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Old 03-17-2008, 10:13 PM   #11 (permalink)
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I doubt the submerged NACA thing is really it... But on the subject of s. NACA's - if they're shedding, they're improperly used/designed... The flow exiting the duct should be sufficient enough to prevent that from happening.... At least, that's what the research we've been doing has said... That, plus shedding makes noise :/

As for the roof line ridges.... One really can't ignore the structural advantages.... The geometry is stronger - so a thinner/lighter material can be used... That on top of aero benefits. Furthermore, flow over the center will be faster than flow over the sides due to interference/spillage over the side of the car.


As for the vette.... I'm sure this isn't hurting

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Old 03-17-2008, 11:06 PM   #12 (permalink)
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vett's do catch a bad rap on mileage. mine has a 2.59 diff. X 0.7 O.D.= 1.81 final but @ 30MPG it's been sent to the wood shed for 6 months now by the VW.

on the longitudinal roof indents, I see them as an air fence.
in a side wind anti-lift.
at the exit, a velocity equalizer.
i was curious about the edges on the side of the hood too.
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Old 03-17-2008, 11:15 PM   #13 (permalink)
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It would also create a smaller wake.
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Old 03-18-2008, 12:32 AM   #14 (permalink)
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Quote:
Originally Posted by trebuchet03 View Post
I doubt the submerged NACA thing is really it... But on the subject of s. NACA's - if they're shedding, they're improperly used/designed... The flow exiting the duct should be sufficient enough to prevent that from happening.... At least, that's what the research we've been doing has said... That, plus shedding makes noise :/
I do agree the NACA roof channel argument is almost certainly wrong, it was just the first thing that came to mind.

As far as the vortex shedding, I first heard about it from a SportAviation (EAA magazine) article. The article reads:

Quote:
Originally Posted by SportAviation
A key and distinguishing feature of the NACA inlet is the curved profile's sharp edges. A common misconception is that the shape functions as a diffuser, but it does not. The edges shed vortices, which entrain air from the free stream flow and deposit it in the inlet.
A figure is given showing vortices that looks like it is from a NACA report (the other figures are clearly ripped from NACA reports). I haven't been able to find the particular report, though. I have no idea whether the author is an aeronautical engineer or just an interested third party like me.

I suppose this could easily be solved with a little tuft testing...

Quote:
Originally Posted by trebuchet03
As for the roof line ridges.... One really can't ignore the structural advantages.... The geometry is stronger - so a thinner/lighter material can be used... That on top of aero benefits. Furthermore, flow over the center will be faster than flow over the sides due to interference/spillage over the side of the car.
Interesting take on the interference idea.

It seems like designers killed a swarm of birds with one stone: decreased frontal area, increased structural stiffness, improved(?) style, + extra advantages (decreased interference drag).

Now the question is how much drag reduction does that feature net? 0.5%, 2%? I wonder...

- LostCause
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Old 03-18-2008, 03:00 AM   #15 (permalink)
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Do I see correct but to me the roof channel seems to be wider at the front and narrower at the rear? If that's the case then it's not doing much in minimizing the frontal area but works as a vortex generator. The sharp corners of the channel also hints that way and the angle also seems to be quite close to 15 deg which is by many sources regarded as the optimal angle for a vortex generator. My not so educated guess is that the main function or this roof channel design is vortex generator while it also offers structural support.
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Old 03-18-2008, 01:40 PM   #16 (permalink)
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Its tallest above the head of the driver and front passenger and gets lower the further you get away from their heads. Quick and dirty way to stiffen the roof panel and reduce frontal area. Its not all aero. On the down side, you can fit a sunroof on this kind of shape.
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Old 03-18-2008, 03:39 PM   #17 (permalink)
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The roof sheet metal is stiffened by the compound curvature of the longitudinal wrinkles. So, lighter gauge sheet metal can be stamped, saving weight while retaining adequate stiffness.

There may be some marginal aero benefit if the ridges make or channel useful vortices.
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Old 03-19-2008, 11:26 PM   #18 (permalink)
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It may well be a simple way to add extra air to the centre of the rear vacuum caused by the truncated tail panel to assist smoothing the airflow in that region.

I suggest the benefit is a combination of adding air from under the car , tapering the sides (in a plan view) to do the same and finally allowing sone extra air from the roof section to make a contribution as well.

It may have a benefit well in excess of the obvious (reduction in frontal area and drag reduction) by helping the "wake" behind the vehicle to be minimised.

As technoshark mentioned...it is the details which matter.

Welcome from me as well by the way.

Cheers , Pete.

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