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Old 01-27-2021, 05:52 PM   #1 (permalink)
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'template' in contemporary literature

Looking at streamlining, I came across the following:
' Aerodynamic Testing and Development of Sunswift eVe ', Simon Ambrose & Graham Doig, 53rd AIAA Aerospace Science Meeting, Proceedings: Kissimmee, Florida, January 5-8, 2015
Sunswift is a World Solar Challenge competitor, Cd 0.16, and Af 0.7875 m-squared.
There are some CFD images. The side elevation indicates fully-attached flow down the roofline profile.
--------------------------------------------------------------------------------------
' Aerodynamics Concept Study of Electric Vehicles, Drag Reduction and Range Increase, Master's Thesis in Automotive Engineering, Umat Aktas, Kristian Abdallah, for National Electric Vehicle Sweden, Chalmers University of Technology, 2017.
As with the Porsche Macan vs Mitsubishi Mirage, Cd 0.37 vs Cd 0.27, ' the largest drag reduction are the diffuser and roof/ trunk spoiler extension concepts.'
--------------------------------------------------------------------------------------
' Aerodynamics Development for a New EV Hatchback Considering Crosswind Sensitivity,' SAE Paper 2018-01-0715, Yusuke et al., Nissan Motor Co., Ltd., April 3, 2018.
For EVs, they attribute 59% of total drag to aerodynamics, at 100-km/h.
In Figure 4, page-4, they give a CFD image of ' lowest Cd streamlined shape.'
It's familiar. Fully-attached flow until truncation, then a very small wake.
--------------------------------------------------------------------------------------
From: INSIDE EVs,
GAC ENO. 146
' Most Aerodynamic EV in The World Being Tested,' May, 27, 2020. GAC R & D Center, China.
They report Cd 0.146.
In plan-view, the car is very much a 1938 'Lange car.'
A very 'low' active splitter.
Seating for six.
Tire width looks reasonably like a 'real' tire.
They include smoke-flow imaging footage from wind tunnel testing.
This car has been reported already here at EcoModder.
--------------------------------------------------------------------------------------
These gentle, continuous curved rooflines tend to show up on the lowest drag vehicles. They provide the pressure recovery which 'IS' the drag reduction.
--------------------------------------------------------------------------------------
From: Modifying the AERODYNAMICS of Your Road Car, by Julian Edgar :
* ' flow separation...explains the low recorded pressures' page 79
* ' You can see that the proposed wing location matches what you'd expect from the smoke testing.' page-83
* ' rear lift... reflects the airflow generating low pressures over the rear half of the car...' page-173
* ' attached flow from nose to tail ' ( Figure 2.45, page-48, [ he's inadvertently describing the 'template', as the Volkswagen XL1 is the 'template' shape ])
* ' world's slipperiest road car', page-48, referring to the 'template' Volkswagen XL1, Cd 0.186. [ It's slippery for Volkswagen, but for no one else, and don't you dare bring it up, because it's 'magical ' and only works at Wolfsburg, Germany ]
* ' it is best if the spoiler is placed in an area of attached flow ', page- 194, ( which suggests that some spoilers are placed in separated flow ( and comparison of some production vehicles with rear spoilers to the 'template' indicate this very thing.)
* ' Another way in which a rear spoiler on a sedan can reduce lift and drag is to simply promote flow reattachment.' page-195 ( but we better be talking about pre-1990 automobiles, as no notchback sedan produced after that date is capable of separated flow, according to the author )[ this rule violated by Audi's 2010 A7 Sportback ( obviously 2010 is 'before' 1990 )]

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Old 01-27-2021, 07:17 PM   #2 (permalink)
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Old 01-27-2021, 07:20 PM   #3 (permalink)
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Old 01-27-2021, 07:26 PM   #4 (permalink)
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Old 01-28-2021, 06:03 AM   #5 (permalink)
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Quote:
Originally Posted by aerohead View Post
typoLooking at streamlining, I came across the following:
' Aerodynamic Testing and Development of Sunswift eVe ', Simon Ambrose & Graham Doig, 53rd AIAA Aerospace Science Meeting, Proceedings: Kissimmee, Florida, January 5-8, 2015
Sunswift is a World Solar Challenge competitor, Cd 0.16, and Af 0.7875 m-squared.
There are some CFD images. The side elevation indicates fully-attached flow down the roofline profile.
I am very familiar with Sunswift eVe, I read that paper a while ago. They don't start with a template and make no mention of a template in the paper. Just in case anyone is mislead by the original post.

