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Old 08-19-2008, 12:40 PM   #1 (permalink)
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Video: discussion of several GM Volt aero features

Most interesting to me, the designer addresses a nagging unresolved question I've had for a long time: what's better, a sharp corner on the side-to-rear transition, or a curve?

Also talks about grille blocking, mirror design, front radii, etc.



And a photo of the rear quarter corner with the crease to promote flow separation there:



Source: ABG Volt gallery

Here's what the designer had to say about that design feature, from the vid:

Quote:
I want to point out the little rear "vertical fin element", I call it, on the production Volt.

Now, air wants to stay attached to the car, but it has to leave the car at some point. And when it does, it needs to leave very quickly. So we want to put an aerodynamic trip device back there so when it leaves the car it separates cleanly and quickly, and doesn't send turbulence into the vehicle's wake.

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Old 08-19-2008, 01:19 PM   #2 (permalink)
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Very cool video. Except where they laughed at the Corbin Sparrow.
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Old 08-19-2008, 01:23 PM   #3 (permalink)
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Great find, Lots of good info
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Old 08-19-2008, 01:24 PM   #4 (permalink)
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I'll have to get to a computer with audio.Look forward to it! Thanks!!!!!!!!!!
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Old 08-19-2008, 01:29 PM   #5 (permalink)
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Quote:
Originally Posted by MetroMPG View Post
what's better, a sharp corner on the side-to-rear transition, or a curve?
Arghh...I'm at work and cannot view videos. So what's the answer? I say sharp cutoff at the point of separation. Did I win?
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Old 08-19-2008, 01:42 PM   #6 (permalink)
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Quote:
Originally Posted by tasdrouille View Post
...So what's the answer? I say sharp cutoff at the point of separation. Did I win?
Yup.
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Old 08-19-2008, 05:25 PM   #7 (permalink)
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curve or straight chop

Hey Darin,Al let me use his wife's computer so I could hear the audio.And here's my shot at your question.-------------- The air wants to follow that ideal pathway we talk about with the teardrop.Without a flow support surface(the back of the car) the air is going to break away.If you round off the trailing edges,the flow will follow to the point,which is 15-degrees of rotation behind the initial origin of the curve,given any radius of curvature ( just like the back side of a golfball ).With this curve,air can enter the wake with more vorticity,which requires more time(and money) to dissipate.------------------------ If you chop the tail off sharply,the air will flow longitudinally,and it has been observed that this increases a pumping action within the wake,creating and strengthening the locked -vortices within the wake,which are associated with the"phantom-tail" as reported by Morrelli and others,which cuts skin friction,while providing form drag reduction approaching that of the full boat tail.In yachting they call it an "embryonic transom." In artillery its called the Aberdeen Projectile.Chrysler aerodynamacists say the flow will "burst" with the hard cut,and that it is preferable,as Kamm and Hoerner and others have commented.There may be advantages unrelated to aero,as progressive crumpling during explosive deformation in a wreck.Also from the video is a clue with respect to the rear spoiler.The little 5mm upkick may direct an upflow,which,when it collides with the flow coming off the roof,blasts any vorticity into a more homogeneous turbulent wake.Tarzan say turbulence"Good!",Frankenstein say vorticity"Bad!".Morrelli said that during the development of the CNR "banana"car,in the windtunnel,they literally sliced pieces off the back of the car like a loaf of bread,and as Kamm had predicted,flow remained attached right up to the chop,and the wake behaved much like a solid surface,with the air skimming along it.Hope I didn't talk this thing to death.
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Old 08-19-2008, 07:02 PM   #8 (permalink)
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Phil, the extend of your knowledge never ceases to amaze me. I need to read more books...
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Old 08-19-2008, 09:11 PM   #9 (permalink)
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BWAHAHA the Corbin joke was the best part.
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Old 08-20-2008, 02:38 PM   #10 (permalink)
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books

Quote:
Originally Posted by tasdrouille View Post
Phil, the extend of your knowledge never ceases to amaze me. I need to read more books...
I was hoping the seminars and installments would save everyone the grief! The books have really helped and I know I'm way behind the cutting edge,however,as Alex Tremulis commented,we're so lucky to ride behind all the aerodynamic pioneers who did all the "pick and shovel work." So much of the fundamental stuff is in the bag,so we're left to play with the details.

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