Want low drag? Don't follow a template.

[JulianEdgar]

Originally Posted by aerohead

I experienced zero examples. Only self-delusion, from a phenomena explained scientifically, but summarily dismissed because it didn't fit the twisted worldview of Mr. Edgar and his 'wrapped flow.'

I use the term 'wrapping around curved surfaces' (and similar) to explain the low pressures that are generated when attached flow follows a convex surface*.

In an email to me last week, former head of Porsche aerodynamics, Dr Thomas Wolf, puts it this way:

"According to the Coanda-Effect the air follows the curved (upper) surface. According to Newton’s first law, there must be a force which pulls the air downwards. This force is exerted to the air by the car body and pulls the air downwards. According to Newton’s third law (action = reaction) there must be a force of the same magnitude in the opposite direction acting on the body."

But hey, what would Dr Wolf know?

Unfortunately, a lot of what Aerohead so steadfastly believes is simply wrong, as some simple on-car measurements very quickly shows. It's just a crying shame that Aerohead has so effectively mislead so many people here for so long.

(*The opposite happens with concave surfaces - high pressures are developed.)

[AeroMcAeroFace]

Originally Posted by aerohead

* Cd 0.15 is for Klemperer's 1922 half-body with wheels.
* Cd 0.10 without wheels, maintaining the same ground clearance.

* 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.
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* Jaray's 'pumpkin seed' of 1922, with diffuser ( his invention ) is Cd 0.13.
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* The baby 'template' car measured Cd 0.121 with 'loose' wheel fairings and chopped tail.
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* Cambridge University's CUER solar racer is 'template'-esque, and with knife-edge trailing surfaces measured Cd 0.11.
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* 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.
It says very clearly, and multiple times in your favourite book that isn't the case, and was found in 1922

* ' 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.

Complete body, meaning not on the ground, I don't see many flying cars

* ' [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 )
Again, that is in free air, most cars can't fly

* 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.
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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.

All I can reasonably take from that is, the template is low drag, especially without wheels, and even more so nowhere near the ground.

But actually, reading through the bits of that book I can access, it says "Klemperer (1922) recognized that flow over a body of revolution... changed drastically and lost symmetry when the body came close to the ground"

Seems pointless to follow something like that to me when designing cars, which live on the ground.

"Despite their extreme length, flow separates from the rear of streamlined cars. By truncating the rear shortly upstream of the location where separation would take place, shapes of acceptable length were generated with no drag penalty"

So that says to me that using a template may actually increase drag, a bit like extending a kamm-back can be pointless.

[JulianEdgar]

Originally Posted by aerohead

* 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.

It's amazing how to a hammer, everything looks like a nail...

The Leaf has far better undercar aero than the Prius - something that perhaps might explain its lower lift? You seem to be forgetting half of the body shape!

As for the Taycan having upper body separation - you are joking, aren't you? Where's your evidence for that - comparison with your make-believe ultimate template? If so many people hadn't been sucked into believing such rubbish, it would be just laughable.

It's really obvious that no matter what anyone shows through testing, or whatever anyone shows through technical references, or what anyone says in citing current car aerodynamicists, you will not change your views. To be honest, that is just gobsmackingly arrogant.

[freebeard]

Anything with front wheels and outside rear-view mirror will have separation?

Originally Posted by AeroMcAeroFace

So that says to me that using a template may actually increase drag, a bit like extending a kamm-back can be pointless.

Aerodynamic utility aside, the full 'default template' offers decreasing utility inside and increasing ride harshness.

[JulianEdgar]

Originally Posted by freebeard

Anything with front wheels and outside rear-view mirror will have separation?

Probably. But the Jaguar XE I tested was pretty darn good.

Aerodynamic utility aside, the full 'default template' offers decreasing utility inside and increasing ride harshness.

High pitch accelerations, you think?

[freebeard]

By that you mean a dog-wagging tail?

[ME_Andy]

A little article on Experts vs Pseudoexperts. So relevant here.

The novice can not tell the difference between experts and pseudo-experts and he is thus much more likely to follow the pseudo-experts simply because there are more of them. Since pseudo-experts do not know what they don’t know and often fashion themselves experts or at least well-informed (perhaps they follow the news?)—after all, have they not accumulated an enormous amount of information—they can appear supremely confident which makes them so much more dangerous and likely to be followed by the novice.

The other thing to realize, when corresponding with a scientist, is they may put their reply in layman's terms so it's easier to understand. It's not necessarily the same way they think of it.

[AeroMcAeroFace]

I think though, that you don't have to be an expert to know the limit of your knowledge.

"they can appear supremely confident which makes them so much more dangerous and likely to be followed by the novice" I think this is the big issue here, that the reserved voices of reason and judgement saying things like, compare your car to other cars, do tests, look at CFD, read papers and books, we don't know the specific solution to your specific problem, essentially get shouted over by the "template is universally applicable" messages. And realistically, it is way easier to follow a template than it is to do all those things.

[JulianEdgar]

I wrote this back in 2008.

Long ago, even before I was a geography teacher, I studied how to teach it. The head of the geography department at college was a very smart person, and a brilliant teacher.

One day we were talking about teaching analogies and models, and the difficulty in simplification of knowledge without introducing straight-out erroneous ideas.

His example of the latter was: Clouds bumping into each other make thunder.

Much better, he pointed out, to say even to the youngest child: Thunder happens because of lightning.

In fact, clouds are a good example of these ideas. My little boy, who is 4 years old, asks what clouds are made of.

