2013 DOE Class-8 semi-trailer research
 

Ran across this, from Lawrence Livermore National Laboratory.
On page-11 they're still using the 10% delta drag / 5% delta mpg relationship.
https://www.energy.gov/sites/prod/fi...ari_2013_o.pdf

2013 is going on two decades but one takes what one can get.

Page 13 gives a relation between the angle measurement and curvature measurement. :thumbup:
Curved is marginally better, but would be structurally more rubust. Curved, horizontal corrugations with a serarated edge?

curves, angles,..............
 

On page-12, they show a 48"- 3-sided tail, and a 32" - 4-sided tail.
And Cds are for the combination gap seal, skirts and tail, together.
On page-13, no dimensions are given for any of the 4-types, and there isn't enough information provided to 'reconstruct' what they've actually done, except for the radii, from which to reverse-engineer. Nothing's straight forward. Bummer.
The Ahmed and modified Ahmed bodies provide all the information we need.
So to, for Rolf Buchheim et al.'s series of prismatic bodies, and curved-roof bodies.
Buchheim's prismatic optimum angles were all a function of the length of the sloped rear panel, as a fraction of the greenhouse length.
Finally, the 4-sided tail isn't shown at all, and we have to guess if the 'floor' is level, or angled up like the top and sides.

serrations on trailing edges
 

If they were serrated, there'd be a small difference in pressure at the 'tips', compared to the 'short' pie-piece voids.
Technically, that's a setup for vorticity and noise at the staggered separation line, and it would rob some pressure recovery overall.
According to NASA Photo: E-38096, from 1981, we'd likely see zero longitudinal vorticity as can be seen ( tufts ) on Project Shoebox.
By the way, NASA'a boat-tail appears to be cribbed from Paul Jaray & Associates' 1921, softened and boat-tailed square-cylinder, tested by Wolgang Klemperer, at the Zeppelin Werkes wind tunnel.
And it's delta-Cd varies almost perfectly as described in SAE Paper # 2020-01-0673, by Jeff Howell et al.. ( Cd 0.2429-predicted, vs Cd 0.242 as measured by NASA ).

On turbojets the serrations are used to order the vorticity and suppress noise.

https://ecomodder.com/forum/member-f...29-0-large.jpg

turbo jet/fan
 

On the jet engine, the dynamic flow is the bypass from the fan interfacing with slower, higher pressure air outside, and then the actual jet thrust interfacing with the slower, higher pressure fan thrust outside, 'backwards' from the box-cavity/ trailer tail which is 'dead' inside.
To reduce drag on the truck, one wants to use every millimeter of length at the tail for pressure recovery and uniform velocity/pressure when the streams meet, minimizing shear.

some numbers
 

There was just enough information to reverse-engineer some values for the semi-trailer:
Length- 576" - 636"
Height- 162"
Width- 102"
Ground clearance- 9.1"
Skirted wake- 93.5-sq-feet
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If 0.175 = 26% drag, then the rig without any mods is Cd 0.67307
With gap-seal and lower skirts, Cd 0.5953
With gap-seal, skirts, and tails, Cd 0.52607 - Cd 0.49807, delta Cd 0.028 ( 5.32% )
If the four tails are extrapolated out to their terminations, they form 'complete' tails of 128", 145.4", 229.6", and 263.3".
With 48" tail with 4-sides, estimated Cd 0.452
With 60" inflatable tail ( longest allowed by DOT ), estimated Cd 0.4185
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@ 65-mph, the drag -to-mpg relationship is delta-10% Cd = delta-5% mpg
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The following boat-tails, if sized to match the Lawrence Livermore semi-trailer width produce lengths of :
1910 Maurice Farman strut ( best ) 145"
1922 Jaray square-cylinder............ 129"
1933 Frey 2D section ................... 141.4"
1963 Walter Korff semi-trailer......... 191.7"
1969 W.A.Mair torpedo................... 193.8"
1976 Wysocki railroad tail.............. 163.2"
1981 NASA Shoebox ( ave. top/side ) 113.4"
1981 NASA semi-trailer.................... 146.2"
2012 Breidenbach SUV..................... 258.6"