[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.