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Old 12-03-2006, 07:27 AM   #1
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Visual representation of airflow behind car

This is a silly video I found on youtube:

http://www.youtube.com/watch?v=xFvx9dKE3tk

It shows a car with a blown head gasket (Nissan Maxima I think). The interesting
thing is you can see how the air is being dragged along behind it, almost
perfectly behind the trunk. This is despite the fact that the smoke is only
exiting from a small point on the exhaust.

This has given me an idea for aerodynamic testing - would it be possible to
get some kind of smoke generator, and mount it in different places while
driving along, and see how the air is flowing? (e.g. if you release the smoke
at the front of the car, you could see how the air flows over and behind
the car....)

Obviously you would need two cars, and it would be illegal, but so is engine
off coasting, and lots of other things that save on fuel!. You would have to
choose a day with no wind, and no cars in the local area.
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Old 12-03-2006, 08:53 AM   #2
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Quote:
Originally Posted by landspeed
This is a silly video I found on youtube:

This has given me an idea for aerodynamic testing - would it be possible to
get some kind of smoke generator, and mount it in different places while
driving along, and see how the air is flowing? (e.g. if you release the smoke
at the front of the car, you could see how the air flows over and behind
the car....)
Smoke in wind tunnels has been done for years and for testing, I think I have seen some early Auto Union or Tatra examples of this in B/W film.

As for the car being in motion, rain mist does the same, my long departed Citroen GS would send two "vortexes" off the back in the rain.

But, if it is not raining, here is your other option:

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Old 12-04-2006, 06:55 AM   #3
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Quote:
Originally Posted by landspeed

This has given me an idea for aerodynamic testing - would it be possible to
get some kind of smoke generator, and mount it in different places while
driving along, and see how the air is flowing? (e.g. if you release the smoke
at the front of the car, you could see how the air flows over and behind
the car....)

Obviously you would need two cars, and it would be illegal, but so is engine
off coasting, and lots of other things that save on fuel!. You would have to
choose a day with no wind, and no cars in the local area.
Hi, "landspeed"!
Idea! Would it be feasible to build a "smoke" generator with a container of a low level of water, a chunk of dry ice (very cold...caution!), and a short piece of hose...to direct the CO2 vapor(condensate?) to whatever point you wish? This "rig" could be pressurized from the ram air of the moving vehicle (shouldn't take much pressure...). Ram air "in" thru one length of garden hose, exposure to furiously bubbling dry ice...tipped into water...and this cold CO2 / water condensate exited from sealed container thru another length of the same size hose...to whatever point you wish to "look at". IMHO, this might work (or a variation of it)! CO2 would not be toxic (just cold!), nor would things go "Boom" if they got out of hand!
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Old 12-04-2006, 01:12 PM   #4
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well co2 is heavier than air could that affect how it reacts in the test. And make sure a dog doesnt chase you cause it could suffocate in the co2 mist. Id be scared to see it on my car cause it has a hatch and I know there will be a small cloud following me very close behind....
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Old 12-04-2006, 02:03 PM   #5
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Look at the shape of my car!. The tailgate is the perfect angle for maximum
drag . I need to figure out how to improve that at some point, without making
the car look too bad! My car also has no aerodynamics at all at the front, has little underbody shielding, and loads of sharp edges at the front.
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Old 12-04-2006, 02:17 PM   #6
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Being an X grocery store employee, I've had plenty of experience with dry ice. It does creat large amounts of vapor, but the amount of vapor it gives of is directly related to how hot the fluid it's placed in. The other thing to consider is, it's doesn't take very long for it too cool down the fluid and stop creating the steam. I think a better bet would be a road flare (VERY HOT) or throwing a few smoke bombs in the muffler, also hot. It seems like it woudl require a large amount of steam/smoke to properly show the drag behind the vehicle. It would work great sticking out the front of the car though! it'd leave real nice trails (although it'd probobly also leave moisture behind)

In our cold climates up here, I think just spraying water into the intake would do the job quite well in -20º F weather. However, I have open exhaust right now (old stuff fell apart) so I couldn't get a very good show from my car.
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Old 12-04-2006, 02:58 PM   #7
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I'm gonna try some form of smoke on my car when the wind is blowing the right way in the alley where I park it. Don't need to be moving just the wind has to be moving. Looks like my Drag CD is .35 which apparently is pretty good - may not be effective putting in a rear belly pan.
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