Problems with BCs in muliphase flow with water falling from a reservoir

Paul_l · March 8, 2011, 09:36

I've been doing the VOF tutorial in the Fluent manual and now I'm trying to modify it to my likings. In the tutorial the flow is driven by a time dependent velocity from the inlet, but I want to do a simulation where the water is falling due to gravity. What I've done is:

-Introduced gravity in positive x-direction (been doing some tests and I think the gravity is fine)

-Made the inlet BC a pressure inlet with 0 gauge pressure

When I patch the nozzle with water and run the simulation, the only thing that happens is that a small strip (the first row of cells) becomes air. It does not seem to be much more movement, neither at the top of the water column nor at the bottom where the water should flow out.

I've also tried patching a small area below the nozzle with water. If the vertical sides of this water column is in contact with air and not the wall, this water falls down as expected. The water in the nozzle however stays inside now as well.

Have I completely misunderstood what kind of boundary conditions I should use?

momech · March 10, 2011, 14:57

hi dear sir.
if the nozzle diameter is too small,i think the physics of problem is such that no water falls down due to gravity,because of adherence!

Paul_l · March 11, 2011, 05:59

Thank you! That is a very good point. I ended up making a grid where the top of the nozzle and the area below is connected so air can flow around. That among with a slightly larger scale and variable time stepping seemed to work well.

Here's a screenshot of my (axisymmetric) grid if somebody is have the same problem:

March 8, 2011, 09:36	Problems with BCs in muliphase flow with water falling from a reservoir	#1
Paul_l New Member Paul Levold Join Date: Feb 2010 Posts: 5 Rep Power: 16	I've been doing the VOF tutorial in the Fluent manual and now I'm trying to modify it to my likings. In the tutorial the flow is driven by a time dependent velocity from the inlet, but I want to do a simulation where the water is falling due to gravity. What I've done is: -Introduced gravity in positive x-direction (been doing some tests and I think the gravity is fine) -Made the inlet BC a pressure inlet with 0 gauge pressure When I patch the nozzle with water and run the simulation, the only thing that happens is that a small strip (the first row of cells) becomes air. It does not seem to be much more movement, neither at the top of the water column nor at the bottom where the water should flow out. I've also tried patching a small area below the nozzle with water. If the vertical sides of this water column is in contact with air and not the wall, this water falls down as expected. The water in the nozzle however stays inside now as well. Have I completely misunderstood what kind of boundary conditions I should use?

March 10, 2011, 14:57		#2
momech New Member mohsen Join Date: Feb 2011 Posts: 13 Rep Power: 15	hi dear sir. if the nozzle diameter is too small,i think the physics of problem is such that no water falls down due to gravity,because of adherence! Last edited by momech; March 12, 2011 at 12:08.

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March 11, 2011, 05:59		#3
Paul_l New Member Paul Levold Join Date: Feb 2010 Posts: 5 Rep Power: 16	Thank you! That is a very good point. I ended up making a grid where the top of the nozzle and the area below is connected so air can flow around. That among with a slightly larger scale and variable time stepping seemed to work well. Here's a screenshot of my (axisymmetric) grid if somebody is have the same problem: