Want low drag? Don't follow a template.

[aerohead]

Originally Posted by JulianEdgar

And journalist - you forgot about that.

We don't need to see all the correspondence from the reviewers - just look at the endorsements. You can be absolutely sure these experts wouldn't have endorsed the book without agreeing with its content!

"The best practical introduction to aerodynamics for the car enthusiast that I've seen. The author combines his own experience with published research to provide useful and reliable insights into the often bewildering world of automotive aerodynamics." Adrian Gaylard, head of aerodynamics, Jaguar Land Rover

"A really good book that should be added to the library of everyone working in automotive aerodynamics, as well as those making car aero modifications at home." - Rob Palin, former Tesla aerodynamicist

"This book covers a wealth of useful car aerodynamic information for the non-engineer." Professor Joseph Katz, author Race Car Aerodynamics

"Your book is unique! It was high time that someone covered vehicle aerodynamics through the practical eyes of someone like you." - Dr Wolf-Heinrich Hucho, the founder of modern vehicle aerodynamics

Unfortunately, from your perspective, a great deal of what you say here is either completely wrong, partly wrong or misleading. People have been sucked into it (or maybe just overwhelmed with the 'wall of noise' approach you take to debates) and so lots of misapprehensions have developed here around car aero. The Template is the most obvious of those misapprehensions.

* Another demonstration of lack of perspicacity.
* Only the spirit of the correspondence will give the world audience the context under which your consultants signed off on your work.
* Don't get me wrong. There's much to commend about your work.
* The sticking point for me is your mishandling of boundary layer theory ( by default ).
* There remain conditions about lift / drag which you've avoided or dismiss out of ignorance. You're in possession of materials that could correct for that. And when given prompts, there appears to be no interest on your part to advance down the field.
* You'll never understand the 'template' without more education.
* Bring your five consultants over to EcoModder.com, Aero Forum if you like. We could better diagnose the situation.

[aerohead]

Originally Posted by kach22i

If anyone wants to take a crack at these questions, feel free to do so.

1. Automotive; downwash is always the result of a vortex, yes, no, maybe.

2. Automotive; downwash is the byproduct of a sudden pressure change via air detachment, yes, no maybe.

I tried to do an image research on "Vortex Downwash" but only aircraft stuff came up.

Then I did a image search on "Automobile Downwash" and found the below.

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/

Aerodynamiks des Kraftfahrzeugs, by Fachsenfeld, and Hucho's
2nd_Edition would be very good sources for the fundamentals.

[aerohead]

Originally Posted by freebeard

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

A vortex is just self-sustaining ordered turbulence?

* Downwash is phenomena associated with attached, upper longitudinal vortices.
* Upper longitudinal vortices are a phenomena associated with transverse viscous shearing forces at the intersection of the top and side flow.
* Viscous shearing forces are associated with the pressure differential between the top and sides of the vehicle.
* Pressure differential is a function of the pressure gradient along top and sides.
* Pressure gradient is dependent upon the aft-body contour, degree of cross-sectional area contraction.
--------------------------------------------------------------------------------------
* Typically, there exists a maximum contour producing an adverse pressure gradient which will just sustain attached flow.( a streamlined contour )
* If this contour is exceeded, the magnitude of the critical, local pressure gradient is also exceeded, triggering flow separation, as per the strictures of boundary layer theory.
* At zero-yaw, a 'streamline' body is incapable of generating separation. One underlying criteria for the 'template.'

[aerohead]

Originally Posted by JulianEdgar

As in, the template is the greatest misapprehension here - but what is the next greatest?

Without a doubt, the significance and action of aero pressures acting on panels.

When Vman455 showed some of the measured pressures on his Prius (data that immediately contradicted much of what Aerohead spouts), I was amused to see no-one actually 'got' the contradiction (or no-one who commented, anyway). I was amused but not surprised - because the same thing happens here when I show my Insight's measured body panel pressures.

Aerohead's ideas in this area are again quite wrong, and again they seem to have been adopted widely here. And - yet again - it's because people don't make measurements - if they did, they'd soon see reality for themselves.

A good example is the person from this group who commented on my 'don't use a template' video. He looked at the Jaguar's CFD pressures (shown in the video) and had them completely reversed! That's sure some misunderstanding.

