Aerodynamic Streamlining Template: Part-C

[COcyclist]

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

[freebeard]

I read it as for any particular shape of varying fineness ratio.

Above 250, a pointed nose is better than a bluff nose.

[aerohead]

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.

[aerohead]

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.

[aerohead]

The following artillery projectiles illustrate the relationship between low-speed subsonic, incompressible flow drag, and supersonic drag:
* The KRUPP ( Pringles potato chip can ) artillery projectile had a subsonic Cd 0.859, which peaks at Cd 1.598 @ MACH 2.5.
* The Charters-Thomas spherical cannon ball is Cd 0.4915, subsonic, rising to a peak Cd 1.014 @ MACH 1.7.
* The Gavre projectile has Cd 0.200, subsonic, peaking @ Cd 0.647, @ MACH 1.35.
* The Aberdeen projectile has Cd 0.158, subsonic, peaking at Cd 0.398 @ MACH 1.0.
* The Streamline Body of Revolution, of L/D= 2.5, from which the 'templates' are based, in free 'flight' is Cd 0.04, subsonic. They are not employed for transonic, nor supersonic velocities.
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* As a streamline 'Half-Body', held in mirror image ground reflection, with the ground clearance cut away, the body is Cd 0.08, subsonic.
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* Adding the wheels of the 1981 Volkswagen 'Flow-Body' long-tail would produce Cd 0.128.
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* Adding Goro Tamai's 70% wheel-drag reducing, 'fully-swept' wheel fairings would bring the drag back down to Cd 0.0944.
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This is where Wolf-Heinrich Hucho's comment about having off-the-shelf technology to build ( a conservative ) Cd 0.09 automobile came from in December of 1986 [ he actually allowed for as low as Cd 0.07 ].
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1) 'Half-bodies' are all he recommended.
2) He showed us the lowest L/D= 2.5:1 drag streamline body, at Reynolds number 10,000,000, Cd 0.04, in Figure 4.119, page- 200, 2nd-Edition.
3) He showed us the lowest drag half-body from which it was made, without ground clearance, in ground proximity, Cd 0.09, in Table 2.1, page- 61, 2nd-Ed.
4) From the ground rules of mirror image we know the half-body, with ground clearance is Cd 0.08.
5) We know what the addition of wheels does to drag.
6) We know what the addition of fully-swept wheel fairings means to wheel-drag reduction.
7) Between Hucho and Tamai, we're given a complete recipe for very-low-drag vehicles, as of December, 1986. Thirty-seven years ago!
8) It's all 'pre-tested', and complete pressure profiles were provided 101-years ago be Georg Fuhrmann: Theoretische und experimentelle Untersuchungen an Ballonmodellen. Jahrbuch der Motorluftschiff Studienges, 1911/12.

[freebeard]

Granted it's a theoretical optimum, how does one intergrate that with the size and shape of people, non-liquid cargo, and with parallel parkling, loading docks, and etc.?

[Piotrsko]

Originally Posted by freebeard

Granted it's a theoretical optimum, how does one intergrate that with the size and shape of people, non-liquid cargo, and with parallel parkling, loading docks, and etc.?

Sonic or subsonic?

[aerohead]

Yes, there are OTHER 'low drag' shapes to consider:
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1) The Morelli shape:
The lowest Cd Professor Morelli was able to achieve for his specialized body, when modified for human passengers was 0.161, and for the body alone. When wheels were added, the drag rose to Cd 0.35. 'Integrating' the wheels into the body allowed the drag to fall back to Cd 0.201.
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2) 'Ellipsoid' bodies:
Yes, ellipsoid bodies have as low a drag, fineness-ratio for fineness ratio, as streamline bodies. What sucks about them though is that, you can't SEE out of them, forwards or rearwards, due to optical distortion and internal reflections created by the nearly 'flat' glazing. So they're knocked out of the competition.
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3) 'Laminar' bodies:
Yes, 'laminar' bodies have very low drag, and if you drive at 33,000, to 41,000-feet above the ground, then, they're your solution for low drag cars.
Should you drive on the 'ground' though, you notice that Earth has it's own turbulent boundary-layer, measured in kilometers, which your 'laminar' body would be completely submerged within, rendering it completely 'turbulent,' possessing zero benefit. They're also associated with the same kinds of outwards vision problems as ellipsoids.
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4) 'Wing' bodies:
If you've followed solar race cars since 1987, you've seen 'wing' cars. You may have also realized that these one, and two-passenger 'cars' are already as 'big' as some of the largest passenger cars ever manufactured. If these cars were 'enlarged' enough in headroom in order to allow the driver and passengers to enter and leave, and 'sit' in a 'conventional' manner, as with 'conventional' vehicles, they would exceed the length of all mass-produced passenger cars.
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5) Very-low-drag bodies capable of accommodating drivers and passengers in the manner to which they are presently accustomed to are very constrained in shape.

[aerohead]

Originally Posted by freebeard

Granted it's a theoretical optimum, how does one intergrate that with the size and shape of people, non-liquid cargo, and with parallel parkling, loading docks, and etc.?

There's nothing theoretical about it, if your using the term out of context, substituting it for 'hypotheses.'
And it's not hypothetical either.
It's just science fact.
The 'baby' template model was the dimensions of the T-100 pickup. It could be a pickup. A CUV. An SUV.
Family of four.
Height- 67.0"
Width- 75.73"
Length ( truncated )- 187.43" ( 1982 Chevrolet Camaro Z/28 )
Length ( longtail )- 265.81" ( Ford F-450 is 266" )

[freebeard]

Originally Posted by Piotrsko

Sonic or subsonic?

Most loading docks are subsonic.