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Old 11-13-2008, 04:26 PM   #11 (permalink)
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Some Eye Candy - I'm personally a very visual learner, and I'm sure someone would be interested in seeing the fun stuff.



Spent an hour to optimize and fix the model - looks like each run will take about 2 hours now But, I failed at getting a rolling road to work :/ I need to look into that.

Next model is with an open back - without an air splitter which is what I'm going to call duct holes to split air between top and bottom).

Model after that will be "stock" followed by open with a splitter, then trailing edge features (zig zag, rounds, etc.), then suction features....

Here's some video


How do I embed video? - nevermind, apparently it does it automatically

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Old 11-13-2008, 05:07 PM   #12 (permalink)
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So the models aren't directly comparable - but from this point on, things should be directly comparable... Remember, this is more qualitative than quantitative....

55mph, open transom, no splitter





those things on the bottom are there just to kinda roughen up the under body so it's not completely smooth... Notice, however, the nice line from the front of the rear wheel back to the bumper...
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Old 11-13-2008, 07:58 PM   #13 (permalink)
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I 'm glad to see you have finally had some time to do some CFD testing.

I'm actually curious if just extending the decklid and curving it downward would decrease your wake size. It would be easier to do and wouldn't be as big a headache. ( And it could be removable if you just bought another trunk and stored the original .

If you remember, your cars profile is almost identical to the Ford Fusion that ran at Bonneville. ( Which had a Cd of .20 while remaining rather stock looking )

Cool pictures ! I'm jealous.
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Old 11-13-2008, 09:43 PM   #14 (permalink)
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Hi,

It seems like the distance from the nose to the windshield is greater than ideal -- they each become separate drag points. Can you try moving the windshield forward and see if it alleviates the high pressure zone at the top somewhat?
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Old 11-13-2008, 10:11 PM   #15 (permalink)
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Quote:
Originally Posted by trebuchet03 View Post
It's Floworks - a part of the Solidworks suite (yay College ) - it's actually doing a 3D analysis, but I'm restricting it to a thin area.
Very cool. Any way to get this program legally? Without being a graphic arts/CAD major?

Quote:
Originally Posted by trebuchet03 View Post
While it may seem I'm putting a lot of time, really, I'm setting up a model and hitting run before I go to bed. The model took me about 15 minutes to build off a photograph and then another 5 minutes to setup the CFD suite.
Good to know someone is getting sleep around here...


Quote:
Originally Posted by trebuchet03 View Post
Theory vs. physical is a valid point... You can have a sound theory, but physical application doesn't work... But, to waste cycles on building something that doesn't work on the theoretical level is not something I'm interested in. Concept -> theory -> optimize -> compromise -> refine -> implement is more or less the linear flow I'm following. Besides, what's good enough to the auto mfr's, Lockheed, Siemens, Boeing, Airbus, Nasa, etc. etc. etc. is good enough for me - Analyze first, then build They didn't build the 787 before modeling and analyzing it
I prefer the OODA loop myself. Observe, Orient, Decide, Act. It is what fighter pilots use in their combat training. The great thing about it is can apply to almost anything. Wiki it...

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Old 11-13-2008, 10:13 PM   #16 (permalink)
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Quote:
Originally Posted by Cd View Post
I 'm glad to see you have finally had some time to do some CFD testing.

I'm actually curious if just extending the decklid and curving it downward would decrease your wake size. It would be easier to do and wouldn't be as big a headache. ( And it could be removable if you just bought another trunk and stored the original .
I totally forgot about that Awhile back I prototyped that very idea using 1" formular foam (pink insulation). Alas, I never got a chance to test - but flutter was a problem in the prototype....
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Old 11-13-2008, 11:47 PM   #17 (permalink)
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Initial Results

Initial Results say.....

VW did a decent job to start with

Here's our baseline


Ignore the fact that there's no ground effect, for a moment... Just use this as a sort of barometer for gross analysis.

Now, with our open back Kamm


Results, thus far.... With the Kamm you get the profile look of a hatchback with all the associated aero downsides. It's my opinion that adding a kammback to a hatchback is effective because you're improving the large cut off zone that is the hatch itself. By adding this to a sedan, starting at the roof - you increase this aforementioned cut off zone from the bumber/trunk to the bumper, trunk and kamm cross section.

Take a look at this, separation occurs at nearly the end- if you can't tell, trust me - I can view the boundary layer in my software, but it doesn't look pretty in a picture.

But look at the overall wake size!

Now, our baseline


Separation doesn't occur until a tad less than halfway down the glass... Which I can vouch as near accurate based on my tuft experiment. Notice how much smaller the wake zone is between the two... All images in this post are directly comparable.

Max Velocities (I can't pinpoint exactly where this is happening just yet - these values could be completely erroneous).
openKamm: 117mph
Baseline: 91mph

------
As always, YMMV. Back to the drawing board - while a bit of a disappointment, I'm not scared away. Next in the pipe, closedKamm and deck lid extension.
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Old 11-14-2008, 12:14 AM   #18 (permalink)
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Interesting!

I know you haven't forgotten you're viewing in 2D, and focusing on the roof/decklid flow.

While the wake cross section may be bigger at the end of the Kamm'ed car, would the Kammback shape reduce the likelihood/magnitude of the pair of drag inducing trailing vortices - more probable on a sedan/notchback?

Also, that entrained separation bubble at the base of the sedan's rear window takes fuel to feed (believe I read Phil making a comment to that effect, anyway).

There's a reason the Prius, Volt, Insight 1&2, A2, and so many others are hatchback/Kammbacks!
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Old 11-14-2008, 01:23 AM   #19 (permalink)
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Quote:
Originally Posted by MetroMPG View Post
Interesting!

I know you haven't forgotten you're viewing in 2D, and focusing on the roof/decklid flow.

While the wake cross section may be bigger at the end of the Kamm'ed car, would the Kammback shape reduce the likelihood/magnitude of the pair of drag inducing trailing vortices - more probable on a sedan/notchback?

Also, that entrained separation bubble at the base of the sedan's rear window takes fuel to feed (believe I read Phil making a comment to that effect, anyway).

There's a reason the Prius, Volt, Insight 1&2, A2, and so many others are hatchback/Kammbacks!

So yes, I've diverted some of my computing power to a better model rather than CFD.... Because I need more than a cut plot to make more valid conclusions.... I am now 95% near a suitable 3D model - it just needs some clean up in a few areas... Those square chunks aren't really there, it's my video card not up to the task of rendering such a large and complex model.






On the subject of the Prius et. al....

The Prius is a rather large car - at least, about the same as my Jetta... The termination of my Kamm (top trailing edge) is much higher than the Prius' termination. I think that will have a significant effect on overall aero performance.

That gives me a good idea though... I need to find either a model of the Prius, or a good side photo in which I can parametrize the shape.... Then we'll see if I can move that over to my model - I'm holding a trace of my car's profile against the Prius - the rear end is very close (which is exciting for me).
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Old 11-14-2008, 04:18 AM   #20 (permalink)
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I'd try to copy the slope of the rear window on the Prius, more or less. The steeper and longer you can make it, the smaller the wake from the kamm back. Provided it is not so steep that you get separation, which will depend on how much turbulence is generated farther forward. It might be worth leaving the new rear window adjustable for a few runs to find the sweet spot. A kamm back is not a substitute for a long tapered tail, it is just a better compromise than a short taper or rounded tail.

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