Quote:
Originally Posted by Vman455
This 22-degree angle comes from Mair's work, as has been correctly stated on this site. But here's the thing: Mair was investigating flow over very long cylindrical bodies (14 times as long as their diameter) and very short, tapered tails (half as long as the diameter) to try and understand the mechanisms of flow attachment and the effect of taper on drag; these are bodies that look more like ICBMs than cars. He also reported on other researchers' work with the same types of cylindrical bodies who found drag minima at different trailing edge angles, as well as minima in side forces in yawed flow at different taper angles, and there are modern articles reporting on work with similar cylindrical bodies that find drag minima at differing taper angles depending on things like the transition between body and tail and other upstream features. 22-degrees was never presented as a hard-and-fast, "beyond this angle airflow detaches" rule because it isn't (except on this site, of course). And every aerodynamic textbook of which I'm aware gives a range of angles for rear taper within which minimum drag can be found. I'm not at home and don't have my books in front of me, but I believe Hucho even uses the word "broad" to describe the range of acceptable backlight angles for low drag, and Barnard writes about the effective angle between backlight upper edge and trunklid trailing edge, saying it should generally be between 10- and 30-degrees for lowest drag--a very wide range!
Contrary to what I believed in the past, the ideal angle for any car has to be found through testing. Depending on the body shape and details, an angle faster or slower than 22-degrees may be allowable or necessary to maintain flow attachment in the widest range of conditions. If you find yourself with a 40- or 50-degree angle, then there's a good chance the attached flow is due to vortex downwash, but the resulting high drag will show up in proper testing ( that's how this phenomenon was discovered in the first place). |
1) Mair's model consisted of an elliptical nose, of 1,3-D in length, A cylindrical main body of 3-D in length, and the boat-tailed section of 1.9-D, which creates a total length of 6.2-diameters.
2) All values reflect zero viscosity effects, only pressure drag. Nothing to do with boundary layer thickness.
3) The take-away for aero modders is the 'lead- in' profile, leading to the final downslope angle. It's a known quantity.
4) Without the boat-tail we're looking at Cd 0.204 for Mair's model with skin friction.
5) With the boat-tail, Cd 0.066.
6) This 'lead-in' profile basically exists on all PGA Regulation golf balls.
7) 'Compound-curvature' roof modifications can entail 800-man-hours of construction time. Some form of Go NoGo might save many hundreds of hours of fabrication time. This would make something like Mair's profile quite
valuable. It may not be perfect, but short of a few hundreds of thousands of dollars for laboratory R & D, it might be handy for some with a more meager budget.
8) This was the original premise for sharing it. And perhaps why Hucho and others have presented it.