Two fluids problem

Birkov · October 29, 2009, 05:43

Hello to all.

Have anyone any experience with with two fluid problem? I tried solve water and air. For each fluid a have one mesh block (I need different velocity inlet for water and air). I create two box(one smaller in z-direction) with small space between them and set water below the air (boxes are in the middle). Water have bigger velocity(5m/s) then air(2m/s) so air is pulling through small space.
(http://img62.imageshack.us/img62/613/flowu.jpg)

I defined two fluids regions, set Newtonian viscosity, k-e turbulent model and gravity. Due to the big density difference I activated "Two-fluid interface slip".
Then I tried lots of configurations for solver, but I still can't find right set up.
(still excessive pressure iteration failures)

If anyone have any tip or suggestion I will be grateful.
Thanks for your time.

MuxaB · October 29, 2009, 12:48

Hi,

Do the two mesh blocks have a gap between them? They really shouldn't. On the other hand having a boundary between two mesh block right at the interface between the fluids is also not a good idea for stability and accuracy issues.

The question here is how to define two different inlet BCs at the left boundary. If the two fluids had the same velocity you could use one mesh block and use fluid height FLHTL to define the location of the fluid interface at the left boundary.

You could use a mass/momentum source to generate the air flow, still using one mesh block. Set the left boundary as a fixed-velocity for the liquid, fluid #1, that is. Block the upper part of the boundary, where air is supposed to come in, with an obstacle. Then define a mass/momentum source immediately to the right of the obstacle, spanning the whole length of it, and define its velocity and mass rates accordingly. The mass source flag will be for fluid #2, IQSR=2, and RHOQSR=density of the second fluid.

Tedious, but i think it should work.

Birkov · November 5, 2009, 07:13

Hi,

thanks for your reply. My picture wasn't accurate. I made another better.
http://img198.imageshack.us/img198/6846/flowc.png

If I create only one mesh block a try set velocity inlet (set velocity and fluid height) only for water (add Fluid region and set height in initial tab) velocity BC is applied not just on set height but on max height. I can't understand why or where is mistake.
But I need different velocity inlet for air and water. So I made two mesh blocks and set Velicity on each block and Pressure as outflow BC. BC on bottom is Wall and on the top is Pressure. BC on the side is set as wall. Remaining BC are set as Symmetry.
From my opinion the biggest problem is if water and air starts mixing together. If I check pressure distribution there are huge pressure jumps and solver give me a error message as I wrote before.
Any suggestions?
Thank you.

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October 29, 2009, 05:43	Two fluids problem	#1
Birkov New Member Join Date: Jul 2009 Location: Prague Posts: 28 Rep Power: 0	Hello to all. Have anyone any experience with with two fluid problem? I tried solve water and air. For each fluid a have one mesh block (I need different velocity inlet for water and air). I create two box(one smaller in z-direction) with small space between them and set water below the air (boxes are in the middle). Water have bigger velocity(5m/s) then air(2m/s) so air is pulling through small space. (http://img62.imageshack.us/img62/613/flowu.jpg) I defined two fluids regions, set Newtonian viscosity, k-e turbulent model and gravity. Due to the big density difference I activated "Two-fluid interface slip". Then I tried lots of configurations for solver, but I still can't find right set up. (still excessive pressure iteration failures) If anyone have any tip or suggestion I will be grateful. Thanks for your time.

October 29, 2009, 12:48		#2
MuxaB Senior Member michael barkhudarov Join Date: Mar 2009 Location: Sante Fe, New Mexico, USA Posts: 337 Rep Power: 18	Hi, Do the two mesh blocks have a gap between them? They really shouldn't. On the other hand having a boundary between two mesh block right at the interface between the fluids is also not a good idea for stability and accuracy issues. The question here is how to define two different inlet BCs at the left boundary. If the two fluids had the same velocity you could use one mesh block and use fluid height FLHTL to define the location of the fluid interface at the left boundary. You could use a mass/momentum source to generate the air flow, still using one mesh block. Set the left boundary as a fixed-velocity for the liquid, fluid #1, that is. Block the upper part of the boundary, where air is supposed to come in, with an obstacle. Then define a mass/momentum source immediately to the right of the obstacle, spanning the whole length of it, and define its velocity and mass rates accordingly. The mass source flag will be for fluid #2, IQSR=2, and RHOQSR=density of the second fluid. Tedious, but i think it should work.

November 5, 2009, 07:13		#3
Birkov New Member Join Date: Jul 2009 Location: Prague Posts: 28 Rep Power: 0	Hi, thanks for your reply. My picture wasn't accurate. I made another better. http://img198.imageshack.us/img198/6846/flowc.png If I create only one mesh block a try set velocity inlet (set velocity and fluid height) only for water (add Fluid region and set height in initial tab) velocity BC is applied not just on set height but on max height. I can't understand why or where is mistake. But I need different velocity inlet for air and water. So I made two mesh blocks and set Velicity on each block and Pressure as outflow BC. BC on bottom is Wall and on the top is Pressure. BC on the side is set as wall. Remaining BC are set as Symmetry. From my opinion the biggest problem is if water and air starts mixing together. If I check pressure distribution there are huge pressure jumps and solver give me a error message as I wrote before. Any suggestions? Thank you.