EcoModder Forum Aerodynamic Streamlining Template: Part-C

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"it doesn't change, regardless of shape"- Speed??? between 20 and 250 mph

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https://ecomodder.com/forum/showthre...tml#post621801

Quote:
 Originally Posted by freebeard The power needed to push an object through a fluid increases as the cube of the velocity. Mechanical friction increases as the square, so increasing speed requires progressively more power.

 The Following User Says Thank You to COcyclist For This Useful Post: aerohead (09-14-2023)
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 09-12-2023, 06:58 PM #752 (permalink) Master EcoModder     Join Date: Aug 2012 Location: northwest of normal Posts: 26,578 Thanks: 7,489 Thanked 8,279 Times in 6,807 Posts I read it as for any particular shape of varying fineness ratio. Above 250, a pointed nose is better than a bluff nose. __________________ . .Without freedom of speech we wouldn't know who all the idiots are. -- anonymous poster ____________________ The best accident is no accident -- Elon Musk
 The Following User Says Thank You to freebeard For This Useful Post: aerohead (09-14-2023)
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' Cd / Shape / Speed'

Quote:
 Originally Posted by COcyclist "it doesn't change, regardless of shape"- Speed??? between 20 and 250 mph
1) the 'shape' determines Cd.
2) the Cd is constant within this velocity range ( this is 'low-speed, subsonic, incompressible-flow aerodynamics ).
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Caveat:
You may see some researches, say, for some Teslas, which were specifically tested for ventilation drag for the different OEM wheels offered, which DOES alter the Cd slightly, however, the difference barely makes it onto to the 'radar screen' in relation to overall drag.
Precise road testing would respect these dynamics in order to parse out actual changes from R-R or aero mods.
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'pointed'

Quote:
 Originally Posted by freebeard I read it as for any particular shape of varying fineness ratio. Above 250, a pointed nose is better than a bluff nose.
Correct!
Above 250-mph ( 405 km/h ) a road vehicle is entering transonic flow, where air begins to compress, and parts of the body may be capable of generating shock-wave drag.
Hucho gives us a glimpse into this world in his 2nd-Edition book, regarding record vehicles.
The rocket-powered 'Blue Flame' is attributed with a subsonic drag of Cd 0.20, which climbs to Cd 0.60, or 0.65 ( if memory serves me ) around Mach-1, in standard air.
You see the same with the Space Shuttle, North American X-15, and NASA lifting bodies.
The U.S. Army's Aberdeen Proving Grounds has published drag figures for a handful of artillery projectiles, which also depicts these exponential drag increases, from subsonic to supersonic velocities, at constant air density.

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