Six - 'Optimum' templates

[aerohead]

From Daugherty and Franzini, drag minimums ( optimums ) occur when both friction drag and pressure drag are at their minimums for a given shape.
R. Barth conducted the dimensional analysis for 5-shapes, and the results are presented in Figure 8.46, page 324, Hucho's 2nd-Edition. ( I presented these materials years ago, but they're now lost to the photo-bucket black-hole.
Barth presented a Cartesian grid, with drag curves for five geometric bodies:
* square-rectangular-cylinder
* circular-cylinder
* 11%-radii, soft-edged square-rectangular cylinder
* round-nosed circular cylinder
* and ellipsoidal cylinder
From the curves, it's easy to find the bottom of the 'bucket' for each shape, where each achieves their drag minimum. Their 'optimum.'
--------------------------------------------------------------------------------------
In order of appearance in the above list, the shapes attain their drag 'optimum' at a Length/ Width ( or Height ) of:
* 2.473
* 2.210
* 4.978
* 5.042
* 6.263
------------------------------------------------------------------------------------
Number- 6 template appears in, ' AERO- HYDRODYNAMICS OF SAILING,' by Marchaj, page 244, for a symmetrical wing section. Again, a Cartesian grid is offered whereas the friction and pressure drag components are given, and the ratio of 'optimum' can be obtained directly from the chart's 'bucket', @ ( 3.92 :1 ) ratio of Chord / section thickness. I published this information also, years ago.
-------------------------------------------------------------------------------------
The take-away, is that because of dimensional analysis, anyone desiring to know the state-of-the-art for some specific body forms, has available to them, off-the-shelf technology, from which to streamline their project from the get-go.
--------------------------------------------------------------------------------------
There are also, 'templates' for piping and duct work in the professional literature, denoting 'optimums' derived from laboratory measurements of contained flows, which illustrate where friction and pressure drags reach their minimums also. These are germane to induction, exhaust, inter-coolers, air-to-air and water -to- air heat exchanger ducting and manifolds.

[Piotrsko]

I don't have the data you state. Pictures perhaps?

[JulianEdgar]

Originally Posted by Piotrsko

I don't have the data you state. Pictures perhaps?

Figure 8.46 from Hucho second edition:



It's my honest belief that such data is absolutely pointless for people modifying their cars. (In fact, worse than pointless because it misleads.)

However, if you were developing a vehicle from scratch, it would be one input.

[JulianEdgar]

Originally Posted by aerohead

-------------------------------------------------------------------------------------
The take-away, is that because of dimensional analysis, anyone desiring to know the state-of-the-art for some specific body forms, has available to them, off-the-shelf technology, from which to streamline their project from the get-go.
--------------------------------------------------------------------------------------

This is the normal Aerohead extrapolation from justifiable data to unjustifiable application.

The shapes Aerohead described are just that - shapes. They are not cars - or even vehicles of any type.

They are not in proximity to the ground; they of course do not have wheels or cooling openings or doors or surface roughness. Three of the shapes have perfectly circular cross-sections.

The lift of the shapes is not shown; nor is the drag when there is a yaw component to the airflow.

To me this is all a bit like showing a picture of a coil spring, stating the equation for coil spring stiffness - and then saying:

The take-away, is that because of this equation, anyone desiring to know the state-of-the-art for coil spring design, has available to them, off-the-shelf technology, from which to design the suspension of their project from the get-go.

[Piotrsko]

Imho, that was kinda like saying the NACA data on wing profiles is pointless because an airplane isn't attached.

[Vman455]

Originally Posted by Piotrsko

Imho, that was kinda like saying the NACA data on wing profiles is pointless because an airplane isn't attached.

I don't think that's analogous; wings are attached to bodies that carry people, cargo, etc. In cars, the bodies are the shapes, and also have to be optimized for carrying people, cargo, etc. and have wheels that roll and turn and move in ground proximity and look good to potential consumers and satisfy federal and/or EU requirements for lighting, bumper heights, and safety and.... By the time all those constraints are applied, you end up very far from theoretically-optimized basic shapes, whereas a wing on an airplane can be very close to the theoretically-optimized profile.

[aerohead]

Originally Posted by Piotrsko

I don't have the data you state. Pictures perhaps?

all 770 of them were extorted in photobucket.

[aerohead]

Originally Posted by JulianEdgar

This is the normal Aerohead extrapolation from justifiable data to unjustifiable application.

The shapes Aerohead described are just that - shapes. They are not cars - or even vehicles of any type.

They are not in proximity to the ground; they of course do not have wheels or cooling openings or doors or surface roughness. Three of the shapes have perfectly circular cross-sections.

The lift of the shapes is not shown; nor is the drag when there is a yaw component to the airflow.

To me this is all a bit like showing a picture of a coil spring, stating the equation for coil spring stiffness - and then saying:

The take-away, is that because of this equation, anyone desiring to know the state-of-the-art for coil spring design, has available to them, off-the-shelf technology, from which to design the suspension of their project from the get-go.

* the premise for their mention is, to introduce to the reader, the concept of 'sweet-spots.'
* if anyone is contemplating construction of an aerodynamic structure, embodying any of these forms in a combination, it's not a loss of value to know that drag minimums do exist.
* TLFcars.com could have known the significance of what towing certain trailers behind a Tesla X would mean to range before the fact.
* Airstream trailers might be considered.
* 'Conventinal' RV trailers.
* Any roof-mounted cargo, ladder, equipment, air-conditioning, Boeing 737 800 Wi-Fi antenna fairing / enclosure, submarine deck-mounted US NAVY SEAL mini-sub, etc..
* this is a portion of the prima facie evidence for the conceptualization of the 'template.'

[aerohead]

Originally Posted by Vman455

I don't think that's analogous; wings are attached to bodies that carry people, cargo, etc. In cars, the bodies are the shapes, and also have to be optimized for carrying people, cargo, etc. and have wheels that roll and turn and move in ground proximity and look good to potential consumers and satisfy federal and/or EU requirements for lighting, bumper heights, and safety and.... By the time all those constraints are applied, you end up very far from theoretically-optimized basic shapes, whereas a wing on an airplane can be very close to the theoretically-optimized profile.

Ignoring pedantics, all the constraints mentioned can be resolved by simply re-configuring the ink on the page.

[freebeard]

* Airstream trailers might be considered.

Eh...Airstream is about half-way there.



I had one of these but I gave it to my brother and he sold it.