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Old 01-02-2021, 08:10 AM   #121 (permalink)
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It is a very interesting paper actually.

Downwards roof slope gives huge lift, 20 degree downwards slope is higher lift and higher drag than 15 degree downwards.

"little data is available on the effects of streamlined tails on non-axisymmetric bodies, whether representative of road vehicles or not" Presumably they need to "read between the lines" more despite reading the same books template enthusiasts have.

Tapering is good, but cannot be done arbitrarily is the main theme of the paper. Angles that work on the roof don't necessarily work on the sides, and vice versa.

I̶f̶ ̶y̶o̶u̶ ̶a̶r̶e̶ ̶g̶o̶i̶n̶g̶ ̶t̶o̶ ̶e̶x̶t̶e̶n̶d̶ ̶y̶o̶u̶r̶ ̶c̶a̶r̶,̶ ̶d̶o̶n̶'̶t̶ ̶e̶x̶p̶e̶c̶t̶ ̶a̶ ̶s̶p̶e̶c̶i̶f̶i̶c̶ ̶a̶n̶g̶l̶e̶,̶ ̶s̶h̶a̶p̶e̶ ̶o̶r̶ ̶p̶r̶o̶f̶i̶l̶e̶ ̶t̶o̶ ̶b̶e̶ ̶o̶p̶t̶i̶m̶a̶l̶.̶

If you are going to extend your car, a profile that works for someone else may not work on your car.


Last edited by AeroMcAeroFace; 01-03-2021 at 06:59 AM.. Reason: See Julians comment
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Old 01-02-2021, 01:37 PM   #122 (permalink)
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Significant aerodynamic drag reduction is obtained on a bluff body by tapering the rear body. In the 1930ís it was found that a practical low drag car body could be achieved by cutting off the tail of a streamlined shape. The rear end of a car with a truncated tail is commonly referred to as a Kamm back. It has often been interpreted as implying that the drag of this type of body is almost the same as that for a fully streamlined shape. From a review of the limited research into truncated streamlined tails it is shown in this paper that, while true for some near axisymmetric bodies, it is not the case for many more car-like shapes. For these shapes the drag reduction from an elongated tail varies almost linearly with the reduction in cross section area. A CFD simulation to determine the drag reduction from a truncated streamlined tail of variable length on the simple Windsor Body is shown by way of confirmation.
My question is how they managed to avoid passive aggressively attacking anyone in this paper? I thought the purpose of science was to incessantly harass individual people for their opinions. They didn't even falsely attribute support of those opinions to others merely seeking civility. Good thing you guys are here to point out these obvious omissions in the research.
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Old 01-02-2021, 02:08 PM   #123 (permalink)
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Quote:
Originally Posted by orange4boy View Post
My question is how they managed to avoid passive aggressively attacking anyone in this paper? I thought the purpose of science was to incessantly harass individual people for their opinions. They didn't even falsely attribute support of those opinions to others merely seeking civility. Good thing you guys are here to point out these obvious omissions in the research.
In science, if there is a different opinion you present evidence that supports your differing opinion. Something along the lines of "contrary to the work of ... the findings of this study suggest this is not the case" or "in the work of ... they write " ..." however this does not agree with our findings"

I think you underestimate the amount of back and forth argument that goes on in science and science papers, argument is good, it gets you closer to the truth. If you believe that disagreement is harassing then don't be a scientist in a contentious area, academic disputes can and do go on for decades.

orange4boy, if you believe that "passive aggressively attacking" people is an issue and you believe you see lots of it here, you could try leading by example and not doing the thing you see as the problem, "be the change you want to see in the world".
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Old 01-02-2021, 03:03 PM   #124 (permalink)
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Old 01-02-2021, 04:02 PM   #125 (permalink)
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My whole point is that disagreement isnít harassment but what goes on here is far beyond disagreement on facts. This isnít a scientific community, itís a forum but the scientific community encourages civility and doesnít generally call people ďfansĒ which is a kind of ridicule in this context. When I came into this ďdebateĒ I simply requested we act as civilly as we agreed to when we joined.

If you think humorous sarcasm is incivility dam you to H E double hockeysticks.

I forgot the winky face last time. My bad.
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Old 01-02-2021, 04:11 PM   #126 (permalink)
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There are examples throughout the paper where many of the rules of thumb so often used here are shown to have quite doubtful validity.

That won't surprise anyone who has done their own research - either through reading or testing.

Just a few examples

1. I've never understood the fascination here for Kamm tails - I could never see a mechanism by which they were supposed to give the results often attributed to them. And it turns out those results are a fallacy.

