FEM ConstraintDisplacement: Difference between revisions

From FreeCAD Documentation
(Grammar)
mNo edit summary
Line 54: Line 54:
* the prefix ''Variable'' specifies that the displacement is not a constant but a variable
* the prefix ''Variable'' specifies that the displacement is not a constant but a variable
* the variable is the current time
* the variable is the current time
* the displacement values are returned as a ''Real'' (floating point value)
* the displacement values are returned as ''Real'' (floating point) values
* ''MATC'' is the prefix for the Elmer solver that the following code is a formula
* ''MATC'' is the prefix for the Elmer solver that the following code is a formula
* ''tx'' is always the name of the variable in ''MATC'' formulas, no matter that ''tx'' is in our case actually ''t''
* ''tx'' is always the name of the variable in ''MATC'' formulas, no matter that ''tx'' is in our case actually ''t''

Revision as of 10:40, 1 April 2023

FEM ConstraintDisplacement

Menu location
Model → Mechanical Constraints → Constraint displacement
Workbenches
FEM
Default shortcut
None
Introduced in version
-
See also
FEM tutorial

Description

Creates a FEM constraint for a prescribed displacement of a selected object for a specified degree of freedom.

Usage

  1. Either press the button FEM FEM_ConstraintDisplacement or select the menu Model → Mechanical Constraints → Constraint displacement.
  2. In the 3D view select the object the constraint should be applied to, which can be a vertex (corner), edge, or face.
  3. Press the Add button.
  4. Uncheck Unspecified to activate the necessary fields for edition.
  5. Set the values or (introduced in version 1.0) specify a formula for the displacements.

Formulas

introduced in version 1.0

General

For the solver Elmer it is possible to define the displacement as a formula. In this case the solver sets the displacement according to the give formula variable.

Take for example the case that we want to perform a transient analysis. For every time step the displacement should be increased by 6 mm:

enter this in the Formula field:
Variable "time"; Real MATC "0.006*tx"

This code has the following syntax:

  • the prefix Variable specifies that the displacement is not a constant but a variable
  • the variable is the current time
  • the displacement values are returned as Real (floating point) values
  • MATC is the prefix for the Elmer solver that the following code is a formula
  • tx is always the name of the variable in MATC formulas, no matter that tx is in our case actually t

Rotations

Elmer only uses the Displacement * fields of the constraint. To define rotations, we need a formula.

For example a face should be rotated according to this condition:

then we need to enter for Displacement x
Variable "time, Coordinate" Real MATC "(cos(tx(0)*pi)-1.0)*tx(1)-sin(tx(0)*pi)*tx(2)

and for Displacement y
Variable "time, Coordinate" Real MATC "(cos(tx(0)*pi)-1.0)*tx(2)+sin(tx(0)*pi)*tx(1)

This code has the following syntax:

  • we have 4 variables, the time and all possible coordinates (x, y z)
  • tx is a vector, tx(0) refers to the first variable, the time while tx(1) is the first coordinate x
  • pi denotes and was added so that after a rotation of 180° is performed

Notes

For the solver CalculiX: