# question about Nested Grid for two fluids

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 May 8, 2012, 05:34 question about Nested Grid for two fluids #1 New Member   Join Date: May 2012 Posts: 3 Rep Power: 6 I started a simple flow example: rectangular block and make it partly nested half water (depth=0.3m) and half air a water flow with constant velocity comes from right boundary, u= 0.1 m/s the simulation will be easily done if 'one fluid' is chosen, whether 2-d or 3-d. However, if I choose two fluids(water and air), while all the other condition and parameters exactly the same with one fluid situation, the errors will occur. 'convective flux exceeded stability limit' will soon come along. After 2 or 3 of them, excessive convection failures make calculation terminated. And only 4e-09 second is calculated. I'm stuck....I need advice. Thank you!

May 10, 2012, 21:10
#2
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michael barkhudarov
Join Date: Mar 2009
Posts: 332
Rep Power: 10
Quote:
 Originally Posted by Zeng I started a simple flow example: rectangular block and make it partly nested half water (depth=0.3m) and half air a water flow with constant velocity comes from right boundary, u= 0.1 m/s the simulation will be easily done if 'one fluid' is chosen, whether 2-d or 3-d. However, if I choose two fluids(water and air), while all the other condition and parameters exactly the same with one fluid situation, the errors will occur. 'convective flux exceeded stability limit' will soon come along. After 2 or 3 of them, excessive convection failures make calculation terminated. And only 4e-09 second is calculated. I'm stuck....I need advice. Thank you!
Is there an outlet at one of the mesh boundaries? For two fluids, if there is an inflow, there also must be an outflow.

 May 10, 2012, 21:55 #3 New Member   Join Date: May 2012 Posts: 3 Rep Power: 6 yes, there is an outlet boundary~ the mesh is as followed: block1 x:0~1.0, nxcelt=50, y:0~0.1, nycelt=5, z:0~0.5,nzcelt=25 ; block2 x:0.5~0.8,nxcelt=30, y:0.02~0.08, nycelt=6, z:0.1~0.4, nzcelt=30 ; Boundaries of block1 are: right: V , left: O , front and back: S , bottom:W , top:P ; for block2 they are all Symmetry

 May 10, 2012, 23:24 #4 Senior Member   michael barkhudarov Join Date: Mar 2009 Posts: 332 Rep Power: 10 hmm. is air modeled as compressible fluid?

 May 11, 2012, 00:00 #5 New Member   Join Date: May 2012 Posts: 3 Rep Power: 6 The flow mode is chosen as incompressible, so both two fluids are calculated as incompressible, right? I thought that inter-block boundary type coefficient(alphamb) might affect, so I changed it from default to 0.5 or 1.0, but still didn't work....

May 19, 2012, 02:08
#6
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michael barkhudarov
Join Date: Mar 2009
Posts: 332
Rep Power: 10
Quote:
 Originally Posted by Zeng The flow mode is chosen as incompressible, so both two fluids are calculated as incompressible, right? I thought that inter-block boundary type coefficient(alphamb) might affect, so I changed it from default to 0.5 or 1.0, but still didn't work....
It is hard to figure out without seeing the problem. Have any ideas yourself?

 May 27, 2012, 09:50 #7 New Member   Mahdiar Farhoudi Join Date: Feb 2012 Location: Tehran, Iran Posts: 17 Rep Power: 6 That is exactly my problem too! did you get what to do? you can also check out: Help > CUSTOM TUTORIALS > Hydraulics Tutorial > Variation: Run as a Two-Fluid Problem

 Tags nested block, two fluids

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