The P. W. Bearman bodies

[aerohead]

* This is a follow-on to the Ahmed Body discussion, germane to the last two comments I made with respect to the NISSAN e200 boat tail.
* Data is from Figure 4.54, page 150, Hucho, 2nd-Edition
and Figure 4.56, page 151, Hucho, 2nd-Edition.
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* The Bearman body of Fig. 4.54 is similar to the Ahmed body, with similar drag @ 25-degree rear slope, and includes the vortex-drag component, parsed out from the overall drag.
* The body is a simple prismatic shape.
* Drag is evaluated from zero-to-45-degrees rear slope.
* The rear slope constitutes 26% of overall body length.
* Vortex drag builds from zero, at 0-degrees, to a maximum at 33-degrees, after which vortex burst occurs, returning to a squareback wake.
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* The second Bearman body, on page 151, is also similar to the Ahmed body.
* It is a simple prismatic shape.
* No 'stilts' are present.
* The entire rear is chamfered at a 45-degree downslope angle.
* In addition to a change to the rear slope, a diffuser is introduced.
* Plan-view taper is introduced.
* The entire body is not represented.
* We do not know the relationship of the length of the 'aft-body' to overall length as a percentage.
* We do not know the starting Cd of the unmodified model.
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* For configuration (1) a 25-degree rear slope is affected, with Cd 0.243 profile drag, plus Cd 0.0575 vortex drag, for Cd 0.3005.
* For configuration (2) a 10-degree diffuser is added, resulting in Cd 0.218 profile drag, plus Cd 0.015 vortex drag, for Cd 2.33.
* For configuration (3), 10-degree plan-taper is introduced, resulting in Cd 0.170 profile drag, plus Cd 0.0065 vortex drag, for Cd 0. 1765.
* For configuration (4) top edge chamfering is introduced, resulting in Cd 0.16 profile drag, plus zero vortex drag, for Cd 0.16.
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* A variant of the second Bearman body showed up at Texas Tech University in the early 1990s, as part of the 'Tailwind Project', with Professor Carver and Funderburk et al..
* This variant was the General Motors Optimum Tail, for Class-8 Tractor- semi-trailers.
* GM chose a 20-degree downslope, 10-degree diffuser, 10-degree plan-taper, and generous radii for all panel edges, equaling 7% of the actual trailer van height ( approx. 8-inches )
* Length of the tail equaled 93.3% of actual van height ( approx. 107.4-inches ).
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* IF an Ahmed body reacted to a diffuser and plan-taper, as in the Bearman body #2:
1) the 25-degree slope would INCREASE drag beyond the original drag.
2) the diffuser would reduce drag, however, the overall drag would still, exceed the zero-modified body.
3) only the addition of the plan-taper would provide for a net drag reduction.
4) there appears to be a requirement for all planar surfaces to provide counter-attacking vortices to burst one another, in order to see any overall drag reduction compared with the 'naked' Ahmed body.
5) should there be something about the performance of the plan-view taper on the NISSAN e200 which is compromised, it could explain a performance deficit, if it exists.
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Funderburk tested for, and recognized also, that, the 'softening' of all edges helped mitigate unwanted characteristics of the X-Y-Z vortices, leading to a drag minimum ( also implied by General Motors choice in edge radii ).
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One benefit which may accrue from the plethora of university Ahmed Body CFD analysis, may be validation of the CFD software used by each independent investigator.
Model parameters are a 'constant.'
All investigations would produce identical output, should the differing software all offer equal efficacy.
CFD software quality validation by default.