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Old 05-29-2020, 12:48 PM   #16 (permalink)
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Originally Posted by JulianEdgar View Post
Your arrogance is just breathtaking. Not only, apparently, am I wrong - but so are Hucho, Barnard, Katz and Skibor-Rylski.

You have developed a theory of lift that is completely wrong as it applies to any car of the last 30-odd years, but rather than admit that, you continue to defend the indefensible.

No wonder so much of the advice that you give here is wrong.

I don't care if your beliefs include conspiracy theories and incorrect understandings of how lift occurs in cars, and that you haven't kept up with any aerodynamic research of the last 25 or 30 years.

But I do care that you are misleading others. In fact, I think it is disgusting - wasting other people's time and energy through, fundamentally, your arrogant belief that you are right and everyone else is wrong.
* I went back through your book and believe I located the source of this controversy:
Here's an evolution of the relevant details:
- page 79- ' wind tunnel photos... smoke trails show flow separation from the rear end of the roof... explains the low recorded pressures across the rear.'
( this is in perfect agreement with Hucho's comments on the relationship between separation,pressure,lift,and drag )
- page 80,Figure 4-13 '... The base pressure of the newer car is twice that of the older cars!' ( in perfect agreement of Hucho's relationship between drag, pressure drag, an separation ).
- page 80, Figure 4-14 ' ... flow separation at the end of the roof of the older car is responsible for the low measured pressures on the rear on the rear of the car.' ( when I comment that separation is responsible for the low pressure, then this statement somehow becomes invalid )
- page 80, ' higher pressures on the rear of the ( 2003 M-B E 500 ) help explain the lower Cd.' ( when I imply the same conclusion, my reasoning is flawed and am attacked for it ).
- page 85, ' [T]he very gentle radius at the top of the windscreen... result(s) in a higher than expected pressure...' ( although an even more gentle radius of the 'template' somehow produces a higher suction peak ).
- page 85, '... implying this area was in a separation bubble.' ( which would not exist with the 'template' contour,as it is incapable of producing flow separation,by definition,implying that the 'template' would lack the energy loss of the E 500's locked-vortex,greater efficiency,and lower drag ).
- page 85, ' ... on the boot lid - a positive pressure..., the base pressure in the wake is much higher...' ( with zero separation, and full pressure recovery ( minus surface friction losses, somehow the 'template', by the very same metric, would not represent an even higher pressure, absolute highest base pressure, and lowest possible pressure drag ).
- page 86, CFD image of the Jaguar XE: the yellow/orange region of the C-Pillar area registers the existence of attached,longitudinal, counter-rotating vortices, agreed upon on page 91: '... with the two rotating vortices that are coming off the C-pillars... likely to leave this area of the car in separated flow' ( somehow this is lost on the Porsche 911 Carrera, with it's even steeper roof contour and no body structure on which to re-attach to [ and remarkably lost on the 'template' which is incapable of generating C-Pillar vorticity,by definition ).
- Finally, on page 173 we find the epicenter of the lift argument :
( The more pronounced rear lift in the case of the Porsche reflects the airflow generating low pressures over the rear half of the car as the air wraps over the long curve.' )
All aerodynamicists would agree with this comment, however not with the implied source of the low pressure.
* The 911 body is 43.8% ft-body.
* The 911 roof contour descends 'early' and too 'steeply' to support attached flow.
* Flow separation on the 911 occurs ahead of the backlight header.
* The rapid pressure rise induced by the steep contour precipitates strong,attached longitudinal vortices, so strong ,that they create the intense fastback downwash, responsible for holding the tufts along the body, giving the observer the illusion of attached flow.
* Should the 911 had had attached flow, it's lift would be at around 79% of the forebody, just ahead of the windshield header, where the 'minimum pressure' resides.'
* Should the 911 body have had fully attached flow, by the time the aft-body flow reached the rear bumper, all the velocity of accelerated flow would have been replaced by pressure recovery,as per the Bernoulli Theorem, leaving only surface friction losses, a fraction of drag compared to pressure drag.
* The only solution to the 911 situation is, to loft the entire body contour,up to the top of the separated flow, which lies within a 22-23-degree slope, off the rear of the car. ( see Hucho, 2nd Edition, pages 281,169,175,141,142,144,124,114,153,154,155,61,15, 16,104,209.
* Solving the 911 issue would violate Ferdinand 'Butzi' Porsche's 'silhouette', and so first the 'duckbill',then the 'whale-tail' and 'tea-tray' spoilers were added as palliatives, protecting the major silhouette, and thereby lessening risk to sales.
The spoilers do not 'spoil' lift-producing airfoil-shaped bodies.Spoilers raise the aft-body upwards out of the turbulence they're already submerged in,which was caused by flow separation of an improper contour in the first place,attempting to reach up into the inviscid flow above, where kinetic energy can attack the structure and facilitate re-attachment, capture a locked-vortex in the case of an non-aspirated spoiler,killing this lift- do -to -separation.
As to the hated 'template,' as a 'streamline body' without any flow separation, positive pressure fore and aft, cancels any lift at the location of minimum pressure, delivering 'neutral' lift,as Spirit of EcoModder registered at DARKO. Just as predicted in the 1930s. Static wheel loading via gravity is all that's necessary for stability at posted speed limits ( in the United States of America,anyway).
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