FEM EquationFlow

    From FreeCAD Documentation

    FEM EquationFlow.svg FEM EquationFlow

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

    This equation calculate 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 FEM EquationFlow.svg 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 constraints can have a specified z-component.


    • DataConvection: The type of convection to be used in the FEM EquationHeat.svg 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).

    Constraint Information

    The electrostatic equation takes the following constraints into account if they are set:


    • Except for calculations in 2D, for all above constraints it is important that they act on a face or body. Constraints for 3D set to lines or vertices are not recognized by the Elmer solver.
    • Since FEM ConstraintPressure.svg Constraint pressure can only be set to faces, pressure constraints cannot be used for calculations in 2D.
    • If there is no FEM ConstraintPressure.svg Constraint pressure set, FEM ConstraintInitialPressure.svg Constraint initial pressure 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 FEM ConstraintInitialPressure.svg Constraint initial pressure and FEM ConstraintPressure.svg Constraint pressure constraint given, the resulting pressure will be relative not absolute. Since a pressure must act on a face, absolute pressure results cannot be obtained in 2D simulations.