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-   -   question about Nested Grid for two fluids (http://www.cfd-online.com/Forums/flow-3d/101577-question-about-nested-grid-two-fluids.html)

 Zeng May 8, 2012 05:34

question about Nested Grid for two fluids

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!

 MuxaB May 10, 2012 21:10

Quote:
 Originally Posted by Zeng (Post 359884) 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.

 Zeng May 10, 2012 21:55

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

 MuxaB May 10, 2012 23:24

hmm. is air modeled as compressible fluid?

 Zeng May 11, 2012 00:00

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....

 MuxaB May 19, 2012 02:08

Quote:
 Originally Posted by Zeng (Post 360456) 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?

 mf_emp May 27, 2012 09:50

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

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