Auf dieser Seite werden die möglichen Einstellungen für den Löser Elmer beschrieben.

Allgemein

Elmer ist ein multiphysikalischer Löser. Daher können mehrere Hauptgleichungen zur Lösung von Problemen verwendet werden. Die verschiedenen Gleichungen sind hier aufgelistet.

Es gibt Löser-Einstellungen, die für alle Gleichungen verfügbar sind. Diese werden hier beschrieben. Einstellungen, die nur für eine bestimmte Gleichung verfügbar sind, werden auf den Seiten der entsprechenden Gleichung beschrieben.

Elmer bietet die Lösungstypen stationärer Zustand und vorrübergehend und zwei weitere Lösungssysteme, lineares System und nichtlineares System. Das nichtlineare System wird verwendet für die Strömungsgleichung and Wärmegleichung.

Einstellungen bearbeiten

Die Löser-Einstellungen findet man in dem Eigenschaften-Editor nachdem man auf eine Gleichung in der Baumansicht geklickt hat. Dort können sie wie jede andere Eigenschaft direkt bearbeitet werden.

Löser

Koordinatensystem

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.

Zeitschritte (Transienten-Analysen)

Hinweis: FreeCAD 0.20.x bietet bereits die folgenden Einstellungen, aber es wird nur das letzte Zeit-Ergebnis ausgegeben. Ab FreeCAD 0.21 erhält man eine Ausgabe für die verschiedenen Zeiten.

Für transiente Analysen müssen die Zeitschritte definiert werden. Dies erfolgt über die folgenden Einstellungen:

• DatenBDFOrder: Order for the method BDF (Backward Differentiation Formula). It is recommended to use the default of 2.
• DatenOutput 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 DatenTimestep Intervals.
  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 DatenOutput Intervals to 5 and DatenTimestep Intervals to 21. introduced in 0.21
• DatenTimestep Intervals: An array of time intervals (number of timesteps). 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].
• DatenTimestep Sizes: An array of timestep sizes. The time unit is second. The array corresponds to the DatenTimestep Intervals.

Notes:

• To achieve the desired total time of a transient analysis, set the proper number and size of timesteps. For example, to simulate 50 seconds of a process in steps of 10 seconds, you need 5 such timesteps. Thus, set Timestep Intervals to [5] and Timestep Sizes to [10].
• Although the terms "times" and "seconds" are used, the times are actually solver progressions if the analysis is not time-dependent.

Informationen zur Visualisierung der Ergebnisse findet man unter Elmer-Visualisierung.

Typ

• DatenSimulation 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.
• DatenSteady State Max Iterations: The maximum number of steady-state solver runs.
• DatenSteady State Min Iterations: The minimum number of steady-state solver runs.

Gleichung

Basis

All equations have these properties:

• DatenLabel: Name of the equation in the Tree View.
• DatenPriority: 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.
• DatenStabilize: 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.

Lineares System

This system has the following properties:

• DatenBiCGstabl Degree: Polynomial degree for the iterative solver method BiCGStabl . This has only an effect if DatenLinear Solver Type is Iterative and DatenLinear Iterative Method is BiCGStabl. Starting with the default of 2 is recommended.
• DatenIdrs Parameter: Parameter for the iterative solver method Idrs . This has only an effect if DatenLinear Solver Type is Iterative and DatenLinear 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.
• DatenLinear Direct Method: Method used for direct solving. This has only an effect if DatenLinear Solver Type is Direct.
  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 (introduced in 0.21) only MUMPS can be used. MUMPS has to be installed manually to Elmer. It is only available as a download per request via email.
• DatenLinear Iterations: Maximal number of iterations for an iterative solver run. This has only an effect if DatenLinear Solver Type is Iterative.
• DatenLinear Iterative Method: Method used for iterative solving. This has only an effect if DatenLinear Solver Type is Iterative.
• DatenLinear Preconditioning: Method used for the preconditioning. For info about preconditioning, see this presentation (page 8) from Elmer.
• DatenLinear Solver Type: If the solving is done Direct or Iterative.
• DatenLinear 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.
• DatenLinear 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 DatenLinear Iterations will be performed.
  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.

Nichtlineares System

Dieses System ist iterativ und hat die folgenden Eigenschaften:

• DatenNonlinear Iterations: Maximal number of iterations.
• DatenNonlinear 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, DatenNonlinear Newton After Iterations or DatenNonlinear Newton After Tolerance.
• DatenNonlinear Newton After Tolerance: The same as DatenNonlinear 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.
• DatenNonlinear 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 DatenNonlinear Iterations will be performed.
  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.
• DatenRelaxation Factor: This is THE most important setting in case the solver does not converge:

Entspannungsfaktor

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

${\displaystyle T_i^{\text{'}}=\lambda T_i+(1-\lambda )T_{i-1}}$

So for the default of 1.0, no damping is used. The smaller ${\displaystyle \lambda }$, 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 ${\displaystyle \lambda }$ above 1.0 to speed the convergence up.

Stationärer Zustand

This part of the settings has only one property:

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

${\displaystyle \Vert u_i-u_{i-1}\Vert <ϵ\Vert u_i\Vert }$

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