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Old 08-11-2010, 07:49 PM   #133 (permalink)
aerohead
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lift/drag

Quote:
Originally Posted by ChazInMT View Post
Morgan, You DO NOT want to keep airflow attached. VGs are used to try and keep flow attached when a rear shape tries to change too quickly for it to remain attached but not quickly enough for it to be a clean break. VG's would be useless here, the slope of the Bugs back is such that flow is remaining attached way too long already.

Understand that when I say you are generating too much lift, it is not all in the straight up direction, a very good portion of the lift is in the backwards direction, which is a huge drag load on your car. This is why you want it to separate at the top of the windshield. If you fear that you are creating a huge area of dead air that is starting at the top of your windshield, you should not worry about it, because this Dead Air drag will be there no matter what you do to your car....I hope that makes sense.

Below is a basic illustration of what Im trying to say. By keeping the air from moving down the rear window, you eliminate the large lift, (Black Arrow) and its associated Drag Vector (Red Arrow).



The "Dead Air" drag is about the same as "Aerodynamically Shaped Perfect" drag. As soon as the air on your bug starts going down past the top of the rear window, it is costing you. So VGs would do no good since the flow is already attached.

Go with a roof spoiler as I suggested earlier, it will yield the best results.

The dead air that forms will be less drag than the lift.
I apologize if I'm getting in out of sequence.
If the air over the backlight were attached,it would be gaining pressure as it moved rearward,actually reducing lift.
With un-attached flow,the wake pressure is at the pressure of the point of flow separation, and since it is acting 'above' the car,it is generating lift.
If you put the back of the Audi TT,with the spoiler onto the Beetle,separation is eliminated and along with it ,lift.
Sportauto-online has a windtunnel shot of the TT and you can compare it to the older Beetle windtunnel shot which is much like the New Beetle would be.
automobilsport.com - your platform for motorsports, photos and lifestyle also has many useful windtunnel photos.
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