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Old   June 22, 2012, 11:40
Question Simulating pipe discharge into air
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Hi,

Has anyone done CFD simulations of water discharge from a pipe into the air? I searched the literature and it seems like there is very scarce information out there, probably since it is not a science problem in these days....

But I really have difficulty simulating this problem when water velocity is high. From everyday observations, when water velocity is low, the discharge stream is smooth and nice and bent by gravity. But when velocity increases above certain limit, the discharged water stream begins to have expanded shape, entraining air, and there is surface breaking resulting in numerous small water droplets. From CFD point of view, this becomes a hard problem to solve.

I have used the free surface model in CFX to try to reproduce what we observe at high velocity, but what I got is still smooth and nice water stream, no wave breaking is obtained. My feeling is that the VOF method in CFX has fundamental limitations in reproducing this turbulent, multi-phase, wave-breaking, and air-entraining problem.

If any of you have done this or related problems, could you educate me how we could do this in CFX? Or are advanced, more academic models needed?

Thanks a lot~
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Old   June 23, 2012, 06:37
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On the contrary, this is an area of lots of studies. Jet flows and spray break up is a major area of CFD research.

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My feeling is that the VOF method in CFX has fundamental limitations in reproducing this turbulent, multi-phase, wave-breaking, and air-entraining problem.
Not at all. You just need a mesh and time step small enough to resolve all the minute features required to create this flow.

Alternately, CFX has some spray break up models designed for things like fuel injectors where the size and distribution of drops in the important thing, not the details of the break up process. It depends on whether you are interested in the macro or micro scale in the spray break up process.
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Old   June 26, 2012, 10:00
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Hi Glenn,

Thanks for your reply. Could you point me to some references on the similar topics?

From my current understanding, most of the "spray breakup" models focus on nozzles (eg fuel injection), where all of the liquid stream is converted to discrete drops, and there is no morphological description of the breakup process (I'm interested in SEEING how a smooth continuous liquid stream breaks into thousands of small droplets).

On the other hand, for jet flows, I have not seen studies focusing on jet interaction with air (free surface problems). Mostly I see literature on jet motion in an infinite liquid reservoir. Different from the spray problem, my concerned subject has a much larger liquid stream, and only outer portions of the stream are converted to droplets (or entraining air) while the bulk of the steam remains an integral one. The breaking up on the outer perimeter of the liquid stream causes the overall shape (angle of the jet) to be expanded.

Thanks.
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Old   June 26, 2012, 19:07
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The break up of a liquid jet in air is a common topic for micro-drop generators and ink jet printers. It includes the physics you describe. Do a literature search for those topics.
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