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Old 09-18-2020, 04:35 PM   #3561 (permalink)
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weak

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Originally Posted by freebeard View Post
This seems weak. What is the mechanism by which the imaginary shape acts [through an impenetrable surface]/[in meat space]?
'[T]he optimum shape in terms of drag is a half-body, which forms a complete body of revolution together with its mirror image- produced through reflection from the roadway.' Wolf Heinrich Hucho, page 15, Aerodynamics of Road Vehicles, 2nd-Edition,Butterworths, 1986.
This has to do with 'effective fineness ratio', a phenomena of aerodynamics, in which a vehicle is twice as 'thick' as what you observe.
In order to achieve the drag minimum of a Length / Diameter of 2.5: 1 streamline body, the car must be 5:1. The template.
A look at early wind tunnel testing reveals models humping, wheels -to-wheels in the air stream to accommodate this phenomena.
I don't have the book with me so I can't go any deeper.

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Old 09-18-2020, 06:21 PM   #3562 (permalink)
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I suspect the mirroring is a simplification to remove the underbody as a consideration.

There's a lot going on in the center of that full body.

Agreed the spats and breakover angles are in order. I'm still a fan of the Magic B*tt Trumpet.
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Old 09-18-2020, 06:35 PM   #3563 (permalink)
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Quote:
Originally Posted by freebeard View Post
I suspect the mirroring is a simplification to remove the underbody as a consideration.

There's a lot going on in the center of that full body.

Agreed the spats and breakover angles are in order. I'm still a fan of the Magic B*tt Trumpet.
The relative motion between underbody and ground is just one of the myriad complications in fluid motion. The template is only the very beginning of wisdom, not the whole science.
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Old 09-23-2020, 11:42 AM   #3564 (permalink)
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mirror-image ground simulation

Quote:
Originally Posted by freebeard View Post
I suspect the mirroring is a simplification to remove the underbody as a consideration.

There's a lot going on in the center of that full body.

Agreed the spats and breakover angles are in order. I'm still a fan of the Magic B*tt Trumpet.
I reread every word in Hucho's book after I got home Friday. I flagged every citation involving the 'mirror-image' ground simulation technique. As well as dozens of other streamlining criteria which failed to make it into Hucho's 'Subject index.'
I packed Julian's book plus Hucho's into my backpack, and managed to get to town this morning without it. I'll try for Friday.
--------------------------------------------------------------------------------------
The earliest I can find a source for the mirror-image technique is pre-1922, from Paul Jaray, of the Zeppelin Werke. Paul was an airship designer for them and is responsible for introducing the longitudinally-asymmetrical, 'teardrop, body of revolution,' airship hull, originally invented by Frenchman, Pierre Julienne, in the 18th-Century.
Jaray's work is presented in 'AERODYNAMIK DES KRAFTFAHRZEUG', self-published by Baron Reinhard von Koenig-Fachsenfeld, 1951, who is actually 'Kamm'.
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Old 09-23-2020, 12:23 PM   #3565 (permalink)
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Quote:
Originally Posted by aerohead View Post
That use of the template would violate the ground reflection, mirror image, and fineness ratio, blowing the Cd all to hell.
There's only one way to use the template.
In another recent thread you mentioned no need for rear diffuser if the underbelly behind the rear wheels were at 2.8 degrees or less.

I assume this rule of thumb is for a range of heights normal to road cars, let's say between 4-8 inches?

The template posted through improperly used by the article's author has a rear departure angle of approx. 2.8 degrees, right?

Right answer using wrong method, or does that Miata need a rear diffuser for an un-yet stated reason?
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Old 09-23-2020, 03:34 PM   #3566 (permalink)
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diffuser

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Originally Posted by kach22i View Post
In another recent thread you mentioned no need for rear diffuser if the underbelly behind the rear wheels were at 2.8 degrees or less.

I assume this rule of thumb is for a range of heights normal to road cars, let's say between 4-8 inches?

The template posted through improperly used by the article's author has a rear departure angle of approx. 2.8 degrees, right?

Right answer using wrong method, or does that Miata need a rear diffuser for an un-yet stated reason?
1) the 2.8-degree upsweep IS the diffuser. It begins at the beginning of the rear wheelhouse. It has the lowest drag observable for a diffuser.
2) as to ground clearance, the body must be able to clear 10-degrees ( approach) at the nose, 10-degrees ( breakover ) between the axles, and 16-degrees ( departure ) at the rear. The 'template' respects the three criteria.
3) the Miata has it's own Sports Car Club of America ( SCCA ) race car category. Many owners take them to the 'TRACK' on weekends. Elapsed time on the course is the primary interest. Winners will have balanced downforce and top speed for the winning edge.
Technically, a diffuser will not function if the onset flow coming to it is perturbed. You'll see mid-engined supercars ducting the exhaust through the body to the wake area just so the can create a really clean belly and diffuser.
I'm not sure what the SCCA rulebook allows in competition.
The Miatas I've seen are all roadsters. No roof! Seems like they'd be going after splitters and rear wings as far as the law allowed.
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Old 09-23-2020, 05:45 PM   #3567 (permalink)
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Quote:
Originally Posted by aerohead View Post
1) the 2.8-degree upsweep IS the diffuser. It begins at the beginning of the rear wheelhouse. It has the lowest drag observable for a diffuser.
Incorrect. The literature is actually all over the place for 'best angle for lowest drag' for a diffuser.

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Technically, a diffuser will not function if the onset flow coming to it is perturbed.
Correct.
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Old 09-24-2020, 05:09 PM   #3568 (permalink)
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best angle

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Originally Posted by JulianEdgar View Post
Incorrect. The literature is actually all over the place for 'best angle for lowest drag' for a diffuser.



Correct.
Of course, the angle is dependent upon the 'length' of the diffuser, as with a prismatic fastback rear slant angle.
From Carr's research, published in Hucho, and an SAE Paper from FIAT, both reported 2.8-degrees as the angle which delivered the drag minimum for the 'long' diffuser. The information is exactly accurate within the context of the source material. That was good enough for me. And a gentleman would allow for the context.
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Last edited by aerohead; 09-24-2020 at 05:12 PM.. Reason: add data
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Old 09-24-2020, 07:10 PM   #3569 (permalink)
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That was good enough for me.
Yes, that's the issue.
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Old 09-24-2020, 07:20 PM   #3570 (permalink)
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So, has anyone compared flat fences to those coupound curve skegs the supercars use?
https://www.thegentlemanracer.com/20...-revealed.html

A limitation of fabrication technique?

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