Quote:
Originally Posted by freebeard
I don't have a good picture handy, but my understanding is that the box cavity needs to touch the Template line, then you get little recirculating eddies on it's outer faces.
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I'd agree with the latter part of the statement (about the stable eddies), but the "Template" (dare I speak it?
) may not be, shall we say,
precisely universally applicable. I believe the base-pressure box cavities that have been found optimal all end well inside where the 'Template' would put them. With the of straight 'offset' cavities on the trucks, IIRC, it was about 0.7m in from the edge and 1m long, the 'full width' straight box also showed drag reduction (nothing remotely Template-like there), and with the 'full width' cavity with panels pointing in the good angles were in the 15-20 degree range - and that's even given a pretty sharp transition. The study on the mercedes A class hatchback also ended up with a box cavity depth of 10cm, with a fairly hefty inset.
<rant>
The template is a very nice rule of thumb which works well for attached (vs. almost-attached) turbulent flow and kammback construction, but it's only optimal / close to optimal for a limited set of circumstances. When you stray outside those, and get things like stable eddy structures which differ from the kammback case (as in this case), or laminar flow (good luck with that
), or cavitation (not a problem for us, dealing with land vehicles), or transsonic flow (not a problem for
most of us) it becomes merely an optimal solution for someone else's problem. By way of analogy, in a free stream, a teardrop is better than a flat-nosed cylinder
sometimes and better than a double cone
sometimes and better than a rounded wedge making 2/3 of the length on the leading edge with a concave tail making 1/3 of the length on the trailing edge
sometimes - but there are situations where the teardrop loses to each of these...
</rant>