SOLVE_IN_LATTICE_FIXED_FRAME
Use this command to solve a diffusion problem in the lattice-fixed frame of reference. This command is available with the HOMOGENIZATION_MODEL as a command within the ADVANCED_HOMOGENIZATION_OPTIONS.
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Syntax |
SOLVE_IN_LATTICE_FIXED_FRAME |
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Prompts |
Solve in the lattice-fixed frame of reference /NO/ Enter |
By default, DICTRA solves diffusion problems in a volume-fixed frame of reference under the assumption that the partial molar volume of all substitutional elements is constant and equal and that the partial molar volume of interstitial elements is zero. When you enable this command, the diffusion problem will instead be solved in a lattice fixed frame of reference. In addition, this option forces the use of the assessed molar volumes read from the current thermodynamic database.
Simulations with moving boundaries as well as cell simulations are currently not available with this feature.
For all geometries the leftmost/lower coordinate is fixed during a simulation (by default it is zero). This command also enforces some constraints for planar and cylindrical geometry. For a planar geometry the size of the domain in the plane orthogonal to the direction of diffusion is fixed. For a cylindrical geometry the length of the cylinder is fixed. For all geometries there may be changes in the size of the domain as a result of the diffusion process, even with closed boundaries, since the assessed molar volumes of the thermodynamic database is used.
All elements are set as substitutional. Though the notion of substitutional/interstitial elements has no meaning when running a simulation with this feature turned on, it does have implications in the POST processor.
The DICTRA POST processor does not use assessed molar volumes, which means that certain plots will be erroneous. There may for example be changes in the total domain size even for a single phase simulation with a closed system due to changes in the molar volume. A plot of the total number of moles in the domain would then, erroneously, show a change over time. Other axis quantities that in general will be incorrect are, for example, IV(*) and VP(*).
Further, when solving in the lattice-fixed frame of reference the unit of the flux is mol*m-2s-1, which differs from the default unit wherein the assumed constant substitutional molar volume (10-5 m3*mol-1) is included in the flux. This must be taken into account for certain types of boundary conditions, that is, the conditions of type
- FIX_FLUX_VALUE
- POTENTIAL_FLUX_FUNCTION
- ACTIVITY_FLUX_FUNCTION
- ITERATIVE_ACTIVITY_FLUX_FUNCTION
See SET_CONDITION for details about the above commands.
In order to roughly get the same result for these boundary conditions the evaluated flux must be a factor 105 higher when solving in the lattice-fixed frame of reference compared to simulations with the default setting.