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Old 11-27-2019, 01:55 PM   #70 (permalink)
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how much

Originally Posted by Xist View Post
Dr. Phil, how much of an aero penalty would the roof have, compared to my version of Hersbird's version?

Wish my skill set were up to the challenge.Either Full-Navier-Stokes Equation CFD,at supercritical Reynolds number,or full-scale tunnel testing would tell the tale.Some thoughts:
*Flattening the roof would reduce frontal area (good!)
*It would structurally weaken the roof,which otherwise may already be optimized for roll-over protection.(it's perfectly triangulated).
*The suction peak at the windshield header would be moved forward,altering aerodynamic axle loading.(may or may not be an issue).
*The 'aftbody' roof portion would be reduced,killing off a portion of the most beneficial deceleration ramp (pressure regain) feature of the vehicle (according to Hucho,Kamm,Fachsenfeld,Lay,etc.)
*There's little question that once the flow encounters the roof apex,we'd see separation (bad),however the downward slope actually appears 'conservative' and we might see rapid flow reattachment over a captured/locked vortex,allowing the inviscid outer flow to proceed to the separation line,undergoing continuous pressure recovery as it transitions;ultimately breaking away to form a wake of relatively high base pressure and overall reduced pressure drag.Just as Hucho applauds.
I'm going to hold judgement on the initial design until the numbers are published.In interviews,Elon Musk has mentioned that aesthetics are important to him.He likes 'beautiful' to be included in the design calculus.As an engineer,he may be attempting to thread a tightly configured needle,balancing between form,function,and subjective visual appeal.
When all the dust settles,I believe that a jury of his peers will consider the Cybertruck an out-of-the-park home run.I do.
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