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-   -   Variable Composition Mixture vs. Fluid Pair (https://www.cfd-online.com/Forums/cfx/27040-variable-composition-mixture-vs-fluid-pair.html)

 Abby February 18, 2009 12:00

Variable Composition Mixture vs. Fluid Pair

I am very new to CFX (and CFD in general) and am running a few test cases to get a feel for how things work. I am modeling a basic tank with a peroxide/water solution (no chemical reactions, no inlet/outlet) and would like to confirm that, due to density differences, the peroxide will settle to the bottom of the tank. Gravity is activated, and I have defined my fluid as a variable composition mixture made up of peroxide and water. I have set the global initial mass fraction to be 8% peroxide. Unfortunately, after running this problem for a relatively long time, things still remain well-mixed.

Even if I calculate the location of where the peroxide/water boundary should be and set up an expression in the global initialization mass fraction box of a mass fraction of 1 below this boundary and zero above the boundary, my solution seems to want to mix the two fluids together. I was under the impression that the variable composition mixture could vary relative amounts of components in space--and the significant density difference of my two fluids would then imply they should settle out. Am I missing some fundamental understanding of how a "variable composition mixture" is defined?

Also, I'm not sure if this is relevant, but this solution has a very hard time converging in any reasonable time period.

As a check, I also set up a (presumably) identical problem with peroxide and water selected as a fluid pair. Again, gravity has been activated, and things again remain well mixed. It is possible--in fact, likely--that there are two separate reasons why my variable composition mixture and my fluid pair remain mixed instead of settling out. I would appreciate any insight regarding either case.

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