Quote:
Originally Posted by Jim Bullis
aerohead,
No, the Morelli shape is not a miracle shape, but the discussion of aerodynamics backed up with test data is quite excellent in the actual 1982 paper by Morelli.
But in that paper, Morelli does not proclaim this shape to be a great answer in the real world, though it gets a Cd of .047 for the low to the road position. He goes on to discuss the essential changes that would be needed, including wheels, and this pushed Cd up to .19, again, as I recall, for wind tunnel tests of that form.
My appraisal is that the Morelli shape is quite inefficient from the volume perspective, even though Cd defined on frontal area is very low.
It might be of interest that Cd for airships in the old test data is defined on a volume basis, specifically, the cube root of volume squared to get area like dimensions.
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Jim,does Morelli go into any of his wind tunnel protocols?
Back in the late 70s the CNR model car created quite a stir with Cd 0.161.
In later articles about the full-scale development car,it was reported that this form achieved this low drag and then "when wheels were added and the car lowered onto the ground the drag jumped to Cd 0.34.
With additional work,and with a cooling system and cabin ventilation they believed that the car might achieve in the range of Cd 0.201-.23.
This is the reason I've had guarded enthusiasm for the new form.
No one else has ever achieved a 3-D technical system which scored lower than Cd 0.04 in free stream,above critical Reynolds Number,and that would translate to Cd 0.08 in ground image reflection,after Jaray/Prandtl.
Morelli's shape is a 3-D variation on the Von Mises wing profile with reflexed-camber tail.
According to boundary layer theory it should be impossible for the flow to remain attached under the tail.It violates everything ever investigated for diffusers on cars with excellent onset flow from ahead and with perfectly smooth bellies.Unlike the wing in 'flight' condition,there is no free stream below the body to pump momentum into the boundary layer.The wing is operating at zero separation and is governed by skin friction,Morelli's form would be governed by pressure drag should separation occur.
The 'banana' car had pontoons blended into the body which acted in part to communicate base pressure forward under the car to cancel lift and also had a ducted rear bumper to also expose underbody flow to the wake's low base pressure.
The Morelli shape as depicted possesses none of these features.
I don't know how it would work.Western Washington State University's Viking series of cars used a milder version of this form and never achieved remarkable Cds.