FEM SolverElmer SolverSettings

This page describes the possible settings for solver Elmer.

General

Elmer is a multiphysics solver. Therefore you can use several main equations to solve problems. The different equations are listed here.

There are solver settings, available for all equations. These are described here. Settings only available for a particular equation are described in the pages of the corresponding equation.

Elmer offers the solving types steady-state and transient and two main solving systems, linear system and nonlinear system. The nonlinear system is used for the Flow equation and Heat equation.

Editing Settings

The solver settings can be found in the property editor after clicking on an equation in the tree view. You can edit them there directly like any other property.

Solver

Coordinate System

The default coordinate system is Cartesian 3D. For some equations, not all coordinate systems can be can be used. This is noted on the Wiki pages of the corresponding equations.

Timestepping (transient analyses)

Note: FreeCAD 0.20.x already provides the following settings but only the last time result is output. Starting with FreeCAD 0.21 you will get an output for the different times.

For transient analyses the time steps need to be defined. This is done by the following settings:

• BDFOrder: Order for the method BDF (Backward Differentiation Formula). It is recommended to use the default of 2.

• Output Intervals: An array of intervals. A solver result file will be output every interval time step. For example, if a result file should be output every third time step, set it to 3. The array corresponds to the Note: The first result in every case will be created for the first time step. To get for example results after 25 % of the total time and if the last result should be the final time, set Output Intervals to 5 and Timestep Intervals to 21. (v0.21)

• Timestep Intervals: An array of time intervals. The solver will perform one time interval after another. For example, if the solver should calculate the first 10 seconds in steps of 0.1 second, then 50 seconds in steps of 1 second and then stop, you need to set the timestep intervals [100, 50] and the timestep size intervals [0.1, 1.0].

• Timestep Sizes: An array of timestep sizes. The time unit is second. The array corresponds to the Timestep Intervals.

Note: Although the terms \"times\" and \"seconds\" are used the times are actually solver progressions if the analysis is not time-dependent.

For how to visualize the results, see the Elmer visualization info.

Type

• Simulation type: If the simulation is Steady state, Transient or just Scanning. Transient means the development over the solver time is calculated. See section Timestepping for the necessary settings.

• Steady State Max Iterations: The maximum number of steady-state solver runs.

• Steady State Min Iterations: The minimum number of steady-state solver runs.

Equation

Base

All equations have these properties:

• Label: Name of the equation in the tree view.

• Priority: Number determining the priority of this equation to the other equations in the analysis. The equation with the highest number in the analysis will be solved as first. If two equations have the same priority number, the one that is first in the tree view will be solved first.

• Stabilize: If set to true, the solver will use the stabilized finite element method when solving the heat equation with a convection term. If set to false, the Residual Free Bubble (RFB) stabilization is used instead. If convection dominates, stabilization must be used to successfully solve the equation.

Linear System

This system has the following properties:

• BiCGstabl Degree: Polynomial degree for the iterative solver method BiCGStabl. This has only an effect if Linear Solver Type is Iterative and Linear Iterative Method is BiCGStabl. Starting with the default of 2 is recommended.

• Idrs Parameter: Parameter for the iterative solver method Idrs. This has only an effect if Linear Solver Type is Iterative and Linear Iterative Method is Idrs. Starting with the default of 2 is recommended. Setting the parameter to 3 might increase the solving speed a bit. For flow analyses the Idrs method is up to 30 % faster than the default BiCGStab method.

• Linear Direct Method: Method used for direct solving. This has only an effect if The possible methods are Banded, MUMPS and Umpfpack. Note that MUMPS usually needs to be installed before you can use it.Note: when you use more than one CPU core for the solver ((v0.21) ) only MUMPS can be used. MUMPS has to be installed manually to Elmer. It is only available as a download per request via email.

• Linear Iterations: Maximal number of iterations for an iterative solver run. This has only an effect if Linear Solver Type is Iterative.

• Linear Iterative Method: Method used for iterative solving. This has only an effect if Linear Solver Type is Iterative.

• Linear Preconditioning: Method used for the preconditioning. For info about preconditioning, see this presentation (page 8) from Elmer.

• Linear Solver Type: If the solving is done Direct or Iterative.

• Linear System Solver Disabled: Disables the linear solver. Only use this for special cases.It can be used to disable temporarily an equation since its solving is then not performed. There are, however cases where the solver is sent into an infinite loop instead.

• Linear Tolerance: The tolerance for the solver to stop. If the error is smaller than the tolerance, the solver run will be finished. Otherwise, the full number of In the Elmer solver log you see how the error is minimized while the solver is running. (Look in the log at the end of every solver iteration for the value behind Relative Change). If it does not go down below a certain value but reaches a value above the current tolerance that is acceptable for you, you can increase the tolerance.

Nonlinear System

This system is iterative and has the following properties:

• Nonlinear Iterations: Maximal number of iterations.

• Nonlinear Newton After Iterations: The nonlinear solver starts with the robust Picard algorithm. After some iterations, the algorithm is changed to the Newton algorithm which converges faster but is less robust if the results temporarily diverge (oscillations might occur). This setting sets the number of iterations after which the switch from the Picard to the Newton algorithm is made.Note: the switch is made whatever is reached first, Nonlinear Newton After Iterations or Nonlinear Newton After Tolerance.

• Nonlinear Newton After Tolerance: The same as Nonlinear Newton After Iterations but here a tolerance is set. The tolerance is the norm of the nonlinear residual. If this is reached, the switch from the Picard to the Newton algorithm is made.

• Nonlinear Tolerance: The tolerance for the solver to stop. If the error is smaller than the tolerance, the solver run will be finished. Otherwise, the full number of In the Elmer output you see how the error is minimized while the solver is running. If it does not go down below a certain value that is acceptable but above the current tolerance, you can increase the tolerance.

• Relaxation Factor: This is THE most important setting in case the solver does not converge:

Relaxation Factor

If the solver iteration results oscillate numerically, the solver results cannot converge to a final, stable value. To avoid that, the calculated variable $T{i}$ of the i-th iteration/solver run is not taken as input for the next iteration, but $T{i}^{'}$, a value that is \"damped\" with the result from the previous iteration. The relaxation factor $\lambda$ is thereby defined as

$\quad T{i}^{'} = \lambda T{i}+\left(1-\lambda\right)T_{i-1}$

So for the default of 1.0, no damping is used. The smaller $\lambda$, the greater the damping and the longer the convergence time. Therefore if the solver does not converge, start changing the relaxation factor to 0.9, then to 0.8 and so on. Values below 0.3 are unusual and if you need this, you should have a closer look to the math of your analysis. For cases, where you get a proper convergence you can set $\lambda$ above 1.0 to speed the convergence up.

Steady State

This part of the settings has only one property:

• Steady State Tolerance: The specific steady state or coupled system convergence tolerance. All the equation solvers must meet their own tolerances for the variable $\omega^2$ they calculate before the whole system is deemed converged. The tolerance criterion is:

$\quad \left\Vert u{i}-u{i-1}\right\Vert <\epsilon\left\Vert u_{i}\right\Vert$

whereas $\epsilon$ is the steady state tolerance and $u_{i}$ is the calculated variable in the i-th iteration/solver run.

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