Quote:
Originally Posted by ChazInMT
Ironically, I posted this simplistic diagram the other day in another forum which illustrates your point, and shows an alternative point. It is a common misconception that the air is "Flung Out" by the windshield & front end, and to a small degree it may be. But the effect is much smaller that you imagine.
In my mind, I see the Template as being the ideal balance between having too aggressive a shape and too flat a shape. An aggressive shape creates a dynamically induced low pressure drag at best, and shreds attached flow at worst creating turbulent flow, both Cd killers. Too flat a curve which will make for a very long vehicle with no appreciable gain, or, more likely, a shortened version of a longer template which will have a larger rear area flat area leaving the air to come up with its own way of dealing with itself, which is never a good thing on the back end of a shape.
So, in effect, air in front of a vehicle is "Smart" and can overcome problems by sort of creating its own shape, going from the ideal crappy front end, to an ideal aero shape front may gain you 20-30% overall gains in Cd.
In the back of a vehicle, the air is "Stupid" (No offense to any stupid people who may be reading this post) and does not manage itself well in the absence of a shape to follow, we must, as a people, as true eco-modders, devise ways to guide the stupid air. One of the easiest most effective ways is to follow the template. It is through proper use and adherence to this template, that 70-80% of the achievable gains may be realized. As always, YMMV (which is really just another way of saying your probably doing it wrong).
Finally, let us ponder the mystery of the shape, and give it our praise.
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In the fore-body of the 'Template' the ram pressure against it provides a 'favorable' pressure gradient which holds the turbulent boundary layer against the 'Template',up to the point of max. camber,where the velocity at the boundary of the 'body' is zero.
Beyond the point of max. camber,there no longer exists a 'favorable' pressure gradient ( it's in the lee of the 'Template' ),the flow is decelerating,and according to Bernouli's Theorem,the velocity would have to slow even more,but it's already at zero!
If the body does not taper at a very gentle rate,the boundary layer will detach ( flow separation ),the flow will reverse directions,beginning eddy flow which will soon grow into full-blown turbulence.
The point of separation determines the base pressure of the wake,which along with the forward stagnation pressure creates the 'pressure drag' we're trying to eliminate.
The 'Template' will not allow separation.It has no wake.It has no pressure drag,only surface friction,which we can't do anything about.
No real mystery,just good empirical science.