Want low drag? Don't follow a template.

[AeroMcAeroFace]

Originally Posted by aerohead

1) boundary layer thickness has nothing to do with passenger car drag.
2) above 20-mph, a passenger car is already at supercritical Reynolds number, with 100% turbulent boundary layer. And that's a 'GOOD' thing.

But doesn't the amount of stuff ahead affect the boundary layer thickness? And doesn't that make the air less "sticky" and more likely to separate?

I would think that a longer vehicle that morphs into a semicircle compared to a shorter vehicle that morphs into a semicircle, would require different rear tapers before flow detaches.

A stretch limo has the same rear profile as the car that it is based on, but will, unless I am mistaken, have different separation characteristics.

[AeroMcAeroFace]

Originally Posted by Vekke

Julian in science opinions should be backed with research papers.

I think Julian and I are still waiting for the research paper showing how the template can:

- 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)

These are the claims, it is not up to us to disprove it, it up to you to prove it.

So far I have not seen, or been shown any papers, or even book chapters, that show application of a template to any car that can show the above.

[aardvarcus]

Originally Posted by AeroMcAeroFace

I think Julian and I are still waiting for the research paper showing how the template can:

- 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)

These are the claims, it is not up to us to disprove it, it up to you to prove it.

So far I have not seen, or been shown any papers, or even book chapters, that show application of a template to any car that can show the above.

Aeromcaeroface,

If you go back to the first post, the entire thread is actually about disproving the AST-I diagram.

To this end, a video was posted in which the AST-I diagram did not align with 6 selected cars. This was apparently considered effective "proof" to disprove the AST-I.

However all of the side-streamlined vehicles (5/6) in the video actually fit the AST-II. If you take screenshots and do overlays, you can easily see this for yourself.

Why would this not be effective "proof," if it can prove it wrong surely it can prove it right?

[Vekke]

- Show where there is separated and attached flow on existing cars

Audi A6 A7 model aero mentioned here many times SAE paper

- Guide the shape of rear extensions

5 of 6 cars in the video match the AST-II template pretty accurate anyone can check themself

- Show how rear spoilers on sedans should be positioned and shaped

Many examples already mentioned lastly the hotrod magazine camaro (ok no a sedan car. Which rear spoiler just touches the AST-II template. First you need to tilt the picture little bit to get it first straigth.

Taycan turbo S also raises its rear spoiler in eco mode to go closer the template. I did not find a side picture to check how high it actually raises.

- Allow the assessment of the ‘aerodynamic purity’ of cars)

This is more tough one to answer but the the most aerodynamic models seem to follow the template even the aerodynamics havent used the template. Like I wrote before you usually cannot know what their limiting factors in the design have been, but the more aerodynamic the car is more likely it happens to follow the template.

Yesterday I checked ligthyear one with Cd under 0,20. Almost perfect match. Sligthly off to give space for rear passenger heads.

Why aerodynamics dont say the use the template--> Does olympic athletics tell what drugs or medizines they use or all the ways they train themself?

No they don`t. In many areas there are trade secrects and common ways how things are done. Maybe aerodynamics want to feel special by making lot of effort to sculpture better shapes in wind tunnel and try different things and then in the end end up exatly the same shape that could have been used in the first place and would have saved a lot of time, money, recources.

Rules of physics don`t change in the wind tunnel or in the road. There is a physical reason behind the template and thats why it works the best.

[AeroMcAeroFace]

Originally Posted by aardvarcus

Aeromcaeroface,

If you go back to the first post, the entire thread is actually about disproving the AST-I diagram.

To this end, a video was posted in which the AST-I diagram did not align with 6 selected cars. This was apparently considered effective "proof" to disprove the AST-I.

However all of the side-streamlined vehicles (5/6) in the video actually fit the AST-II. If you take screenshots and do overlays, you can easily see this for yourself.

Why would this not be effective "proof," if it can prove it wrong surely it can prove it right?

Because the claims are about application to any vehicle (with attached flow) not that there are cars that fit the template.

There is a joke " why are you throwing spaghetti in the pond? to stop sharks. but there aren't sharks in the pond. that is because of the spaghetti."

I am sure we can all agree that if there were sharks in the pond then spaghetti would be proved useless, but just because there are no sharks doesn't mean it proves effectiveness of spaghetti.

Same thing here, if there is attached flow, that is not necessarily because it fits the template.

If the template were universally applicable, then it would be 6/6 and no contrary evidence would have ever been found.

Nobody is dismissing that there are cars that fit the AST-II template on the centreline that have attached flow, and I don't doubt that there are many cars that do fit the template on the centreline, but that doesn't mean that following a template is the best for your car.

[ME_Andy]

However all of the side-streamlined vehicles (5/6) in the video actually fit

Five of six have a similar shape? What a coincidence! It's almost like their designers started from some kind of template.

[aardvarcus]

Aeromcaeroface,

If "any" aka infinite is required to prove, should six be sufficient to disprove?