And I might add, the reason for that complete misunderstanding can be related back to the silly template - you could see that's what he was trying to build his ideas on. But crap in = crap out.

* Without a zero-separation reference pressure profile datum, from which to compare, there's no 'context' with which to qualify the significance of any pressure measurements you provide.
* From CAR and DRIVER's 'Drag Queens' article, the Nissan LEAF is the only 'template' car tested.
* The LEAF had lower rear lift than the PRIUS ( a sub-'template' form )
* The lower lift of the LEAF can be explained only by higher pressure acting over the aft-body.
* Had the flow actually been attached on the PRIUS, it's rear lift would have been less than the LEAF.
* The only explanation possible is, that the PRIUS is experiencing flow separation. Just like the 2010 Audi A7 Sportback. 2020 Porsche Taycan.

[aerohead]

Please bear in mind that, a pressure profile for the entire surface area of the panel ( many hundreds of measurements ) is a prerequisite for ascertaining any lift / drag data. Not a dozen or so centerline readings.

[aerohead]

Originally Posted by AeroMcAeroFace

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."

* Cd 0.15 is for Klemperer's 1922 half-body with wheels.
* Cd 0.10 without wheels, maintaining the same ground clearance.
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* The 1981 Volkswagen 'Flow' body long-tail, by Buchheim et al., is Cd 0.14.
Its half-body, at maintained ground clearance is Cd 0.0913.
--------------------------------------------------------------------------------------
* Jaray's 'pumpkin seed' of 1922, with diffuser ( his invention ) is Cd 0.13.
--------------------------------------------------------------------------------------
* The baby 'template' car measured Cd 0.121 with 'loose' wheel fairings and chopped tail.
-------------------------------------------------------------------------------------
* Cambridge University's CUER solar racer is 'template'-esque, and with knife-edge trailing surfaces measured Cd 0.11.
--------------------------------------------------------------------------------------
* The flow on the sides of the 'template' is essentially the same over the top. It's a half-body. That's what half-bodies do.
* ' The main contribution to the drag force originate from the rear part of the body.' Hucho, page-61, 2nd-Edition.
* ' [I]t is very important to design a rear body surface which brings the divided streamlines smoothly together. Optimum shapes are ' streamlined' bodies having a very slender rear part.' Hucho, page 61, ditto.
* ' [T]he optimum shape in terms of drag is a half-body, which forms a complete body of revolution together with its mirror image- produced through reflection from the roadway.' Hucho, page 15, ditto.
* ' [L]ow drag can only be achieved when the separation at the rear is eliminated.' Hucho, page 16, ditto.
* ' [A]n effective fineness ratio in free air of 2.27..... approaches the drag minimum recognizable.' Hucho, page 210, ditto. ( 'template' is 2.5:1 )
* The pressure recovery... provides for the reduction of the drag.' Hucho, page 144, ditto.
* ' [P]ressure drag is the largest component in the aerodynamic drag. Its minimization is the true objective of motor vehicle aerodynamics.' Hucho, page- 119, ditto.
* ' A closer approach to the value of the basic body without wheels is only achievable through further integration of the wheels into the body.' Hucho, page- 201. ditto.
--------------------------------------------------------------------------------------
All the above are defining the 'template.'
* Hucho shows it in Table 2.1, page 61, from Horner's book of 1951. Cd 0.04 in free flight ( actually lower ). Half-body would technically be Cd 0.08.

[aerohead]

Originally Posted by JulianEdgar

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.

In the Preface of his 2nd-Edition, Hucho admonishes us, advising us to learn everything about road vehicle aerodynamics, plus that from other disciplines, such that we may draw on the entire body of knowledge to draw on for solutions.
You'll be in an information desert unless you've explored biomimicry, airships, blimps, blisters, canopies, fairings, radomes, bullet-valves, nozzles, self-powered rail-cars, submarine exterior hulls, torpedoes, projectiles, belly tanks, drop-tanks, towed electronics arrays, antenna housings, etc..
If you'll continue to learn, you'll eventually arrive at a reality which overturns your present rudimentary understanding.

