Quote:
Originally Posted by ennored
Something else I see missing around here...
AST (I or II) ≠ attached flow ≠ ideal
That is, just because there is attached flow, doesn't make a shape ideal. And you don't have to follow the template for attached flow.
I did some playing a bit ago with the template. As an engineer (being nothing more than a cup of coffee and a spreadsheet) I fired up Excel. Plotted and fussed with the template. I arrived at this:
At this point, the math/numbers are irrelevant. It let me draw the template shape (in 2D obviously).
We know that a sphere in a wind tunnel has attached flow for a few degrees past tangent. It's often expressed as 3 or 4 seconds on the clock face. Knowing that, and recalling the oft mentioned 22° (as happens to be found on "the template" too.) I drew this:
What do you know, 22° lands at 3.67 seconds. Hmmm....
Is this second shape a "minimum" shape to achieve attached flow? Data I've seen suggest it is. Or at least very close. Would this shape be ideal? No, we already know that. But, it ends up pretty close to the template, and perhaps explains why many "less than ideal" tails do so well. Here they are together:
Not all that different.
SO.
Less than ideal tail for a Prius, that follows the Toyota lines? It'll be less than ideal, but very good chance it'll exhibit attached flow. |
You're barking up the right tree.We went over this a couple years ago with 'Rolex meets Big Ben'.
*The TBL golf ball established the separation-free contour out to 4-seconds
*I held the forebody constant,and then began increasing the radius of the aftbody until the entire span was the 'midnight-to-4-seconds after' contour.to create a default 'Template.'
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*It's extremely important that the contour leading to the 22-degree tangent be no 'faster' than it is with the 'Template'.The boundary layer is already in a hostile environment and if you throw too much curvature at it,the rapid pressure rise will trigger separation,and they'll be no reattachment.
*Here's W.A.Mair's wind tunnel-tested aft-body.Notice the very gentle lead-in to the 22-degree slope.
*According to boundary layer theory we can't cheat on this,that's why the 'Template' is as 'slow' as it is.
*A 2.5:1 fineness ratio streamline body of revolution has the only contour which both minimizes separation,while also minimizing surface friction (wetted area).
*This 2.5:1 body is the basis of the 'Template.'
*But is all you have to go by is an analog clock or watch,it's not a bad contour.
