FEM Architecture & Solvers

Relevant source files

The following files were used as context for generating this wiki page:

• src/Mod/Fem/coding_conventions.md
• src/Mod/Fem/femsolver/elmer/sifio.py
• src/Mod/Fem/femsolver/calculix/writer.py
• src/Mod/Fem/femexamples/manager.py
• src/Mod/Fem/femtest/test_information.md
• src/Mod/Fem/femexamples/elmer_nonguitutorial01_eigenvalue_of_elastic_beam.py
• src/Mod/Fem/Gui/Resources/translations/Fem.ts

The Finite Element Method (FEM) module provides a robust framework for performing mechanical, thermal, and fluid simulations. It serves as a bridge between geometric models and external numerical solvers such as CalculiX, Elmer, and Z88. The architecture is designed to handle mesh generation, constraint definition, solver input writing, and post-processing of results within a unified Analysis container.

Sources: src/Mod/Fem/coding_conventions.md, src/Mod/Fem/femexamples/manager.py:126-140

Module Architecture and Conventions

The FEM module follows strict coding conventions to maintain a separation between the application logic (App) and the graphical user interface (Gui). The core logic is often implemented in Python for solver interfacing and C++ for performance-critical data structures.

Component Structure

• Analysis Container: The Fem::FemAnalysis object acts as the primary parent for all simulation-related objects, including meshes, solver settings, and material properties.
• Naming Policies: Python code utilizes snake_case for modules and functions, while CamelCase is used for classes. C++ code exclusively uses CamelCase.
• Type Checking: The module avoids hasattr(obj, "Proxy") in favor of specialized utility methods like is_of_type(obj, "TypeString") from the femtool.femutils module.

Sources: src/Mod/Fem/coding_conventions.md:58-94

General Simulation Workflow

The following diagram illustrates the standard data flow from geometric model to simulation results.

Solver Interfacing

The FEM module uses dedicated "Writer" classes to translate FreeCAD internal data into solver-specific input files.

CalculiX Solver (Ccx)

The CalculiX writer translates analysis data into .inp files. It enforces a consistent unit system (mm, N, s, K) because the underlying solver is unit-agnostic.

Parameter	Unit Used in .inp
Length	mm
Mass	t (tonnes)
Force	N
Pressure	MPa (N/mm²)
Temperature	K

The FemInputWriterCcx class manages the export of node sets, element sets, and steps (Static, Frequency, Buckling, or Thermomech).

Sources: src/Mod/Fem/femsolver/calculix/writer.py:59-100

Elmer Solver

For the Elmer solver, the module utilizes a Builder class to construct the Solver Input File (SIF). The SIF is organized into logical sections.

Valid SIF Sections

The Elmer interface recognizes specific sections required by the ElmerSolver:

• Simulation, Constants
• Body, Material, Body Force
• Equation, Solver
• Boundary Condition, Initial Condition

Sources: src/Mod/Fem/femsolver/elmer/sifio.py:34-58

Constraints and Boundary Conditions

Constraints define the physical environment of the simulation. They are mapped from geometric references (faces, edges, vertices) to mesh nodes or surfaces during the input writing phase.

Supported Constraint Types

The FEM workbench supports a wide range of physical constraints identified in the GUI and translation resources:

Category	Specific Types
Mechanical	Fixed, Displacement, Force, Pressure, Bearing, Gear, Pulley, Spring
Thermal	Initial Temperature, Heat Flux, Temperature Boundary
Fluid	Fluid Boundary Condition (for CFD)
Numerical	Tie Constraint, Plane Multi-Point Constraint (MPC)

Sources: src/Mod/Fem/Gui/Resources/translations/Fem.ts:10-1043

Testing and Example Management

The femexamples.manager provides a programmatic interface to set up and validate simulation scenarios.

• Example Setup: Uses ObjectsFem.makeAnalysis and specialized factory methods like ObjectsFem.makeSolverCalculiXCcxTools to build test documents.
• Automated Execution: The run_analysis function handles document recomputation, mesh generation (via Gmsh or Netgen), and solver execution.
• Unit Testing: Tests are organized by solver and object type. Documentation specifies that standard test files are located in Mod/Fem/femtest/data/.

Sources: src/Mod/Fem/femexamples/manager.py:126-212, src/Mod/Fem/femtest/test_information.md:89-102, src/Mod/Fem/femexamples/elmer_nonguitutorial01_eigenvalue_of_elastic_beam.py:61-118

Mesh Generation Flow

When running an automated analysis, the system identifies mesh generators within the analysis group and invokes the appropriate tools.

Summary

The FEM module architecture emphasizes a structured translation of geometric and physical data into solver-specific formats. By utilizing a common Analysis container and specialized input writers for CalculiX and Elmer, it ensures that complex multi-physics simulations can be defined once and executed across different numerical engines. The system is supported by an extensive example management framework that validates solver integration and architectural consistency.