Quote:
Originally Posted by freebeard
Thanks Talos Woten.
I'll contemplate the 1/4-1" range vs dihedral anglesI notice for a prolated shape, the more severe angles are at the front end. The rear would be inverted for a box cavity.
What do you think of this one. I made it for another thread in 2015 -- a bellmouth difusser.
One big vortex generator to order the wake.
edit:
I didn't say you can shove it.  |
It's the angled triangular facets which would impel the flow sideways off their optimum path.
The sharp angles around the corners are also 'spikes'.
They're also deflecting the flow 'away' from where the local streamline would 'go' if on a streamlined contour.
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If you can imagine an air molecule above the body, as a roller lifter, the atmospheric pressure as the valve-spring tension, and the body as a camshaft lobe passing under the roller lifter; there is a limit to the 'contour' of the camshaft lobe, after which the spring tension is insufficient to hold the roller follower in contact against the cam. The roller 'floats'. It 'separates.'
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The lowest pressure on the car is just ahead of the windshield header.
All the air would like to go there.
That's okay in the 'front' of the car.
It's the worst thing for the back of the car.
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Beyond the roof apex, even on a perfectly streamlined body, the air is moving towards a higher pressure than where it's at, at the rear of the body.
The only thing that keeps the air moving rearwards is, shearing forces from the local streamline, just above the boundary layer 'strafing' kinetic energy into the boundary layer, 'pinning it down', like a machine gunner, against the body's surface.
If the body surface falls away too radically, the shearing force from above just richochets off the top of the boundary layer, without imparting any energy.
When this happens, the boundary shrugs its shoulders and says ---- it! It always wanted to go where the windshield was, and that's exactly what it attempts to do, rolling up into eddies as it tries to climb backwards up the back of the car, against the flow, then completely lifting off the surface as the local streamline attacks it, blooming into full turbulence.
The energy balance necessary to keep the flow attached is a very delicate affair.
'(C)hange in a pressure distribution is highly significant for the origin of drag.'
Hucho, 2nd-Ed, page-117.
' The pressure drag is explained by the deviations of the pressure distribution in actual flow.' Hucho, page-124.
' (I)t is very important to design a rear body surface which brings the divided streamlines smoothly together .' Hucho, page 61.
'(P)ressure drag is the largest component in the aerodynamic drag. Its minimization is the true objective of motor vehicle aerodynamics.' Hucho, p.119