a change in wake pressure in the SAE paper (or my book)

So most of the post is just Aerohead fitting a car to his theory. To a hammer, everything looks like a nail.

Note: The description of the Audi notchback / hatchback / fastback models was included in my book in the context of the required different rear diffusers for each body shape, so my description of the fastback spoiler was brief. Aerohead hasn't read the SAE paper.

[aerohead]

Originally Posted by JulianEdgar

Just to be clear, there is no mention whatsoever in the SAE paper about a template, standardised shape, etc. There isn't in any SAE paper that I have ever seen on the aerodynamic development of a road car.

And, as is so common with what Aerohead writes here, he correctly quotes, then misquotes, makes up stuff, and applies his own interpretations to what was actually written. Some examples:

• in the SAE paper (and my book) the spoiler is described as having one position, not two.
  
• the way the spoiler works, as described in the SAE paper, is substantially different to Aerohead's theory
  
• the change in drag is not related to a change in wake pressure in the SAE paper (or my book)

So most of the post is just Aerohead fitting a car to his theory. To a hammer, everything looks like a nail.

Note: The description of the Audi notchback / hatchback / fastback models was included in my book in the context of the required different rear diffusers for each body shape, so my description of the fastback spoiler was brief. Aerohead hasn't read the SAE paper.

Specific mention or not of the 'template' in a SAE Paper does not detract from it's efficacy in predicting certain aerodynamic phenomena.
We know it from the work of Walter E. Lay, at Michigan University in 1933.
We know it from the work of Carl Breer, at Chrysler, in 1934.
We know it from the work of Kamm and Fachsenfeld at the FKFS, between 1935 and 1939.
We know it from the work of Elliott G. Reid, at Stanford University, in 1935.
We know it from Sighard Horner's text of 1951.

We know it from the work of R.G.S. White, at MIRA in 1969.
We know it from the work of W.A. Mair, in 1969.
We know it from Robert Cumberford's work in 1974.
We know it from the work of Dr. Hans Leibold, at Daimler-Benz/ Mecedes-Benz, in 1978.
We know it from the work of NASA, and their 'Project Shoebox,' 1980.
We know it from NASA's Space Shuttle research at Edward's Air Force Base, California, 1980.
I know it from my CRX HF, USFRA LSR in 1990,at Bonneville.
We know it from General Motors' Aero 2000 and Aero 2002, of 1983.
We know it from the work of General Motors' Oldsmobile Division, and their OLDS Aerotech, 1987 closed-course LSR at Ft. Stockton, Texas
We know it from Wolf Heinrich Hucho's text of 1987.
We know it from Texas Tech University's research of 1988.
My Chrysler Proving Grounds, CRX HF coast-down record, 1991.
We know it from General Motors/ EV1 LSR, 1993.
We know it from Texas Tech University's 'Tailwind Project' of 1995.
My 'Spirit of EcoModder.com' LSR at Bonneville, 2012.
We know it from Mercedes-Benz' 2016 IAA.
'Templates' are all around, and have been for 87-years.

[aerohead]

Is anything I've ascribed to your language about the SAE Paper misrepresentative of what you published ?
For $ 67 I got your take on it. There was no shortage of information from which to draw my conclusions. The direct 'template' comparison spoke for itself. If I've made a factual mistake I apologize.
Just let me know, so I may make the appropriate changes. Thanks in advance.

[aerohead]

This was addressed elsewhere.
Audi has provided a fine CGI representation of the A7's dual-mode, rear spoiler and operation.
Freeze-framing provides an opportunity to examine its operation in detail. And provides a sense of scale, for comparison to the 'template' as well.

[JulianEdgar]

Originally Posted by aerohead

Specific mention or not of the 'template' in a SAE Paper does not detract from it's efficacy in predicting certain aerodynamic phenomena.
We know it from the work of Walter E. Lay, at Michigan University in 1933.
We know it from the work of Carl Breer, at Chrysler, in 1934.
We know it from the work of Kamm and Fachsenfeld at the FKFS, between 1935 and 1939.
We know it from the work of Elliott G. Reid, at Stanford University, in 1935.
We know it from Sighard Horner's text of 1951.

We know it from the work of R.G.S. White, at MIRA in 1969.
We know it from the work of W.A. Mair, in 1969.
We know it from Robert Cumberford's work in 1974.
We know it from the work of Dr. Hans Leibold, at Daimler-Benz/ Mecedes-Benz, in 1978.
We know it from the work of NASA, and their 'Project Shoebox,' 1980.
We know it from NASA's Space Shuttle research at Edward's Air Force Base, California, 1980.
I know it from my CRX HF, USFRA LSR in 1990,at Bonneville.
We know it from General Motors' Aero 2000 and Aero 2002, of 1983.
We know it from the work of General Motors' Oldsmobile Division, and their OLDS Aerotech, 1987 closed-course LSR at Ft. Stockton, Texas
We know it from Wolf Heinrich Hucho's text of 1987.
We know it from Texas Tech University's research of 1988.
My Chrysler Proving Grounds, CRX HF coast-down record, 1991.
We know it from General Motors/ EV1 LSR, 1993.
We know it from Texas Tech University's 'Tailwind Project' of 1995.
My 'Spirit of EcoModder.com' LSR at Bonneville, 2012.
We know it from Mercedes-Benz' 2016 IAA.
'Templates' are all around, and have been for 87-years.

Just to be clear, there is no mention whatsoever in the nominated SAE paper about a template, standardised shape, etc. There isn't in any SAE paper that I have ever seen on the aerodynamic development of a road car.

The current car aerodynamics standard reference textbook (Aerodynamics of Road Vehicles, 5th edition) gives 'template shapes' 2 pages in 1300.

If only it were as easy as using a template!

[JulianEdgar]

Originally Posted by aerohead

Is anything I've ascribed to your language about the SAE Paper misrepresentative of what you published ?
For $ 67 I got your take on it. There was no shortage of information from which to draw my conclusions. The direct 'template' comparison spoke for itself. If I've made a factual mistake I apologize.
Just let me know, so I may make the appropriate changes. Thanks in advance.

I have already pointed out three errors in what you have written.

[JulianEdgar]

Originally Posted by aerohead

This was addressed elsewhere.
Audi has provided a fine CGI representation of the A7's dual-mode, rear spoiler and operation.
Freeze-framing provides an opportunity to examine its operation in detail. And provides a sense of scale, for comparison to the 'template' as well.

The SAE paper, and my book, makes no mention of a dual mode spoiler. So you certainly aren't quoting either my book or the paper.

If you wish to find other information, and build a theory from that - fine. But that makes ostensibly quoting my book and the SAE paper as your sources rather deceptive.