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Old 02-24-2018, 04:30 PM   #25 (permalink)
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Reynolds number

Originally Posted by All Darc View Post
Reynolds number ??
I confess I don't know what is it.

Drag it's much more relevant in high speed :

Some solar cells, like monocrystaline, works better tha others (like pollycrystaline) for now ideals of lower angles. The top can have solar cells, ans some of the laterals too, for when the sun reach a low angle and rit such area. The solar array in general would not be ideal for all cells at same time even in the best sun'angle possible, but it would help to get more power.

My idea (maybe crazy) is about to use the principle of torpedo low drag, maybe carring two persons, like ecotracer/Monotracer, since it would have a low CD and a low CDA, due small area and aerodynamic, and at same iome combine with the principle of flat plate in horizontal, following the wind. The plate would create a large area for solar cells and add very little drag. But the plate would not be completely flat, but with a curvature , specially near the torpedo shell, to follow the wind curve relted to the shell.
Imagine a torpedo with bat wings or something like it.
This would create a vehicle for two, with lower drag than APtera or Solarworld No.1, and with similar solar cells area to Solarworld No.1
It would miss the skill to performe low angle inclined 45 degree curves like Monotracer, but would be very economic. Not sure if it could be 2 wheel or would require at leat 3 wheels (more probable).
Motor vehicle aero drag is basically a function of pressure drag.
Pressure drag is a function of flow separation,or lack thereof.
Flow separation is closely associated with the health of the boundary layer.
The best attached flow for 'real' vehicles is with a fully turbulent boundary layer.
This will occur when the Reynolds number goes super-critical,which for 'real' cars, occurs at about 20-mph.
If you keep a car below 20-mph,you can keep it in a laminar boundary layer,which really reduces turbulence-induced surface friction.
Surface friction isn't a problem with 'real' cars and a laminar boundary layer would actually increase pressure drag (overall drag) in a 'real' car.
The Ecorunner,by being 'short' ,and driving 'slow' can claim a very low 'laminar' Cd,but this would never possible with a 'full-size car,moving at real highway velocity.
The Reynolds number is associated with length,kinematic viscosity,and velocity.Once you get to around Rn = 500,000,the boundary layer will transition,from laminar,to turbulent (TBL).Golf balls are artificially roughened to trigger the immediate transition to TBL,allowing flow to remain attached longer before separating,thereby reducing the low pressure wake (pressure drag),and extending the distance of the drive.
The Rn of cars is measured in millions.
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