# How fluid is air, or what is the right question?

• posted on February 17, 2007, 10:00 pm

Water seeks its own level. This means that when one pours a bunch of water on a table it won't end up like a little mountain, but it will very quickly spread out pretty widely (subject to surface tension, etc.)
When you pour a lot of sugar or sand, or dirt it won't spread out as much, I presume because of friction between the sugar, sand, and dirt particles. If you have a box of the stuff and open a door at the bottom, some of the stuff will come out the door
If you have a heated building and open an overhead door 12 to 16 feet high, for trucks, how fast will the cold air outside pour into the building, and how fast will the hot air nearer the ceiling be forced out.
I was recently at such a building, when it was below freezing out, and I was close to the door and every time the door was opened, I'd feel a bit of cold air, but not as much as I expected, and when the door was shut, the building seemed back to normal very quickly. If it had been full of water, in a twentieth of the time it took a car to drive in or out, all the water would have run out. Does air not flow even at 1/20th the speed of water? What am I missing?

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• posted on February 17, 2007, 10:19 pm

What you're missing is the density of the air just inside the door opening is only a little less dense (because it's warm), than the outside air that's on the other side. If the room were full of water, there would be a huge density difference and gravity would cause it to pour out.
Or create a water example, where you had a pool with a vertical divider, seperating warmer water from cooler water. If you pull up the divider for a minute, the water will begin to mix, but the warm water won't just rush over in a minute.

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• posted on February 17, 2007, 10:23 pm

So does air, the pressure across the earth varies by 10% max.

It is spread out evenly (more or less), see above.

With just one opening, the cold air has to get in and the arm air get out. A nice circulatory arrangement may occur, but it's better if there are two openings, especially if they are arranged in the right places. Check out solar chimneys

Air can flow at 100 m/s or so on earth, in jet streams, can you find water moving at anywhere near that speed?

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• posted on February 17, 2007, 10:40 pm

Doesn't the tital waves move neat this speed ? I don't think in meters/second but miles per hour. Thought the tital waves could hit several humdred mph.
Air will expand to fill all the space, but cold and hot air are almost the same density. Take a clear class and heat some water so it is about 150 deg F and drop in a couple of drops of food coloring and see how slow the cold fluid moves if you don't sitr it.

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• posted on February 18, 2007, 2:47 am

Time to get educated and join the rest of the world.

The wave moves, not the water. In the middle of the ocean, the tsunami does not consist of water moving fast like in river rapids. As it comes on shore, that changes, of course, as all the video shots from 2005 demonstrate.
R.G. Vickson

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• posted on February 17, 2007, 10:23 pm

You need something to move the air. When you open the door, there is already air on both sides so there is little exchange. If there is a breeze, or temperature differences, there will be some air movement, but it is not like spilling water onto a table. To prove this, put a divider in a container of water. Color the water on one side,let it stand and be very still, then pull up the divider. It will eventually mix, but not very fast. Just as some cocktails are poured in layers.
On a windy day, open the door and you will feel more air, but if you have openings at the other end of the building, no matter how large, you will get a draft through the building. At one of our buildings on a breezy day, I can tell when a big door is opened as far as 1000 feet away. On a calm day, you don't feel anything even 50 feet away.

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• posted on February 18, 2007, 4:07 pm
wrote:

Air movement in side a closed or semiclosed area is extremely complex. Although air densitytemperature gradients play an important role, air mixing is highly dependent on the existing HVAC and placement of furniture or equipment in the room. Although I could not find the report in a short imte, I have seen a Department of Energy study from quite a few years ago that discussed the result of smoke tests in a variety of room configuratoins. What amazed me is that in some parts of rooms with what might be consdered good circulation, there were dead air spots that persisted for very long periods.

