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:
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.
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.
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.
i can't watch the vid with audio for the moment (no sound @ work and no internet at my appartment yet) so i'm just looking at the images.
there's an interesting bit about the rear spoiler, where the first version is straight and the second seems to have something which reminds me of a gurney flap.
i've seen certain BMW's wich also have a small gurney flap like spoiler added. and quite a number of cars with a slight upward sweep to the rear spoiler lip.
given it's shown in this video i wonder what it's use is and how it works to aid FE. it would be a relative simple and good looking addition to my car, but i'd like to understand better what it's supposed to do.
So 10 counts would be 0.01 Cd. And they got a half mile range extension from that? Wow. Pretty good. I think I'll go ahead and remove the roof rack from my Vibe, even though I wanted to get my SG calibrated first. Too impatient.
So 10 counts would be 0.01 Cd. And they got a half mile range extension from that? Wow. Pretty good. I think I'll go ahead and remove the roof rack from my Vibe, even though I wanted to get my SG calibrated first. Too impatient.
That's my biggest issue right now. I didn't start a fuel log until I started modding, and now I want to do everything all at once. I have no clue how much each mod or driving change is actually affecting my MPG. I guess cumulative is good enough to know though.
I recently became friendly with an owner of multiple dealerships in MA, one of which is a Chevy dealership. We talked this week and I asked him if had heard anything new on the Volt. He'd seen this video and told me that an e-mail had gone around to sales departments weeks ago talking about "substantial" changes in the final design. He was worried it was going to look more like a Malibu since all the engine/drivetrain mules so far are Malibus. That would be a huge mistake. People want the cutting edge styling to match the cutting edge powerplant.
So 10 counts would be 0.01 Cd. And they got a half mile range extension from that? Wow. Pretty good. I think I'll go ahead and remove the roof rack from my Vibe, even though I wanted to get my SG calibrated first. Too impatient.
Be warned that you will have some nasty holes in the back of the Vibe that you cannot cover. I saw more benefit than problem by removing it, so have at it.
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Thx NoCO2; "The biggest FE mod you can make is to adjust the nut behind the wheel"
I am a precisional instrument of speed and aeromatics
If your knees bent in the opposite direction......what would a chair look like???
Trikkonceptz: I thought you were going to fabricate something to smooth over those gaps?
SuperTrooper: I think the substantial changes they're talking about are addressed in the video. The car is only going to have some of the features and "design language" of the concept car which was a disaster, aerodynamically.
