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Old 02-04-2012, 04:19 PM   #1 (permalink)
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Beetles in the Rain

Lucky me, I was driving back from work in the rain the other day and was right behind a 2005ish Beetle and noticed the air patterns highlighted in the mist. My main observation was the 2 large trash can sized counter rotating vortex's spinning behind the rear fenders. I mean it was very noticeable. They were quite energetic and rotating fast with the insides of each vortex going downwards. To clarify, the shapes were like trash cans horizontal to the road with the open end of them pointing at the back of the car. There only appeared to be a foot in between the 2. Looking at other vehicles, I saw nothing of the sort, or very subdued versions of the horizontal tornadoes.

This has me thinking that a lot of the low Cd problem with the car is the energy required to generate these swirls of air. I think it would be interesting to make 2 models which were identical in the front, with the same frontal area, but design the backs so one creates these large vortices, and the other it's just "dirty air" and see the difference.

Something else was apparent, in order for these vortexes to be so close to the rear end of the car, the flow on the back centerline must be remaining attached quite far down the back enhancing the strength of vortices.

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Old 02-04-2012, 04:34 PM   #2 (permalink)
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Originally Posted by ChazInMT View Post
Lucky me, I was driving back from work in the rain the other day and was right behind a 2005ish Beetle and noticed the air patterns highlighted in the mist. My main observation was the 2 large trash can sized counter rotating vortex's spinning behind the rear fenders. I mean it was very noticeable. They were quite energetic and rotating fast with the insides of each vortex going downwards. To clarify, the shapes were like trash cans horizontal to the road with the open end of them pointing at the back of the car. There only appeared to be a foot in between the 2. Looking at other vehicles, I saw nothing of the sort, or very subdued versions of the horizontal tornadoes.

This has me thinking that a lot of the low Cd problem with the car is the energy required to generate these swirls of air. I think it would be interesting to make 2 models which were identical in the front, with the same frontal area, but design the backs so one creates these large vortices, and the other it's just "dirty air" and see the difference.

Something else was apparent, in order for these vortexes to be so close to the rear end of the car, the flow on the back centerline must be remaining attached quite far down the back enhancing the strength of vortices.
Yeah,this is exactly what Hucho and others warn us about.All that kinetic energy is forever lost to atmospheric heating.Literally!The vortices could be the poster child for the 2nd Law of Thermodynamics!
This is also why it's very hard to discern these sorts of things without a smoke-generator.Tufts just don't reveal all that's needed.
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Old 02-04-2012, 04:56 PM   #3 (permalink)
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I have noticed that just after a rain, you can see these vortces very easily if you position yourself low and with a light source behind the vehicles on dim lighting conditions.
I noticed that air will swirl behind the cab of trailerless semis and have so much upward force that it forms a mini tornado ( vortex ) going upwards.
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Old 02-04-2012, 04:58 PM   #4 (permalink)
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Yeah,this is exactly what Hucho and others warn us about.All that kinetic energy is forever lost to atmospheric heating.Literally!The vortices could be the poster child for the 2nd Law of Thermodynamics!
So are you suggesting next time I'm on the road I should put a frankfurter on a stick and hold it 3 feet behind a Beetle to cook it?
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Old 02-04-2012, 05:16 PM   #5 (permalink)
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That would definitely work with a classic Beetle!
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Old 02-04-2012, 11:03 PM   #6 (permalink)
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Perhaps that's why Ernie Rogers had such dramatic results with his flat spoiler.
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Old 02-07-2012, 05:24 PM   #7 (permalink)
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So are you suggesting next time I'm on the road I should put a frankfurter on a stick and hold it 3 feet behind a Beetle to cook it?
Yes,and you risk jail time if you forget the mustard,pickle relish, and diced onions!
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Old 02-07-2012, 06:11 PM   #8 (permalink)
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Yeah,this is exactly what Hucho and others warn us about.All that kinetic energy is forever lost to atmospheric heating.Literally!The vortices could be the poster child for the 2nd Law of Thermodynamics!
This is also why it's very hard to discern these sorts of things without a smoke-generator.Tufts just don't reveal all that's needed.
These vortices are also easy to see in your rear view mirror when you are driving in a light snow shower on a windless day. My 68 VW Beetle used to develop such large vortices that it used to sweep the road behind it clear of blowing snow. By contrast, the Aerocivic just displaces the snow upwards a bit as the car passes under it, then sets the snow back down to its original level and the snow keeps on falling behind the car as if the car had never even been there.
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Old 04-22-2012, 09:14 PM   #9 (permalink)
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So, does this mean that the air is staying attached too far down? If so, what do we do to eliminate, harness, reduce, or improve it, what ever is the best case. I've gotten a bit better mileage since i added my VGs at the top of the rear glass, and can certainly see their effects in the mist patterns down the glass, but the increased mpg could be learning better driving habits too. I've wondered if there were better placements for them, but I don't have a 3d model of the beetle or powerful enough cfd software to do much virtual testing on something that large.
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Old 04-23-2012, 04:54 AM   #10 (permalink)
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So, does this mean that the air is staying attached too far down?
Yep

It's probably helpful to look at the other design derived from the same original designer - Porsche
Most Porsches, particularly pre-2000 have a similar profile of a sharp bonnet to windscreen transition and forward glasshouse that allows 3D flow around and over it, yet the aerodynamically superior porsches consistently have either a more gradual glasshouse/tail incline (less verticle energy in the airflow) or a larger dead area at the rear.
The second seems easier to achieve, probably with a horizontal deflector

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