' It ain't what you know, it's what you think you know that just ain't so.' Mark Twain

[Vman455]

There's a fundamental disconnect in all of the above between idealized theory and reality. Idealized theory is useful for understanding physical phenomena and mechanisms. In 2nd semester physics we were given many calculation problems dealing with charged surfaces of infinite area or wires of infinite length because such idealized setups simplify the problem and isolate one or a few variables. The previous semester we dealt with many problem setups of systems with massless pulleys or balls flying through the air with no aerodynamic resistance--situations that do not exist in the real world but nevertheless are useful to build understanding. Similarly, idealized aerodynamic shapes are useful for understanding the mechanisms that give rise to drag and lift, not prescriptions for real cars. Further, in the current literature there is a great deal to be learned about idealized shapes which Hucho's book, especially the very old 2nd edition, doesn't talk about--especially lift, ground clearance and underside shape.

Originally Posted by aerohead

* The only explanation possible is, that the PRIUS is experiencing flow separation. Just like the 2010 Audi A7 Sportback. 2020 Porsche Taycan.

This theory, drawn from simplified ideals, that the 2010 Prius suffers from flow detachment because it does not conform to the "template" is easily disproven with a few wool tufts and some tape:



The flow over the rear of a Prius is nicely attached. And it should be, given that the Prius underwent hundreds of hours of wind tunnel testing--more than any other Toyota at the time, according to the company--was optimized for low drag, and at the time it went on sale was the lowest-drag production car on the US market.

You can easily see the difference by fitting a spoiler at the top of the window. Now the flow is forced to separate and looks very different from the stock configuration above:



All this should make a reasonable person step back, look at the examples of real-world testing (here and in numerous other threads), and assess the theory in light of them. In this case, the theory that "flow attachment is dictated by conformity of rear body shape in profile to this particular idealized template" clearly needs some work.

[JulianEdgar]

Originally Posted by aerohead

In the Preface of his 2nd-Edition, Hucho admonishes us, advising us to learn everything about road vehicle aerodynamics, plus that from other disciplines, such that we may draw on the entire body of knowledge to draw on for solutions.
You'll be in an information desert unless you've explored biomimicry, airships, blimps, blisters, canopies, fairings, radomes, bullet-valves, nozzles, self-powered rail-cars, submarine exterior hulls, torpedoes, projectiles, belly tanks, drop-tanks, towed electronics arrays, antenna housings, etc..
If you'll continue to learn, you'll eventually arrive at a reality which overturns your present rudimentary understanding.

' It ain't what you know, it's what you think you know that just ain't so.' Mark Twain

Yeah, well it was Dr Hucho himself who suggested I buy the fifth edition - that's the book with just two pages out of 1300 on 'ideal shapes'!

[aerohead]

Originally Posted by Vman455

There's a fundamental disconnect in all of the above between idealized theory and reality. Idealized theory is useful for understanding physical phenomena and mechanisms. In 2nd semester physics we were given many calculation problems dealing with charged surfaces of infinite area or wires of infinite length because such idealized setups simplify the problem and isolate one or a few variables. The previous semester we dealt with many problem setups of systems with massless pulleys or balls flying through the air with no aerodynamic resistance--situations that do not exist in the real world but nevertheless are useful to build understanding. Similarly, idealized aerodynamic shapes are useful for understanding the mechanisms that give rise to drag and lift, not prescriptions for real cars. Further, in the current literature there is a great deal to be learned about idealized shapes which Hucho's book, especially the very old 2nd edition, doesn't talk about--especially lift, ground clearance and underside shape.



This theory, drawn from simplified ideals, that the 2010 Prius suffers from flow detachment because it does not conform to the "template" is easily disproven with a few wool tufts and some tape:



The flow over the rear of a Prius is nicely attached. And it should be, given that the Prius underwent hundreds of hours of wind tunnel testing--more than any other Toyota at the time, according to the company--was optimized for low drag, and at the time it went on sale was the lowest-drag production car on the US market.

You can easily see the difference by fitting a spoiler at the top of the window. Now the flow is forced to separate and looks very different from the stock configuration above:



All this should make a reasonable person step back, look at the examples of real-world testing (here and in numerous other threads), and assess the theory in light of them. In this case, the theory that "flow attachment is dictated by conformity of rear body shape in profile to this particular idealized template" clearly needs some work.

Keep studying. You may eventually understand.