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Old 12-11-2011, 02:10 PM   #31 (permalink)
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Quote:
Originally Posted by Sven7 View Post
I will posit that the sharp edges on the NB only generate very large vortexes in the wake.
I've never assumed that the new Bug created vortex's at the rear corners. I would like to see some images of that. Below is some similar information which may or may not apply.

http://ecomodder.com/forum/showthrea...ook-10304.html


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Old 12-11-2011, 03:44 PM   #32 (permalink)
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Ba is what I'm talking about

Here, I drew a picture. this is what Phil calls "attached vorticity"



Quote:
Originally Posted by aerohead
Attached-vorticity is a different animal.

The front of a vehicle defines how the atmosphere will be distributed as it passes the vehicles body.

In order to comply with conservation of mass,the windshield and roof cause the air to move at a greater velocity than down the sides of a vehicle,or underneath,due to the greater distance traveled by the air in the same time.
To comply with Daniel Bournouli's (sp?) Theorem,this accelerated flow must exist at a lower pressure than elsewhere around the vehicles body.

If the body designer is competent,the roof curvature,tumblehome, and upper radius of the greenhouse/C-pillar region will be such that when the flow off the roof rejoins the slower flow off the sides,these flows are at comparable velocity and pressure,the they simply continue on in a laminar flow fashion.
If the flows should meet beyond a critical velocity or pressure differential ( delta-V/delta-P) the slower,higher pressure flow will seek out the faster,lower pressure flow,and the two streams will spin up into a vortex as they attempt to blend together.

The vacuum created at the center of the vortex creates tremendous drag,and the kinetic energy required to feed the vortex robs precious kinetic energy from the flow,which,again,can never be converted to useful static pressure,hence the high drag and lower mpg.

If you've ever had a window seat on an airliner you may have seen wingtip vortices form during landing,caused when the higher pressure air below the wing is bleeding over the wingtip into the lower pressure air above,spinning it into a vortex.The pressure drop is extreme enough to create the refrigeration effect which brings the temperature of the water vapor in the airstream below dew-point,creating the clearly visible water fog condensing at the center.
So in effect, the air traveling along the top of the Beetle's body side detaches sooner and has to go somewhere. It pushes inward because of the atmospheric pressure around the moving vehicle. The air coming around near the beltline detaches about a foot later and starts the same pattern. The offset rhythm of these two areas creates a rotating mass of air on each side of the New Beetle. The car's specific shape is geometric enough that the top can be treated almost as a half cylinder. There probably aren't counter rotating vortexes coming off the bottom side to even them out. Herein lies the problem. Large, unbalanced vortexes.

Or, that's what I figure
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Last edited by Sven7; 12-11-2011 at 04:24 PM..
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Old 12-12-2011, 10:21 AM   #33 (permalink)
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Are winglets the answer to the vortex problem?

Industrial Design - Transportation pictures by kach22i - Photobucket



Aviation Partners Inc. - Seattle WA - Blended Winglet Technology and Performance Enhancement Systems for Winglets


Boeing 737 Advanced Blended Winglets

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Old 12-12-2011, 11:00 PM   #34 (permalink)
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Kach22i - are you recommending I consider some winglets instead of a kammback or other design?

Very intriguing -- you guys are the pros - I would be happy to try some different designs if you guys can help me sort out the best choices --- and I could try to do some tuft testing too.
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Old 12-12-2011, 11:11 PM   #35 (permalink)
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Proposed Kammback

Considering the kammback approach - will keep the angles consistent with the areo template. Will taper towards center somewhat as well to follow natural lines on car. My neighbor has a Honda Fit so I'll take some cues from the rear detail on it - it seems abrupt corners are OK. Will also await your reply on the fins you are proposing two posts up... very interesting stuff guys -- many thanks!

PS Averaged just over 45 MPG on last tank - so hoping with reading the site and a slew of small aeromods, fine-tuning my driving, etc., hoping to get in the 55+ MPG category...
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Old 12-12-2011, 11:12 PM   #36 (permalink)
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If I post some pretty detailed mock ups would someone be able to analyze the flow on them with that cool software? Can you also do that from a rear view or from a top down view? Just curious... many thanks!!!
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Old 12-13-2011, 01:07 AM   #37 (permalink)
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I think the best thing you could do would be a kammback. That way it will make the aerodynamically "ugly" shape into a usable wagon form. If you look at "A" in the diagram it has no vortexes coming off the back.

Winglets would be interesting and if you feel like experimenting by all means do it. I'd like to see what happens. But I've never heard of a ground vehicle using them.
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Old 12-13-2011, 01:25 AM   #38 (permalink)
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Shoulda just got a Golf huh.
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Old 12-13-2011, 08:11 AM   #39 (permalink)
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Quote:
Originally Posted by Frank Lee View Post
Shoulda just got a Golf huh.
Mr. Positive strikes again.
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1977 Porsche 911s Targa
1998 Chevy S-10 Pick-Up truck
1989 Scat II HP Hovercraft

Chin Spoiler:
http://forums.pelicanparts.com/off-t...effective.html

Rear Spoiler Pick Up Truck
http://forums.pelicanparts.com/off-t...xperiment.html

Roof Wing
http://ecomodder.com/forum/showthrea...1-a-19525.html
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Old 12-13-2011, 08:12 AM   #40 (permalink)
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Ha ha! Thanks for the laugh (and stomach punch) Frank! Actually I did plan to buy a Golf TDI. Drove 7 hours to buy a "pristine" Golf, only to get there and find out it was covered in hail damage, etc., etc. Super buzz kill. Needless to say, with that cash burning a hole in my pocket, once I found any reliable TDI for sale in Kansas City I bought it. Wasn't even looking at Beetles at all --- was just focused on finding a TDI again...

Sven7 - many thanks for the ideas here - can't wait to get to building!

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