A vehicle being optimized with tapered sides (top view) being compared to a roofline template (side view) isn't contrary evidence, more just a question of why someone thought it should be done....

I won't argue that it might not be "the best" since I obviously have not tested infinite possibilities. Who here has the wind tunnel and/or computing resources to design near infinite possibilities from scratch? If so, can I come over? I have a 1994 Suburban with a few more details to work out.

I still am waiting for people to build aero add on devices better than the things I have built for the vehicles I built them for. And when they do, I will applaud them and ask how they designed them. And that would definately influence my next build.

When you are pouring hundreds to thousands of hours into a build, you take these design details seriously. You read literature. You buy books. You do the best you can with the resources you have.

Then you post what you did on the internet to "give back." Where you are told your design is stupid, its basis is flawed, you are lying about your MPG, you picked the wrong vehicle, you arent using the vehicle right, etceteta. Its great fun to be insulted and called a liar... Meanwhile nobody has built anything better...

[AeroMcAeroFace]

Originally Posted by aardvarcus

Aeromcaeroface,

If "any" aka infinite is required to prove, should six be sufficient to disprove?

Infinite is required to prove yes, however no evidence supporting disproval is enough for a theory to be accepted, and really we don't care if it is a theory that works or proven.

The evidence that I have seen six or even a million cows that are black and white does not prove my theory that all cows are black and white. It certainly supports my theory, however just one cow that is not black and white is sufficient to disprove.

If 5/6 vehicles fit the pattern and one doesn't then that means that the pattern is not universal, if 999,999/1,000,000 fit the pattern and one doesn't then that means that the pattern is not universal.

"It's almost like their designers started from some kind of template." Maybe they did, but I haven't seen any evidence supporting that, or that any modern car has started or used a template at any point.
It is a valid theory that they used a template as a starting point, but without supporting evidence.

Unfortunately the template is unlikely to ever be proven, even if there is no contrary evidence, it may be a valid and usable theory though.

I have never called anyone stupid, wrong or a liar. And I certainly don't doubt that the template has its uses or may work well for you. I am here to argue against the template, it should stand up to reasonable scrutiny. However that does not mean that anyone who believes in it is wrong.

"When you are pouring hundreds to thousands of hours into a build, you take these design details seriously. You read literature. You buy books. You do the best you can with the resources you have." I agree, of course, but whatever you think about the template, it is not mentioned in any literature or book I have ever read, (unless as a low drag shape, but no mention of applicability to other cars). So even if it is great and universally usable or not, I struggle to see how doing so much reading would lead you to a use template, which, as far as I can tell, is only ever mentioned on an internet forum.

I would welcome a scientific paper demonstrating use of a template, I want to see it, in fact if anyone is currently writing one I would like to co-author it, but so far there is not enough evidence for me to be convinced.

[kach22i]

Originally Posted by aerohead

......
It could be that the committee for the Taycan also had standards for rear vision which limited the rear contour. Just a guess.

I agree with your guess, a combination of customer approval and rear vision goals sounds reasonable.

A wing in free flow and rear spoiler acting with attached flow preceding it will be more effective than wings/rear spoilers in disturbed flow (I think).

Problem is selling said designs in mass.

[aardvarcus]

Since people like analogies so much… This conversation reminds me of something else…

Person A:
Did you know you can heat your house from the sun? South facing windows, size the overhangs to reject summer sun, extra insulation, etcetera.

Person B:
No. I do not believe it. Unless you pay extra for a special house with a special certificate from a governing body regulating low energy construction, no house can be made or modified to be heated by the sun. You can’t mimic details to get there. You cannot copy construction techniques to get there.

Person A:
But wait, it works. I applied some rules of thumb and some hard math and calculated…

Person B:
Rules of thumb don’t work. The sun does not work. You see, not every house is heated by the sun- it must be wrong! Other houses have south windows, overhangs, insulation but they require supplemental heat. Houses can not be heated by the sun. You must have central HVAC, you see all houses have central HVAC. All architects design to have central HVAC.

Person A:
But I actually built one, and it works. See, here are some plans.

Person B:
You might have accidentally found something that worked for you, but it will not work for anyone else. Nobody listen to this other guy, he is wrong. See this plan has 12% glazing and this one has 10% glazing. Inconsistent. They don’t agree. Must be wrong. Everything is wrong. Install central HVAC or I will taunt you some more.

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You guys are free to believe what you want to believe. But it is 72.5 degrees inside, 43 degrees outside. Lows in the teens to twenties all week, but it hasn’t fell below 69.5 degrees at the coldest inside (the morning after a cloudy day). No heat going whatsoever, unless you count the sunlight.

Go ahead and don’t believe me, it doesn’t hurt my feelings. Have to have thick skin on the internet. My proof is in my pocketbook, when those utility bills come in that is my proof. Just like my aero devices, proof is at the pump. And I know my house is not designed perfectly, but it “works” and works pretty darn well in my book. Just like my aero add on devices.

My suggestion to anyone watching this thread- just go build something. It's fun! Design it the best you can, and get started.