[aerohead]

Originally Posted by Tesla

Ok,
As I understand it you may be suggesting a taller vehicle may need an even longer proportional taper profile.

Would this be the same whether that vehicle had a longer body or shorter one with the same frontal area?

This then also contradicts the use the narrower section to govern the taper as has been mentioned on numerous posts in regard to boattail design.

Got the 22-23° rule, never break it under any circumstances.

Got the Laminar thing, not relevant to road going vehicles.

Got the VG thing, many questions still remain unanswered and no conclusive evidence of benefits shown except in rare specialised cases.

But things still don't add up:
The Dryden van boattail, now they reported significant benefits and yes they did get seperation further down the on the original tail before they truncated it, but they most likely went way past the 22° point then as well.

As I understand the template theory, they should have had seperation on that tail in the first 6", yet flow remained attached for a good 4'.



from this thread:
http://ecomodder.com/forum/showthrea...n-21952-7.html

And then there is all the designs, patents and fitted boattails to Semi Trailers, none of these are anywhere near the template, and most are in the same range as the Dryden Van 10-15°, I assume they have had some proven testing over the last 10years, otherwise we would not be seeing an increase in their application.

I don't dispute the effectiveness of the template profile in boattail design, but am just trying to answer some of the contradictions.

I do know now that in fluid motion the rules change with size, materials and conditions, infact it seems we really don't know what the rules are.
We have a set of rules that can effectively describe an outcome under a fixed set of conditions, then as conditions change we need to change the rules.
Doesn't seem to be far from quantum mechanics, everything is just a model for an observed effect, but actual governing laws are ever elusive.

*I don't recommend anything other than the 'Template.'
*I offered the plan view 4:1 section profile only as an illustration of something 'different' which has been used to good effect.
*NASA's Ford Econoline boat tail lowered the drag from Cd 0.47,to 0.238.That's really good,but they left 50% of the drag remaining on the van.
*In fluid mechanics the shape defines the drag.Size makes no difference as long as Reynolds numbers are equal.
*We do know what the rules are:
*1 you can base a design around the 'Template' aft-body contour for low drag in a vehicle you can see out of.
*2 you can base a vehicle on a 'Reverse-Template' and achieve drag within 15% of the 'Template',but you won't be able to see out of it.
*3 you can base a vehicle on an ellipsoid and achieve identical drag as the 'Template' but you won't be able to see out of it,park it in a garage,go over a hump,around a corner,etc..
*4 you can base a vehicle on arbitrary legal length requirements and do your best to optimize for that length only,knowing you'll never actually achieve the lowest drag.