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Nasibullayev I.Sh. Application of free software FreeFem++/Gmsh and FreeCAD/CalculiX for simulation of static elasticity problems. Multiphase Systems. 15 (2020) 3–4. 183–200 (in Russian).
2020. Vol. 15. Issue 3–4, Pp. 183–200
URL: http://mfs.uimech.org/mfs2020.3.129
DOI: 10.21662/mfs2020.3.129
Application of free software FreeFem++/Gmsh and FreeCAD/CalculiX for simulation of static elasticity problems
Nasibullayev I.Sh.
Mavlyutov Institute of Mechanics UFRC RAS, Ufa, Russia

Abstract

The paper discusses the stages of computer numerical simulation of engineering problems and ways to improve the accuracy of simulation; provides a brief overview of free software for simulation elasticity problems by the finite element method, as well as trends in the development of free CAD and CAE software. For a successful engineering study, it is necessary to choose a convenient tool that takes into account all the features of the problem being solved. Based on the solution of a test static problem of linear elasticity, two approaches to engineering modeling were demonstrated. The first approach requires programming skills - the full modeling cycle was written in the programming language of the FreeFem++ software. Additionally, the method mesh generating in the Gmsh program with subsequent use in the FreeFem++ program is shown. In the second approach, the full cycle of modeling is carried out through the interface of the FreeCAD program with the built-in CalculiX solver, which does not require programming skills. A way to parameterize the task using the Python interpreter built into FreeCAD is also proposed. The simulation results obtained using both approaches are compared for an object to which an external action is applied, determined by the Dirichlet or Neumann boundary conditions, and two types of object fastening are analyzed: rigid embedding and limitation by a plane with zero friction. The analysis of the use of computing resources by various direct and iterative methods is carried out. Within the framework of the considered test problem of static linear elasticity, the most optimal method in FreeFem++ is the iterative method of conjugate gradients CG both in terms of computation time and in terms of the memory used. The highest speed of calculations is provided by the Cholesky iterative method with conditioning by the incomplete Cholesky expansion in the CalculiX program.

Keywords

static elasticity,
free engineering software,
FreeFem++,
Gmsh,
FreeCAD,
CalculiX,
direct and iterative solution methods of the system of linear equations

Article outline

The paper discusses the stages of computer numerical modeling of engineering problems and related programs (preprocessor, solver, and postprocessor), features of direct and iterative methods for solving of linear equations system (LES) and ways to improve the accuracy of modeling using the finite element method; provides a brief overview of free programs for modeling elasticity problems by the finite element method, as well as trends in the development of free computer-aided design and engineering analysis.

For successful engineering research, it is necessary to choose a convenient tool that takes into account all the features of the problem being solved. Based on the solution of the test static problem of linear elasticity, two approaches to engineering modeling were demonstrated. The first approach requires programming skills, and the full simulation cycle is written in the FreeFem++programming language. Additionally, the method of creating a calculated grid in the Gmsh program and then using it in the FreeFem++program is shown. In the second approach, the full simulation cycle is carried out through the interface of the FreeCAD program with the built-in CalculiX solver, no programming skills are required. A method for parameterizing the problem using the Python interpreter built into FreeCAD is also proposed. In the FreeFem++ program, the simulation was carried out using the following direct and iterative methods for solving LES: LU-decomposition, the Crout algorithm, the Cholesky decomposition; UMFPACK (Unsymmetric MultiFrontal method), the CG conjugate gradient method, and the generalized minimal residual method GMRES. The CalculiX program used a direct method for equations with sparse SPOOLS matrices and an internal iterative method with preconditioning by scaling the diagonal elements of the LES matrix ITERATIVE SCALING and with preconditioning with incomplete Cholesky decomposition ITERATIVE CHOLESKY.

The results of modeling obtained using both approaches are compared for an object to which an external influence is applied, determined by the Dirichlet or Neumann boundary conditions, and two types of fixing of the object are analyzed: rigid sealing and restriction by a plane with zero friction. The analysis of the use of computing resources by various direct and iterative methods is carried out. Iterative methods for solving LES for the problem under consideration have demonstrated a higher computational speed compared to direct methods. This is due to the fact that the three-dimensional problem was solved, and even for first-order elements, the number of equations in LES is significant. In the framework of the considered test problem of static linear elasticity, the most optimal method in FreeFem++ is the iterative method of conjugate gradients CG both in terms of calculation time and the amount of memory used. The highest calculation speed is provided by the Cholesky iterative method with the condition of incomplete Cholesky decomposition in the CalculiX program. It is shown that at the same computational cost, the accuracy of calculations for second-order elements on a coarse mesh is higher than for first-order elements on a denser mesh.

Based on the analysis of the computational capabilities of the programs, their functionality and ease of use, we will identify the features that can affect the choice of a numerical simulation program. Features of FreeFem++: programming skills are required; pre/postprocessor functions are available (creating geometry 2D/3D and generating mesh; loading a mesh created in Gmsh; visualization of calculation results); available direct methods use more computational resources compared to CalculiX; iterative methods are used with the ability to connect custom preconditioners; FreeFem++ allows you to create, rebuild, and adapt computational grids during code execution, perform full parameterization of the task, run a series of calculations based on sets of parameters, and process the results of calculations. Features of FreeCAD / CalculiX: programming skills are not required; FreeCAD / CalculiX allows you to conduct a full cycle of research within a single program, as well as parameterization of the problem in the Python programming language; as a separate program, it contains a pre/postprocessor (creating a geometry and mesh from a command file, visualizing data from a calculation results file); it has a fast and cost-effective direct SPOOLS method (compared to direct FreeFem++ methods); it has fast iterative methods with preconditioning.

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