Fully attached flow, I completely agree with, that is the goal, getting there is rarely if ever done using a "template" in any form.
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Old 01-28-2021, 11:42 AM   #6 (permalink)
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I wish we'd get past this "template" fetish.

I believe it's been shown that our famous aerodynamic templates are generalizations at best, and as such can lead to misinterpretations, misapplications and mistakes.

No question that our templates are a good general guide to consider amongst other aerodynamic information, but it appears that too much reliance has been placed on them in some circles, which do not seem to include most of today's experts in the field.
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Old 01-29-2021, 09:24 AM   #7 (permalink)
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The problem here has been one of interpretation. The "template" is fine in that it is a low drag shape, an example of the maxim that long, tapered shapes generally give lowest drag. However, even a cursory read through any modern aerodynamics text will show that:

1) There isn't one ideal shape

2) when you take an ideal shape in free air and move it close to the ground, the effect of the ground introduces complexities to the flow

3) because of this, an ideal shape in ground proximity is not its free-air ideal shape cut in half (although this is used as a thought exercise in several aerodynamics textbooks--to understand mechanisms of both drag and lift)

4) the flow over a car is three-dimensional (highly three-dimensional, most textbooks put it), and can't be considered only in two-dimensional slices or profiles

5) the flow over an ideal shape depends on the entire body i.e. you can't take part of it (the tail) and apply it to a completely different body and expect that it will still be the lowest-drag solution. What happens at the front affects flow at the rear, and because of pressure transmission through air at the speed of sound, what happens at the rear affects flow at the front

6) unfortunately, through intent or not, the template has come to be seen here as all of these: the single ideal shape, cut in half with no concern for underside shaping aside from a simple diffuser, used as a "dimensional analysis" tool by considering a 2D profile only, applied to cars of various body geometries willy-nilly with little to no testing or verification and none (that I have seen) comparison testing of tapers of varying side angles, diffuser angles, top angles, and length on the same car

7) we now have tools for simple testing of drag changes. I'm getting a new vehicle this spring (well, old vehicle--my late grandfather's truck that I used to drive and has been sitting unused at my parents' house for the past several years) with a mechanical throttle connection specifically so I can test drag changes, which I can't do easily in my Prius. We can measure changes in ride height at speed, or panel pressures and see what body changes do to lift and airflow. There's no need to sit behind a keyboard and guess! I'm as guilty of that in years past as anyone here, but I've at least learned that much (between all the doom-scrolling of the last year or so).
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Old 01-29-2021, 12:12 PM   #8 (permalink)
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Old 01-29-2021, 12:38 PM   #9 (permalink)
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Quote:
Originally Posted by MeteorGray View Post
I wish we'd get past this "template" fetish.

I believe it's been shown that our famous aerodynamic templates are generalizations at best, and as such can lead to misinterpretations, misapplications and mistakes.

No question that our templates are a good general guide to consider amongst other aerodynamic information, but it appears that too much reliance has been placed on them in some circles, which do not seem to include most of today's experts in the field.
* This is a curious phenomena.
* Hucho reported that a Cd 0.09 vehicle was doable in December, 1986. It would require the 'template' as it's the only form he published which satisfied the initial condition for such a vehicle, which could be a passenger vehicle.
* The issue was the ' vehicle specification', set by the committee with the only authority to set specifications.
* It never had anything to do with 'aerodynamics.'
--------------------------------------------------------------------------------------
* Today, no automotive manufacturer, excepting the Chinese, are even exploring really low drag.
* So its makes sense that, no legacy carmaker would even be paying attention to really low drag.
- Michael Mauer decides what a Porsche looks like.
- Marc Lichte decides what an Audi looks like.
- Walter Maria de Silva determines what a SEAT looks like.
- Klaus Bischoff determines what a Volkswagen looks like.
- it goes on and on........................
-------------------------------------------------------------------------------------
* An aerodynamics engineer can measure and recommend 'til the cows come home, and it can all be for naught, if it's vetoed by the designer/ stylist. It's been this way since 1926.
--------------------------------------------------------------------------------------
* So, in light of today's 'reality' of SUVs driving product planning, aerodynamic 'experts' might not even be swimming in waters, even remotely associated with low drag.
* And not to be disrespectful, but being an aerodynamics engineer for a legacy carmaker today may come down to a matter of putting lipstick on a pig.
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Old 01-29-2021, 01:08 PM   #10 (permalink)
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modern aerodynamics text

