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Originally Posted by freebeard
Actually just the opposite. The 4v octahedral dome has 512 vertexes, and the shape I evolved has a void on the bottom so it's only 448 vertexes. That would fit in a spreadheet, higher frequency means smoother shapes although the count goes up fast. I think at 4v it would be like the dimples on a golf ball, but I can't find any info on frequency vs Reynolds number. Think F-117.
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My comment was more related to complexity of construction, not the aerodynamics of the shape, I'm not up to speed with all the programs used or the knowledge of those shapes, I know the basics of geodisic domes, but only in the sense of dwellings. As for the F117, I expect the primary aim was stealth, not aerodynamics and we all know that if you put enough thrust behind it a house brick will break the sound barrier.
Quote:
Originally Posted by freebeard
Your most recent post is all about the limitations of 3rd hand Cartesian coordinates. The solution used in naval Architecture is loft lines. Those are still only accurate to the limitations of the drafting methods. One could define the shape geodesically, then use software to slice it into layers like NeilBlanchard has. The octa symmetry has 90° corners at the middle sides so you could stretch it a little to the front to get the bluff body, then droop the nose to lower the stagnation point and stretch more to more to the rear with a truncation. Width and height can be de-coupled. And the 'equator' can be lowered, all because of the octahedral symmetry.
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Understand about the limitations you mention, but I think it is just a simple observation, the angles along the spine are changing rate up and down, this will create a wiggly line, not a continuous curve in one direction, in the table from 20to 40% you go from 4 to 3 to 3.5 then 2 in rate change, surely this should be all in one direction, either up or down for the template type curve.
Quote:
Originally Posted by freebeard
As for the 22° vs 25° debate, I start blocking it out. A gathering angle at the rear of 45° means 22 1/2° and that's close enough for me. I don't get what you mean by the cylinder to reattach flow, but that's OK.
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I think you referenced the sphere and 25* seperation at 4 seconds analogy, then suggested a 25* cone behind this, my point was it probably wouldn't matter if you attached a 1* cone behind that the air is gone. From what I have read & as I understand things so far, there are some ideal shapes to keep flow attached and the template is one of them, but in general they are all in the same family. With a sharp bend the air can hold on for a bit, but there will be payback, once you pass the 4 second mark you're into the turbulance and this will not settle or re-attach for quite some distance.
With freight trailers, they are limited to length, so they dive in hard with the angle and can get a good benefit, most likely better than if they followed the template to the same distance, but I suspect if they extended the full length with that steeper angle, there would be nothing further to be gained and the result may even be worse. So for a short cut off tail, you can go a bit steeper, for a longer tail greater benefits will be seen by sticking to the template. That,s what I think I know, but still learning.