Quote:
Originally Posted by JulianEdgar
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!)
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* 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?