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Old 06-21-2008, 04:59 PM   #1
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An A-ha moment...

Went to the quarter car wash today to wash the Geo. (OK, it's actually a buck fifty and that's only for four minutes...put in three bucks for eight and some change minutes.) Aimed the high pressure wash nozzle at the headlight...wanted to wash off the bugs I picked up from around the sewage treatment plant. Noticed an interesting thing...aiming the nozzle at various places on the car gives a really good visual indication of airflow over the car.

From this I was able to tell: Wipers and antenna didn't seem to make any difference. Recessed headlights did, as did the grille openings. License plate, not much. Mirrors parted the water very nicely.

Seems like this might be a really good way to test aero mods before taking them on the road. One could re-create this with a brass nozzle with a small hole, or a slit rather than a round hole. Plus, it would be a fun way to cool off on hot summer days!

I did a search and could not find any evidence of anyone having done this...if this is old hat to you all, then I apologize. It was an aha moment for me, though!
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Old 06-21-2008, 05:45 PM   #2
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i rainx/wax my car and drive it aftewr a rainstorm(beads up everywhere) i notice the water on the hood parts down th emiddle hump in the hood and off to the sides.anternna does nothing nor does the mirror(bullet shaped) some water goes around it some goes between the mirror and window. it then gets blown up around th etop of the window and off the back.

the rearend is really interesting, i notice the wind/water from the sides of the car meeting with the stuff comming over the top and kinda comes back together to the central point on the car.
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Old 06-21-2008, 08:41 PM   #3
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I did the Rain-X test on the windshield of a Ford Escape. Slope is about 45 degrees. Driving in a light rain that didn't require wipers, I was able to see the pattern on the windshield created by the drops moving with the wind. It produced a very neat radial pattern starting at the bottom center of the windshield. The middle line goes straight up. The bottom flow goes laterally straight off the sides. And it made fairly straight lines from the stagnation point to the top corners.

This was an apple dropping moment in that it illustrated that the airflow over the top is not a simple 2-dimensional flow like you see in side view diagrams with the air flowing over the top. It is a complex 3-dimensional flow, which unfortunately makes it more difficult control.
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Old 06-21-2008, 08:44 PM   #4
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Oh, yeah. Almost forgot. I was thinking of trying to use a leaf blower to try to highlight localized flow patterns. But I need to develop a smoke source that isn't dissipated by the wind. A water spray might do the trick.
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Old 06-22-2008, 05:59 AM   #5
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For a smoke source, why not a smoke bomb? The fireworks stand down the street from my house must have loads of 'em, I ought to buy a bunch.
http://blogs.smarter.com/gadgets/200...ed-smoke-bomb/
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Old 06-22-2008, 08:02 AM   #6
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I have an old Holloween smoke generator that will blanket the neighborhood. But I want to see the flow, not fly IFR. The fuel for that is some form of glycerin. I just need to rig a hot plate to drip it on and it will generate smoke. Hard part is rigging a dispenser to produce a steady stream, probably from some kind of wand that I can control by hand. Sounds easy. Never is. And it has to get prioritized with the house remodeling project I've working on (slowly) for the last four years.
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Old 06-22-2008, 11:49 AM   #7
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Quote:
Originally Posted by theholycow View Post
For a smoke source, why not a smoke bomb? The fireworks stand down the street from my house must have loads of 'em, I ought to buy a bunch.
http://blogs.smarter.com/gadgets/200...ed-smoke-bomb/
First problem with that is that you don't see airflow (maybe if I set up a box fan...).

Second problem is that I live where all fireworks are outlawed. Mind you, that hasn't always stopped us from going to the next town over and getting them anyway, but, to paraphrase Orson Welles, fireworks are not a secret vice. Now...I could always Seafoam the Cadillac, then set a fan behind it...I think the water jet would give me the same information, be more environmentally friendly, more fun and a lot cooler when it's 100 degrees outside!
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Old 06-22-2008, 11:54 PM   #8
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Smoke testing would be hard, right?
- Need to blow air with big fans to move the air around the car
- Need to blow the smoke to check the wind patterns (leafblower?)
- Need to shape the smoke into a streak/line...

Looking online there's a few favorite ways to make smoke, but they all seem to involve fire and heat, and not really obvious how to make a thin thick smoke stream:

- Ping pong balls, cut up in a closed aluminum wrapper, poke hole, set fire, blow fire, get smoke.

Small enough, but seems to burn fast. Dunno if it's healthy.

- Chemical: This is what I found on youtube (I never tried it!)
60gr Potassium Nitrate
40gr Sugar
Mix and cook on low fire, till sugar caramelizes, will look like peanut butter
1 big TBSP Sodium Bicarbonate
Mix. (Slows combustion)
Fill in a cardboard tube, leave a hole in center (molded using the body of a large pen)
Let dry an hour
Drop in a fuse line, held with cotton.
Wrap in tape, leave a holewhere fuse is for smoke.

Note: The result seems overwhemingly potent, and likely heats up a lot, but might be hard to use for smoke testing.

There's also chloridric acid with aluminum? 90gr Sugar and 100gr Salpetre (???)... 50gr sugar and 50gr chlorydrate...
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Old 06-23-2008, 07:29 PM   #9
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Water droplets do have inertia of their own. The smaller the droplets, the more they will follow the air flow. Smoke after all is just a colloidal solution of air and something you can see.

I was stuck in a store during a storm once. There was a car, a Celebrity if I recall correctly, directly in front of the door sideways to the building. It was facing directly into a 50-60 mph gale and the rain going over it looked like a commercial of a car in a wind tunnel. Unfortunately, it's difficult to schedule wind tunnel tests if you need a storm. But then there was the Ben Franklin guy...
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Old 06-23-2008, 10:14 PM   #10
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Where am I going to find a 60mph gale with rain in northern california? The weather is always perfect. LOL!
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