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Old 11-11-2007, 03:00 AM   #11
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The only way you'd gain from drafting that semi would be to sit 5' to 10' max off his bumper...something I don't recommend. Otherwise you are just being buffeted by the big swirls of air.
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Old 11-11-2007, 08:48 PM   #12
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The only way you'd gain from drafting that semi would be to sit 5' to 10' max off his bumper...something I don't recommend.
Mythbusters had some cool testing done at NASA Ames Research in a scale tunnel.... At 7 car lengths (~100 feet from the truck) - they found a drag reduction of 11%. At 10 feet, 39% drag reduction. I never got as close as 10 feet, but at the distances shown in my pictures - I was getting a fuel economy increase (37.8 versus ~41.6 - a/c off, resetting the SG on long flats using CC).

My overall FE for the entire trip (200 mi) was 39.0 - dropping in the end from 39.3 when I was forced by my passenger to turn the a/c on

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Otherwise you are just being buffeted by the big swirls of air.
Even very close, you're being hit by tangential air flow (but lower energy, slower). That will not be drag against forward motion - that is, instead of the energy being in the parallel direction to motion, it's tangential - and the only fluid velocity that matters (in our case) is flow parallel to the direction of motion I should build an anemometer to get some data
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Old 11-11-2007, 09:06 PM   #13
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What drafting I've done was not very close and ScanGauge does show an increase in MPG's. There is turbulence from the sides but the straight on air resistance is reduced. I've even drafted a tiny little Nissan truck with a camper shell at least a car length back and got just as good an increase.
An all or nothing attitude will normally yield the later.

I bet cats would have a ball playing on that car with the yarn.
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Old 11-12-2007, 06:41 PM   #14
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So, I have an interesting update

Unfortunately, I didn't get pictures - due to low light conditions on the ride home. BUT - the difference between cracking the window open and having it shut is huge!

With the window shut, the rear tufts flicker back and forth as I would expect. BUT, crack the window open (Even just an inch) and those tufts go crazy - fluttering back and forth much faster and occasionally separating from the glass (but only very briefly).

Just food for though I'm sure cracking is better than full open, but I didn't realize the effect was that great.
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Old 11-12-2007, 10:32 PM   #15
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trebuchet03,

When you're taking pictures ... are you taking them yourself, or do you have someone else in the car behind the lens?
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Old 11-13-2007, 07:52 AM   #16
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trebuchet03,

When you're taking pictures ... are you taking them yourself, or do you have someone else in the car behind the lens?
A passenger took almost all of them... I took a couple - but really, that's dangerous All the ones behind the semi were done by a passenger
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Old 11-13-2007, 04:24 PM   #17
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I was more worried that there was the chance it was you taking the ones of the back window
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Old 11-27-2007, 09:48 AM   #18
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When looking at the car alone versus drafting the semi, doesn't the change in behavior lend itself to the change in drag coefficient wrt speed I've read about?
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I think if i could get that type of FE i would have no problem driving a dildo shaped car.
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Old 11-27-2007, 10:16 AM   #19
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When looking at the car alone versus drafting the semi, doesn't the change in behavior lend itself to the change in drag coefficient wrt speed I've read about?
I'm cautious in saying yes... Tufts, smoke, oil etc. are used to visualize flow interaction and really shouldn't be used as a metric to compare cD values.

There is a relation between cD and Reynolds number (which is a function of velocity, length et. al.)

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Old 11-27-2007, 10:40 AM   #20
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Can't we get away w/ it for high drag/low pressure areas like behind a car? I mean, lets say I'm Ernie and I do tuft tests on my Beetle in order to see if there's a difference via the spoiler. Wouldn't less eddies as indicated by the tuft imply there's a decent chance of a drop in Cd? Or for that matter, if the tufts eddy at one speed but not another, wouldn't this imply that the flow hasn't detached at the point and some speed but may detach higher up at some higher speed, leading to greater drag?

I feel it should be like load wrt efficiency in an engine, but I suppose I'm just grossly oversimplifying it.
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