Tiny, tiny water droplets, I say.

So, how does rain happen, he asks?

I say: The tiny droplets run into each other and join together. When they are big enough, they fall to the ground.

While I am saying this, sometimes I think of a much more sophisticated model: water vapour, latent heat of evaporation and condensation, relative humidity, dew-point, hygroscopic nuclei – and other concepts.

A meteorologist would probably think of vapour pressure, a chemist might think at a molecular level, a physicist might consider terminal velocities, a climatologist might consider climate change, a minister of religion might think of God, an agnostic might think of the magnificence of nature.

In the description of clouds and rainfall that I say to my son, I am conscious of the gross simplifications I am making.

But that’s OK: every single thing I know about the world is a gross simplification of reality.

The intellectual models I use to make sense of what occurs around me are just reducible approximations of what really happens.

When I write technical articles, I am conscious that all the time I am presenting fundamentally simplistic models. I hope that they’re not of the ‘clouds bumping into each other make thunder’ type: but they may be.

Recently, I wrote an article on suspension roll centres, virtual pivot points and other ways of analysing suspension designs. In doing so, I consulted five different suspension design textbooks, and also considered very carefully the experience I have in developing human-powered vehicle suspensions, and modifying car suspensions.

As always, I was quite conscious during the writing of the article that the model I was presenting of reality was likely to be flawed: as I have already implied, every model we have of reality is, to a greater or lesser degree, flawed. However, I hoped that the information would benefit people’s understandings, especially in practical outcomes.

The day after finishing the article, I looked through a complex SAE paper on suspension roll centres. This paper immediately debunked several suspension ‘myths’, most of which I had implicitly or explicitly promulgated in the article I had written.

However, the paper was working at a level analogous to the ‘vapour pressure and hygroscopic nuclei’ theory of why rain falls: if I based my article on the SAE paper in question, perhaps less than half of one percent of my readers would understand anything I wrote. (If in fact I could understand it myself!)

So I could easily decide not to write anything at all: if it’s not ‘right’ and ‘correct’, surely it shouldn’t be written?

But that would be like saying to my son: I cannot tell you why rain falls; it’s too hard to understand.

I cannot tell you what a roll centre is; it’s too hard to understand.

Or I cannot tell you what a voltage is; it’s too hard to understand.

I cannot tell you what engine detonation is; it’s too hard to understand.

I cannot tell you how a tyre behaves when cornering, it’s too hard to understand.

And so on.

And these things – and all things – really are too hard to understand… if you want as ‘correct’ an understanding as it is currently possible to have.

Are my articles full of errors? So by definition, very likely.

Anyone who suggests that the technical articles they present for general readers are perfectly correct – or do not mislead in the slightest – just do not understand the nature of knowledge – and how all our descriptions of what goes on around us are just relatively simplistic models.

Me? I try to use the simplest model that’s consistent with not being grossly misleading…

(And 'using the simplest model that’s consistent with not being grossly misleading' doesn't include using a template to:

- 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)

[Stubby79]

I never trust what others "know"...takes a few sources saying the same thing for it to become a "maybe" or an "if they are right". Then I go do my own reading/research/testing, the whole while keeping in mind it's only an "if". If it happens to work, then I need to figure out why it works. Sometimes I can't figure it out, and it sits there annoying me, gets shelved mentally, and often, when new information comes to light, it all clicks, and viola! I file it away as "proven" and the "why" of it, until proven otherwise.

I recently went through someone's arduino script, which I had no clue how to read in the first place, and worked my way through it until I understood what the program was doing, step by step. The only mystery was the bit math - not the math itself, but why he chose to use it the way he did to affect the results. It was obvious that it made sense to him, and was a very efficient way of producing the results needed, but without being able to see the numbers changing under what circumstances, I'm still in the dark on it...soooooooo bugging me! Going to have to run through it carefully...

The process applies to more than technical mumbo-jumbo. I make up my own mind on...everything. I don't take anything as gospel just because it's been said. Even laws, rules, etc. I've got to tear it apart, look at the gains vs consequences, and decide for myself if I'm going to follow or believe it.

At work, I hate being told how to do things. Just tell me what needs doing, and let me figure out the most efficient way to get the best results. Show me how you do it, sure, but don't tell me that's the best way, or the way I have to do it. I usually take what I've been taught, adapt it to work best with my skills, and end up with something that resembles what I was taught but is a lot more efficient (or easy). Or I throw it out, try something completely different, and if I was too hard or didn't work out, I try a different approach next time, until I hone in on a "better way".

I'm rather tired of seeing people never putting any thought, any personal twist on their approach. Lemmings. I even show them other ways, and they still run in to the same headache the next time. Oy.

This is rather why I am here...there is a better way for me to get to work every day. One that takes less fuel, costs me less resources (money, generally, and/or time). I decided long ago that this is something worth pursuing. When work isn't demanding my full attention (which it rarely does), my thoughts wander quickly on to how to make things better. As in build/modify them to be better - faster or cheaper, or both. It's an ongoing jigsaw puzzle in my head that I'm seemingly always piecing together. Apparently I must like this line of thinking, since I do it a lot and enjoy figuring out everything along the way.

Welp, nice to have an excuse to explain why I'm so messed up compared to most people. It does leave me wondering how many others spend their time dissecting things - whatever interests them - in their minds, and, once they understand it, figure out how they could improve upon it. Anyone? Anyone?

Back to watching Mustie1 dissect machines on youtube.