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 2d or 3d. 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 4e09 second is calculated. I'm stuck....I need advice. Thank you! 
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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 
hmm. is air modeled as compressible fluid?

The flow mode is chosen as incompressible, so both two fluids are calculated as incompressible, right?
I thought that interblock boundary type coefficient(alphamb) might affect, so I changed it from default to 0.5 or 1.0, but still didn't work.... 
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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 TwoFluid Problem 
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