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• posted on February 17, 2007, 10:35 pm

cfm = 16.6Avsqrt(HdT), for 2 Av ft^2 vents with an H' height diff and dT (F) temp diff.
Nick

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• posted on February 17, 2007, 10:52 pm

How fast would water have run out of the building if you had water outside, at same level as inside? The building is not surrounded by vacuum.
Mati Meron | "When you argue with a fool, snipped-for-privacy@cars.uchicago.edu | chances are he is doing just the same"

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• posted on February 18, 2007, 1:26 am

How about this: I know of a grocery store that had a somewhat larger opening, as its main entrance, that was open maybe 14 hours a day (during normal open hours). Or was it open 24 hours? I forget. The difference between this entrance and a simple opening is there was a substantial downward blast of air as you entered or left through it. The air blast somehow kept warm inside air and cold outside air separated. (How?) And since this was in a suburb of Buffalo NY, the outside air could get rather cold (and windy!)

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• posted on February 18, 2007, 2:29 am
On Sun, 18 Feb 2007 01:26:56 +0000 (UTC), snipped-for-privacy@world.std.spaamtrap.com (Michael Moroney) wrote:

I have seen that arrangement too. I was never sure if it was meant to work like you say, or only to give people a warm feeling when they walked in.
It like you say, I don't know how either.

Maybe there is a force field like Captain Wonder would have, if he had his own comic book.

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• posted on February 18, 2007, 4:22 am

Yes is will.
http://www.shiromi.com/gallery2/d/16996-1/water3_std.jpg
http://static.flickr.com/21/24681862_be8d89ed03.jpg
http://www.corporate.basf.com/basfcorp/copsfiles/pressefotodb/1208_wassertropfen-water_droplets.jpg

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<%-name%>
• posted on February 18, 2007, 5:53 am
In sci.physics, Androcles
wrote on Sun, 18 Feb 2007 04:22:46 GMT

Androcles does have a point here; if the water doesn't adhere to the tabletop (e.g., wax), it will bead up. These are definitely "little mountains", although more in the US Eastern sense, not the Sierra Nevada. :-)
Even if the water does adhere to the tabletop, it will still bead up, although not as much.
To be sure, surface tension can only go so far; if one pours a cubic meter (1 metric tonne) of water on a 1 m^2 wax-coated tabletop, it won't all fit as little beads.
However, neither will it all fall off the edges, either. Most of it, yes -- but not all. The rest will have to be wiped off with a towel or left to evaporate; if one is working with impure water, one gets salt -- a fact noticed by many after a rainfall on their car rooftops, windshields, hoods, etc.
--
Useless C++ Programming Idea #8830129:

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• posted on February 18, 2007, 5:00 am

Actually, air and water are pretty much the same thing when you study the dynamics of fluids and gasses. The physical properties of both are a bit different, water is for instance much less compressible than air, water is more dense and the specific heat differs. Yet, there isn't really a lot of difference when the dynamics of fluids and gasses are modeled, it is all Navier Stokes maybe in a slightly different form. Maybe a good book is "Ocean Atmosphere Dynamics" by Gill which treats both topics next to each other.
Your building example is not really fair, a building filled with water in an air environment is something different than a building filled with air in an air environment. Why not a building filled with warm water in a cold water environment?
Ejo
Ejo

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• posted on February 18, 2007, 8:45 pm
Gas behaves similar to liquid--both move faster with higher temperatures. But there is one important difference: gas (air) is spongy meaning that it easily expands and contracts in volume, depending on temperature and pressure. Liquids (and solids) stay about the same volume. Molecules are close together in solids, far apart in gases. Water will move faster than a gas in an air environment due to the differences in specific gravity. Also, nature tends to dissipate substances.
wrote:

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• posted on February 19, 2007, 5:18 pm

If you have enough gravity that the kinetic velocities of the gas molecules are "small" then air will fall to lower levels like this. In general the gas will expand to make its sound speed roughly equal to the square root of the layer thickness times gravity. Of course that's nothing but hydrostatic equilibrium.
The other consideration is the mean free path between inter-particle scattering events be small compared to any dynamical scale.
On both counts, air is treatable as a fluid (and is treated with fluid equations).
--
ciao,
Bruce

drift wave turbulence: http://www.rzg.mpg.de/~bds /