Initial and Boundary Conditions

Setting up initial and boundary conditions is crucial to ensure simulations are physically meaningful and achieve proper convergence. In OFFBEAT, as in all OpenFOAM applications, these conditions are defined in the field dictionary files located in the 0/ folder (or the starting time folder). Each required field must have a corresponding file containing the initial and boundary conditions.

Required Fields

Most OFFBEAT simulations require initial and boundary conditions for the following fields:

• Temperature field (T)
• Displacement field, which differs based on the solver:
  • D for total-field displacement solvers (e.g., smallStrain, largeStrainTotLag).
  • DD for incremental solvers (e.g., smallStrainIncrementalUpdated, largeStrainUpdLag).

Additional fields may need to be defined depending on the activated physics. For instance:

• Neutronics solvers require initial and boundary conditions for neutronFlux.
• Element transport solvers may involve additional field definitions.

Note

When restarting a simulation, the code reads all field files and their initial/boundary conditions from the latest available time step. However, when starting a new simulation, only the mandatory fields (e.g., T and D) are typically required. Defining additional fields is generally unnecessary and not recommended.

This guide focuses on the primary fields T and D. Future updates will expand coverage to other fields.

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Field File Structure

An example of a typical field file for T is provided below:

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.3.0                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     9.0;
    format      ascii;
    class       volScalarField;
    location    "0";
    object      T;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dimensions      [0 0 0 1 0 0 0];

internalField   uniform 293;

boundaryField
{
    "wedge.*|.*Front|.*Back"
    {
        type            wedge;
    }

    cladOuter
    {
        type           fixedValue;
        value          uniform 500;
    }

    ".*Top.*|.*Bottom.*"
    {
        type            zeroGradient; 
    }

    etc...
}

The key sections in the T field file are:

1. The header: Contains metadata about the field, including:

   • Field name (object entry)
   • Field type (e.g., volScalarField, volVectorField, volTensorField, volSymmTensorField)
   • File location (e.g., "0")

2. dimensions: Specifies the field's dimensional units in SI format (e.g., [mass length time temperature quantity current luminousIntensity]). These must align with OFFBEAT libraries and solvers.

3. internalField: Defines the field's internal values. Most cases use uniform values (e.g., uniform 293 for a scalar field like temperature, or uniform (0 0 0) for a vector field like displacement). Non-uniform fields can be defined using OpenFOAM tools like topoSet or codeStream. Refer to the OpenFOAM documentation for details.

4. boundaryField: This subdictionary defines the boundary conditions for all patches in the domain. Typically, one patchField dictionary is required per patch. However, users can simplify the setup by using wildcards (e.g., top.*) and the | symbol (e.g., top.*|bottom.*) to group patches with similar names into a single definition. This can reduce redundancy and streamline the configuration process of the boundaryField subdictionary.

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Boundary Conditions

OFFBEAT uses boundary condition definitions similar to standard OpenFOAM cases. For general instructions, consult the relevant OpenFOAM user guide for your version:

• OpenFOAM.org user guide
• OpenFOAM.com user guide

Boundary conditions in OFFBEAT are referred to as fvPatchField classes in OpenFOAM terminology. Users can choose from standard OpenFOAM boundary conditions (e.g., fixedValue, zeroGradient) or use specific conditions developed in OFFBEAT for fuel performance simulations. These specialized boundary conditions are detailed in:

• Temperature BCs
• Displacement BCs

Warning

For fuel-rod simulations, it is tipically required to define the gap gas composition and initial pressure in the gapGas file, located in the starting time folder. See the Gap Gas Model documentation for details.