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
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.
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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.