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FEM_ConstraintDisplacement.md

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GuiCommand
Name MenuLocation Workbenches Shortcut SeeAlso
FEM ConstraintDisplacement
Model , Mechanical Constraints , Constraint displacement
FEM_Workbench
FEM_tutorial

FEM ConstraintDisplacement

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 ((v0.21) ) specify a formula for the displacements.

Formulas

(v0.21)

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 given formula variable.

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

$\quad d(t)=0.006\cdot t$

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 a prefix for the Elmer solver indicating that the following code is a formula
  • tx is always the name of the variable in MATC formulas, no matter that tx in our case is actually t

Rotations

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

If for example a face should be rotated according to this condition:

$\quad \begin{align} d_{x}(t)= & \left(\cos(\phi)-1\right)x-\sin(\phi)y\ d_{y}(t)= & \left(\cos(\phi)-1\right)y+\sin(\phi)x \end{align}$

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) refers to the first coordinate x
  • pi denotes $\pi$ and was added so that after $t=1\rm, s$ a rotation of 180° is performed

Notes

For the solver CalculiX:

{{FEM Tools navi}}

⏵ documentation index > FEM > FEM ConstraintDisplacement