FEM EquationFlow

From FreeCAD Documentation

FEM EquationFlow

Menu location
Solve → Flow equation
Default shortcut
Introduced in version
See also

This equation calculates viscous fluid flows using the Navier-Stokes equations.

For info about the math of the equation, see the Elmer models manual, section Navier-Stokes Equations.


  1. After adding an Elmer solver as described here, select it in the tree view.
  2. Now either use the toolbar button or the menu Solve → Flow equation.
  3. Change the equation's solver settings or the general solver settings if necessary.

Solver Settings

For the general solver settings, see the Elmer solver settings.

The flow equation provides these special settings:

  • DataDiv Discretization: To be set to true for incompressible flow for more stable discretization when the Reynolds number increases.
  • DataFlow Model: The flow model that should be used. The default Full includes convection and time derivative terms in the model. No convection switches off the convection terms and the Stokes model switches off the convection terms and the (explicit) time derivative terms.
  • DataGradp Discretization: If set to true pressure Dirichlet boundary conditions can be used. Also the mass flux is available as a natural boundary condition.
  • DataVariable: Optional only for calculations in 2D: You can change the default of 3 to 2.
    Note: In this case none of the flow velocity boundary conditions can have a specified z-component.


  • DataConvection: The type of convection to be used in the Heat equation.
    Note: For thermal flows it must be set to Computed (the default).
  • DataMagnetic Induction: If set to true the magnetic induction equation will be solved along with the Navier-Stokes equations.

Notes for Convergence

If the solver results do not converge, you can try these things (in the given order):

  1. Reduce the DataRelaxation Factor, see the nonlinear system settings.
  2. Increase the value for DataNonlinear Newton After Iterations, see the nonlinear system settings.
  3. Reduce the number of CPU cores used, see the FEM preferences.
  4. Increase the mesh density (make it more fine).

Analysis Feature Information

The flow equation takes the following analysis features into account if they are set:


  • Except for calculations in 2D, for all above boundary conditions it is important that they act on a face or body. Boundary conditions for 3D set to lines or vertices are not recognized by the Elmer solver.
  • Since Pressure load can only be set to faces, pressure loads cannot be used for calculations in 2D.
  • If there is no Pressure load set, Initial pressure condition will only be taken into account if DataGradp Discretization is set to true.


The results are the velocity in and the pressure in . If there is no Initial pressure condition and Pressure load given, the resulting pressure will be relative not absolute. Since pressure must act on a face, absolute pressure results cannot be obtained in 2D simulations.