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Not quite sure what you meant with the pressures, but,
As I understand it,
Theoretically, without any outer boundary's, a flat plate moving through the air (or any fluid),has a boundary layer that increases proportionally to the length of the plate, I haven't studied the equations to determine if it is linear, exponential or whatever, but it is a proportional relationship.
So if we assume a linear relationship,
a small car with 3' of roof has say, 3/8" of boundary layer at the end of the roof,
A van with 12' of roof would have 1 1/2" of boundary layer and,
A Semi Trailer with 40' of flat roof would have about 5" of boundary layer.
So this then begs the question,
If a longer vehicle has a naturally thicker boundary layer, and a thicker boundary layer is more stable around a curve than a thinner one, can the taper at the rear of a longer vehicle be sharper than that on a shorter vehicle and still have the airflow attached?
We see the Semi Trailer boattail flaps at quite sharp angles of around 10-15°, but the template suggests anything greater than 3-5° at the start of taper and you will get immediate seperation.
So then I have to ask is the template application dependant on vehicle length and shape, how long are the flat sections before tapering, does this have a bearing on how aggressive the taper can be?
We must remember the template taper from the point of greatest camber to the tail, is based on the frontal profile of the template which is a bit of an elongated hemisphere, this profile will roughly fit most road vehicles, like sedans and hatches, but when you start to apply it to wagons, vans and trucks there is an extended section of flat roof and sides which I am starting to think has a significant bearing on template application.
For these long vehicles, it may not be necesary to trip the boundary layer to thicken it, as it is already thicker, it may be that we just need to compress the template a little and actually use a more agressive taper.
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