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Old 06-25-2021, 03:26 PM   #1 (permalink)
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the Ahmed body

A comment was directed towards a # 19 ( permalink ) I posted at the thread:
eNV200 aero mods, by vianney, presently on page-2 here at the Aerodynamics Forum.
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While I couldn't locate my original work sheets from 2012, from which my comments were drawn, I've been able to locate the specific work I published in 2012, and essentially retrace my steps in the evolution of the published quanta, and the context from which they were derived.
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If anyone is interested, I'll be happy to discuss it to the best of my abilities.

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Old 06-25-2021, 05:13 PM   #2 (permalink)
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The Ahmed body, and the lessons drawn from it, are now little used in modern car aerodynamics.

In fact, testing of detailed car models in wind tunnels with a moving floor and wheels has shown that the lessons from the Ahmed body on real cars is quite hit and miss. A good example is the current Porsche Cayenne, that specifically goes against traditional Ahmed body ideas and records lower drag in doing so.

I cover the Porsche in my Century of Car aerodynamics book. When Professor Howell was reading that section, he said that the idea that Ahmed bodies couldn't be relied on to guide car shape was now quite old-hat - he actually mentioned a paper he had written (maybe a decade ago?) that had made that point.

To suggest that you can calculate real-world car Cd figures* from Ahmed body variations, as you appear to do in the post you cite, is completely and utterly wrong.

(*let alone to three decimal places!)

Last edited by JulianEdgar; 06-25-2021 at 06:03 PM.. Reason: typo
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Old 06-25-2021, 06:04 PM   #3 (permalink)
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little used

Quote:
Originally Posted by JulianEdgar View Post
The Ahmed body, and the lessons drawn from it, is now little used in modern car aerodynamics.

In fact, testing of detailed car models in wind tunnels with a moving floor and wheels has shown that the lessons from the Ahmed body on real cars is quite hit and miss. A good example is the current Porsche Cayenne, that specifically goes against traditional Ahmed body ideas and records lower drag in doing so.

I cover the Porsche in my Century of Car aerodynamics book. When Professor Howell was reading that section, he said that the idea that Ahmed bodies couldn't be relied on to guide car shape was now quite old-hat - he actually mentioned a paper he had written (maybe a decade ago?) that had made that point.

To suggest that you can calculate real-world car Cd figures* from Ahmed body variations, as you appear to do in the post you cite, is completely and utterly wrong.

(*let alone to three decimal places!)
* No argument there. I've never seen it used other than in CFD analysis and the wind tunnel.
* My comment to the eNV200 owner had to to do with his boat tail performance.
* In my 2012 ' Comparative Anatomy vs. Cd Adapted from : Hucho, 2nd Ed., Fig. 4.14, I followed the Ahmed body reported data for the 1st seven images. Using my drafting instruments with a photo-enlarged image of Ahmed's chart, I was able to measure the Cds which were not directly reported. Ahmed himself reported to three decimal places ( please see Figure 4.14, at 30-degrees ) and for the last two, incorporated features from the research of Bearman, Potthoff, Carr, GM, FIAT, Hucho, NASA, dealing with vortex drag.
* When vianney mentioned that the sides of his tail might be experiencing flow separation, it triggered a thought directed towards the 2012 drag chart and research.
In California, a full-scale Ahmed body vehicle was constructed an tested by NASA.
The Cd 0.25 Ahmed body translated into Cd 0.41 as an actual motor vehicle when with a conventional body on frame undercarriage.
A full belly pan with diffuser dropped it to Cd 0.347 ( NASA's numbers, not mine)
The truncated boat tail lowered the drag to Cd 0.242 ( NASA's numbers , not mine )
NASA's truncated tail was good for a 30.2% drag reduction, although was of curved surface geometry, much like the W. A. Mair tail of the late 1960s, and Walter H. Korff's boat tail of 1963 ( SAE Paper # 649B, January 1963 ).
Would you like to show us where I ever even mentioned calculating real world drag coefficients in my comment to vianney?
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Last edited by aerohead; 06-25-2021 at 06:09 PM.. Reason: add data
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Old 06-25-2021, 06:24 PM   #4 (permalink)
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The Ahmed body was used in the mid 1980s as a very simple shape to work out some relationships between airflow and drag. It is still used today when basic relationships need to be explored, eg in model wind tunnel research.