2. The oft repeated statement made here that we're interested only in what is happening at the back of the car isn't supported by the paper - and furthermore, what works best at the back of the car is heavily dependent on what is happening further forwards. So applying a template shape to just the back of the car - done here dozens of times - is misleading.

On the other hand, the paper does support some things that have been done here - notably the use of added long, tapering tails to a small base area.

It's a very significant paper for this group, and the advice that's often given in it.
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Old 01-02-2021, 04:20 PM   #127 (permalink)
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My whole point is that disagreement isnít harassment but what goes on here is far beyond disagreement on facts. This isnít a scientific community, itís a forum but the scientific community encourages civility and doesnít generally call people ďfansĒ which is a kind of ridicule in this context. When I came into this ďdebateĒ I simply requested we act as civilly as we agreed to when we joined.

If you think humorous sarcasm is incivility dam you to H E double hockeysticks.

I forgot the winky face last time. My bad.
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Old 01-02-2021, 05:14 PM   #128 (permalink)
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Quote:
Originally Posted by AeroMcAeroFace View Post
It is a very interesting paper actually.

Downwards roof slope gives huge lift, 20 degree downwards slope is higher lift and higher drag than 15 degree downwards.

"little data is available on the effects of streamlined tails on non-axisymmetric bodies, whether representative of road vehicles or not" Presumably they need to "read between the lines" more despite reading the same books template enthusiasts have.

Tapering is good, but cannot be done arbitrarily is the main theme of the paper. Angles that work on the roof don't necessarily work on the sides, and vice versa.

If you are going to extend your car, don't expect a specific angle, shape or profile to be optimal.
Perhaps, don't expect a predetermined angle, shape or profile to be optimal?

There will be specific angles, shapes and profiles that are optimal, but they will be able to found only through testing on your particular car.

And yes, it is certainly a very interesting paper.
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Old 01-06-2021, 12:15 PM   #129 (permalink)
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And regarding the original topic, I intend doing a video soon on Kamm tails.

The latest SAE paper* on the topic (2020 published! - very rare to find new stuff published on the original low drag shapes), basically debunks so-called Kamm tails as a way to low drag.

(*I found the paper through a reference that Aerohead made to it here - thanks very much for that.)

The SAE guards their copyright fiercely but there is no copyright on facts, so I will be able to summarise the paper in my own words without issues. (But not use any of their diagrams, unfortunately.)
1) They do mention that, ' the drag reduction from the elongated tail varies almost linearly with the reduction in cross section area' ( which is in keeping with Lay's, Kamm's, Reid's, Heald's, and other investigators ).
2) their comment about ' car-like' shapes implies that they don't know about Jaray's body cross-section morphology.
3) The Kamm-Tail did produce Cd 0.12, and that's without any active suspension/ variable ground clearance or wheel fairings.
4) They also discuss how, ' little data is available on the effects of streamlined tails on non-axisymmetric bodies, whether representative of road vehicles or not.' Guess they don't know about EcoModer.com.
5) The 'Windsor' body they investigated doesn't represent a 'real' automobile either. So that's problematic.
6) I wouldn't recommends NASA's Project Shoebox as an example of boat-tailed 'squareback' vehicles, as it's architecture is problematic.
7) Spirit of EcoModder demonstrated up to a 62% drag reduction by just following the old timers.
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Old 01-07-2021, 06:04 AM   #130 (permalink)
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4) They also discuss how, ' little data is available on the effects of streamlined tails on non-axisymmetric bodies, whether representative of road vehicles or not.' Guess they don't know about EcoModer.com.
5) The 'Windsor' body they investigated doesn't represent a 'real' automobile either. So that's problematic.
6) I wouldn't recommends NASA's Project Shoebox as an example of boat-tailed 'squareback' vehicles, as it's architecture is problematic.
7) Spirit of EcoModder demonstrated up to a 62% drag reduction by just following the old timers.
4) Or maybe they dismiss it due to results not being scientifically rigorous, not thought out well, not founded in science, based on flawed assumptions? Many reasons other than ignorance I can think of.

5) It is strange that you keep telling us that the template(s) (of which there are now 8) are universally applicable, but all of a sudden the windsor body is the wrong one?
6)same answer as 5)

7) do you have actual evidence for that claim? As far as I can tell, your evidence is based on a highly blocked wind tunnel, which is okay for streamlined shapes but not spirit of ecomodder. You believe it to be streamlined (streamlined means no separation) so that evidence relies on your belief.

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