Quote:
Originally Posted by Vman455 View Post
The problem here has been one of interpretation. The "template" is fine in that it is a low drag shape, an example of the maxim that long, tapered shapes generally give lowest drag. However, even a cursory read through any modern aerodynamics text will show that:

1) There isn't one ideal shape

2) when you take an ideal shape in free air and move it close to the ground, the effect of the ground introduces complexities to the flow

3) because of this, an ideal shape in ground proximity is not its free-air ideal shape cut in half (although this is used as a thought exercise in several aerodynamics textbooks--to understand mechanisms of both drag and lift)

4) the flow over a car is three-dimensional (highly three-dimensional, most textbooks put it), and can't be considered only in two-dimensional slices or profiles

5) the flow over an ideal shape depends on the entire body i.e. you can't take part of it (the tail) and apply it to a completely different body and expect that it will still be the lowest-drag solution. What happens at the front affects flow at the rear, and because of pressure transmission through air at the speed of sound, what happens at the rear affects flow at the front

6) unfortunately, through intent or not, the template has come to be seen here as all of these: the single ideal shape, cut in half with no concern for underside shaping aside from a simple diffuser, used as a "dimensional analysis" tool by considering a 2D profile only, applied to cars of various body geometries willy-nilly with little to no testing or verification and none (that I have seen) comparison testing of tapers of varying side angles, diffuser angles, top angles, and length on the same car

7) we now have tools for simple testing of drag changes. I'm getting a new vehicle this spring (well, old vehicle--my late grandfather's truck that I used to drive and has been sitting unused at my parents' house for the past several years) with a mechanical throttle connection specifically so I can test drag changes, which I can't do easily in my Prius. We can measure changes in ride height at speed, or panel pressures and see what body changes do to lift and airflow. There's no need to sit behind a keyboard and guess! I'm as guilty of that in years past as anyone here, but I've at least learned that much (between all the doom-scrolling of the last year or so).
1) I respect your opinion.
2) I'm unsure as to whether or not you've actually studied the evolution of the aerodynamic streamlining template.
3) The text addresses everything you've mentioned.
4) Hucho used the term 'ideal'. If you take exception to that language, I recommend you take it up with him.
5) The fact is, that for half-bodies, the 'template' is the lowest drag form ever measured. If you take exception to that, I recommend that you take it up with those who measured it.
6) Anyone who understands the concept of a 'streamlined' shape, understands that it is the 'shape' which determines the aerodynamic outcome.
7) Should any particular vehicle demonstrate an initial 'streamlined' contour, and then 'diverge' from the contour, the immediate solution is, to correct the architecture to conform to the streamlined contour.
8) 'Complex' body shapes can be re-'simplified', and restored to the 'simple' body plan.
9) As 'modders', we take the 'starting' conditions and work from there.
10) Only until one has put an 800-hour project behind them, do they realize the benefit of a 'template', as a 'first approximation'.
11) Evidence exists that, not only has the 'template' succeeded as the 'first approximation', it has also, in many cases, satisfied the 'final solution' as well.
12) Some of the most expensive and high-performance, boutique automobiles have used the 'template' wittingly or not, in the final design of their product.
13) It's happened too many times to be a statistical outlier 'black swan.'
14) I am not a pedant, relying on a textbook to 'tell ' me what to do.
15) And I personally know enough PhDs to know that some PhDs aren't worth the parchment their PhD was printed on. They don't get a free pass. They have to earn it.

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