It is never used to predict the Cd of a full-size car, and is certainly not extrapolated to an estimate of the Cd of a full-size car. To even imply this is possible is completely wrong.

This is what an Ahmed body looks like:



You said:

Quote:
Originally Posted by aerohead View Post
Since you mentioned this, I re-visited the research done on the Ahmed body.
Some data points:
* when the rear is modified, this new 'aft-body' constitutes 21.8% of total body length
1) @ zero-degree rear downslope only, Cd 0.25
2) @ 5-degree downslope only, Cd 0.233
3) @ 9-degrees downslope only, Cd 0.229
4) @ 12.5-degrees downslope only, Cd 0.23
5) @ 15-degrees downslope only, Cd 0. 236
6) @ 20-degrees downslope only, Cd 0.253
7) @ 25-degrees downslope only, Cd 0.287
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8) #7 plus a 10-degree upswept diffuser, Cd 0.262
9) #7 and #8, plus 10-degree plan-view boat-tailing. Cd 0.214
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* As the numbers reveal, the top rear slant initially lowers drag until just before the 20-degree angle, then rises above the original Cd.
* Adding the diffuser lowers drag, although it's still above the original Cd.
* It isn't until the side boat-tailing is introduced that, drag finally falls below the original Cd, for an overall 14.4% drag reduction, @ Cd 0.214.
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* This 25-degree, 10-degree, 10-degree tail relies on a series of vortices, in 3- planes, attacking one another, to burst each other.
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If it were true that the side flow on your tail WAS compromised, the above illustration might provide insight into the performance.
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I did a rough calculation using the as-shown, truncated W.A. Mair boat tail:
* Ignoring the wheels, the wake area was reduced to 15.38 square-feet ( 1.428 meters-squared. A 51% reduction.
* With rear belly pan, 'slow' diffuser, rear wheel skirts, and tail, we have a probability for a 45% drag reduction.
You were giving advice about the shape of a car modification, and as part of that advice, quoting Cd figures based on an Ahmed body. To do this is highly misleading.

The Ahmed body, and measurements resulting from it, cannot be used to guide the shape of car modification.

In fact, as the Porsche Cayenne example shows, lower drag may be able to be achieved by adopting a shape that the Ahmed body indicates is high in drag.
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Old 06-25-2021, 06:28 PM   #5 (permalink)
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discussion

Thanks, stores closing. If I'm around next week I'll catch up.
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Old 06-25-2021, 07:12 PM   #6 (permalink)
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Using very simple shapes to provide guidance for car modifications is invalid. Those simple shapes (whether they be The Template or an Ahmed body) are aerodynamics painted with the broadest possible brush. In the case of the Ahmed body, particularly, a lot has been learned since 1984.

It's rather like reading in an old book that for best power, the engine's air/fuel ratio should be 12.5:1, and best economy, 16:1. Of course, neither figure may actually be correct on a given engine: the best ratios can only be found through testing.

With the Ahmed body, and enlarging a diagram from an old book to get Cd figures in an attempt to guide car modification, it is doubly misleading.

First, the data applies only to an Ahmed body, a very simplified shape. Second, as recent research has shown, the Ahmed body results (and specifically drag changes from altering rear body angles) may in fact suggest the very opposite of what gives lowest drag!

And if the Ahmed body gives no actual guidance of use to car modifiers, why muddy the waters by mentioning it in the context of someone modifying their car?

Any time someone mentions simplified shapes and attempts to draw lessons from them for car modification, it's wise to be highly sceptical. Car aerodynamics is just not that easy.
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Old 06-25-2021, 07:17 PM   #7 (permalink)
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Old 06-25-2021, 07:18 PM   #8 (permalink)
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Quote:
Originally Posted by JulianEdgar View Post
The [old idea] and the lessons drawn from it, are now little used in the modern [thing].
Don't get tired of saying it.

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