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Process block is defined with <process> </process> tags.

This section defines the process, that is modelled in the project. It has to contain ’name’ and ’type’ tags:


Name can be defined freely, type has to contain one of following strings list of available processes. More details about some of them can be found in the Process-dependent configuration section.

Integration order

The tag <integration_order> </integration_order> is mandatory and defines the integration order of the Gauss-Legendre integration over an element. Currently orders 1 to 4 are supported.

Process variables

An important part of this section is defined in “process_variables” tag. It is important, because in later parts of the project files (e.g.: in definition of errors). Those variables and their order are specific to each type of process. The order in which those variables have been defined in each process plays an important role. For more details see time loop and linear solvers. Information of which variables are required for which process can be found in appropriate sections of the Doxygen documentation. For example, for THM process following parameters are required:


Secondary variables

This tag is optional. It allows to define which secondary variables will be computed and saved in the output files. Which secondary variables are available is dependent on a specific process. They can be defined as follows:

    <secondary_variable internal_name="internal_name" output_name="output_name"/>

where internal_name has to match the name of one of available secondary variables and output_name can be defined by the user. output_name will be used as the name of the field into which a specific variable will be written into an output files.

Error tolerances

In this section, within <convergence\_criterion> tag, relative tolerances for the error have to be defined - using tag <reltols>. Each value in this tag defines the tolerance for errors with respect to one process variable. The order of variables is process specific. For the process variables listed above, relative error tolerances can be defined as follows:


The order can differ based on the order in which the processes are defined and on dimensionality of the process (e.g., number of components in displacement, or number of chemical constituents). Keep in mind that some process variables have more than one value like displacement in the example above. In such a case, a matching number of reltols has to be defined.

Constitutive relations

Constitutive relation can be one of the existing relations implemented in OpenGeoSys or it can be defined by user using MFront. They are used with one of the following mechanical processes:

  • Hydro-Mechanics
  • Phase-Field
  • Richards-Mechanics
  • Small Deformation
  • Thermo-Mechanics
  • Thermo-Hydro-Mechanics
  • Thermo-Richards-Mechanics
  • TH2M

To define constitutive relation, tags <constitutive_relation> </constitutive_relation> are used. The fixed, minimum requirement is presence of <type> </type> tag. Other tags depend on the chosen relation.

There can be more than one constitutive relation used in one project file but only one for each material id, similar to how each medium is defined in media block. OpenGeoSys selects constitutive relation for a material id based on the content of the attribute id in tags <constitutive_relation> </constitutive_relation> and <medium> </medium>.

Source terms

There are four available types of source terms.

Classical types for defining source terms in space are:

  • Nodal (point-like source terms)
  • Line (1D features in space)
  • Volumetric (2D or 3D features in space)

For complex boundary conditions (like transient ones), the boundary condition can be defined as type:

  • Python (complex boundary conditions reflecting e.g. measurements and time series data)

Remember that the source term has to be scaled by the volume as explained in Basics and in this Example. Source terms can also be manipulated via Python.

Specific body force

The tag <specific_body_force> can be used to define gravitational force. This tag is optional. By default it is not considered in the simulation, it has to be explicitly enabled by setting an appropriate value in the project file. Presence of this tag should be reflected also in the initial stress conditions. It can improve the convergence in the first time step, if the correct stress field is defined.

    <specific_body_force>in_X_direction in_Y_direction in_Z_direction

In a 3D with gravity along the z-axis pointing downwards one would write the following definition for the specific body force:

    <specific_body_force>0 0 -9.81</specific_body_force>

Warning! If specific body force is added, it can be reflected within the initial state of stresses.

Stress is a somewhat unique, as the initial conditions for stress are not defined in the block “Process variables”, but in “Process”. It requires setting up a constant or function definition in the “Parameters” block where initial stress is defined. It has to match the dimension of the experiment. OGS expects it to be the effective stress (total stress with pore pressure subtracted), this is in contrast to total stress that is expected to be provided as boundary condition. If meshes are defined as axially symmetric, the stress has to be provided in polar or cylindrical coordinates.

Jacobian Assembler

The global non-linear equation system can be solved either with Picard fix-point iterations or a Newton scheme.

For the latter the process has to provide a Jacobian. If the analytical Jacobian is not implemented, one can use a quasi-Newton scheme where the Jacobian is approximated by numerical differentiation using a central differences or a forward difference scheme.

For the Picard fix-point iterations this section is not used.

This article was written by Feliks Kiszkurno. If you are missing something or you find an error please let us know.
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