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Old 07-16-2008, 03:37 PM   #1
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Lightbulb Feel the draft....FEEL the DRAFT!

I got on the highway as usual today, and there was an unusually large traffic quantity. Soon I entered the left lane in the large space between a large tractor towing a short shipping container and a sports car with 4 excessively bright headlights inexplicably turned on, actually managing to provide awful glare on a bright sunny day.

I found that the truck was steadily going about the speed I'd go, and was staying in the left lane instead of moving right as most truckers would do, so I decided I'd try to draft. I figured I'd need to draft closer than usual for such a short truck, and drafted at 1.5-2 seconds.

If you're familiar with containers on trucks, their bodies sit higher than a normal box trailer. The trailer is an independent frame that's pretty high to begin with, and the box sits on top of the frame. This particular one had an abnormally high frame. Looking at it I figured there wouldn't be a good draft since there was almost no truck down at my height, the bulk of it being higher than my hood.

I drafted him for what seemed like forever, until he seemed like he was getting out of my way, thinking I was tailgating pissed off. I felt bad for him, some jerk pulled out of the breakdown lane in front of him, nowhere near highway speed.

Anyway...here's what I noticed about the wind. I drive with my windows open, and usually at 70mph I have steady, comfortable wind. Behind this truck, I was getting rhythmically whacked in the head with bursts of air in a 1 second cycle. Was I in bad turbulence, or was I in a good draft?
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Old 07-16-2008, 03:58 PM   #2
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I think there's more turbulence on those container trucks due to the ribbed sides on the containers.
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Old 07-16-2008, 04:00 PM   #3
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I know when I would draft in my VX, which i had fairly low to the ground, behind a normal tractor trailer i could feel burst and the car would feel like it was being pushed side to side...I always assumed this was what a true draft felt like since the air seemed like it came around the sides of the truck, not straight on as if no one was in front of me...wouldn't mind to hear some input from someone who has knowledge on this...
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Old 07-16-2008, 04:06 PM   #4
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Vortex shedding off a bluff body...

http://www.maths.manchester.ac.uk/~m...ons/vtran2.mpg
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Old 07-16-2008, 04:22 PM   #5
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Ah, that certainly explains what I felt. Was I too far back for a decent draft?
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Old 07-16-2008, 04:34 PM   #6
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It would be interesting to carry an accurate barometer with you to see where you got the lowest pressure.

Next time try different distances to see what effects you notice.

regards
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Old 07-16-2008, 06:07 PM   #7
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That animation is at a fairly low reynolds number I think, the wake would be 3-4x as long in the 10^5 to 10^6 range.
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Old 07-16-2008, 07:26 PM   #8
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Von Karman strikes again!
http://www.en.wikipedia.org/wiki/Von..._vortex_street
A Karman vortex can occur after any size object. The airflow meter on my car (and many Mitsubishis) is a Karman type.
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Old 07-16-2008, 08:30 PM   #9
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How does nobody talk about "surfing"?

Seriously it is not as dangerous, no rock chips, and still brings about a lot of gains just look at the sides right around the middle of the shape is where the air separates the most...
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Old 07-17-2008, 03:31 AM   #10
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Quote:
Originally Posted by trautotuning View Post
Seriously it is not as dangerous,
Since I've nearly been merged into by large vehicles over 20 times that I can remember in the last few years, and have never come close to running in the back of one, or being outbraked by one, I'll take that with a pinch of salt.
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I remember The RoadWarrior..To understand who he was, you have to go back to another time..the world was powered by the black fuel & the desert sprouted great cities..Gone now, swept away..two mighty warrior tribes went to war & touched off a blaze which engulfed them all. Without fuel, they were nothing..thundering machines sputtered & stopped..Only those mobile enough to scavenge, brutal enough to pillage would survive. The gangs took over the highways, ready to wage war for a tank of juice
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