# FEM EquationFlow/ru

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Системное название FEM EquationFlow FEM EquationFlow Solve → Equation flow FEM Нет - FEM tutorial

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.

## Usage

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:

• ДанныеDiv Discretization: To be set to true for incompressible flow for more stable discretization when the Reynolds number increases.
• ДанныеFlow 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.
• ДанныеGradp Discretization: If set to true pressure Dirichlet boundary conditions can be used. Also the mass flux is available as a natural boundary condition.
• ДанныеVariable: 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.

Equation:

• ДанныеConvection: The type of convection to be used in the Heat equation.
Note: For thermal flows it must be set to Computed (the default).
• ДанныеMagnetic 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 ДанныеRelaxation Factor, see the nonlinear system settings.
2. Increase the value for ДанныеNonlinear 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:

### Notes

• 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 Constraint pressure can only be set to faces, pressure constraints cannot be used for calculations in 2D.
• If there is no Constraint pressure set, Constraint initial pressure will only be taken into account if ДанныеGradp Discretization is set to true.

## Results

The results are the velocity in ${\displaystyle {\rm {m/s}}}$ and the pressure in ${\displaystyle {\rm {Pa}}}$. If there is no Constraint initial pressure and 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.