Prescribed Displacement

The node adds a condition where the displacements are prescribed in one or more directions to the geometric entity (domain, boundary, edge, or point). It is also possible set maximum and minimum limits for the displacements, so that for example a one-sided support can be modeled.

If a displacement is prescribed in one direction, this leaves the structure free to deform in the orthogonal directions.


	If a zero displacement is applied in all directions, this is the same as a Fixed Constraint.


	For details, see Prescribed Displacements, Velocities, and Accelerations.

Shell Properties


	This section is only present in the Layered Shell interface, where it is described in the documentation for the Prescribed Displacement node.

Interface Selection


	This section is only present in the Layered Shell interface, where it is described in the documentation for the Prescribed Displacement, Interface node.

Coordinate System Selection

The constraints are applied in the axis directions of the coordinate system selected in this section. You should only use a coordinate system that has fixed spatial directions.

Prescribed Displacement

For the displacement in each direction, select a setting from the list — , , or . Select:

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(the default) to leave the displacement component unconstrained.

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to constrain the displacement component to a given value. Enter a scalar value for the component of the prescribed displacement u0.

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to set a maximum and a minimum limit for the displacement component. Enter values for the maximum displacement u0,max and the minimum displacement u0,min. By default, they are set to Inf and -Inf, which corresponds to no active constraint.

If any displacement component is set to , an additional section is visible. Select the used to implement the weak inequality constraint — or . For both methods, enter a kp.

By default, the method is suggested, which in principle enforces the maximum and minimum limits for the displacement by adding nonlinear springs with a stiffness equal to kp when the limits are exceeded. This method is usually robust, but the accuracy is directly dependent on the chosen penalty factor.

The method adds extra degrees of freedom to improve the accuracy of the constraint. Here, the penalty factor is a numerical parameter, and has less impact on the accuracy of the constraint compared to when using the penalty method. The implementation of the augmented Lagrangian method puts no restrictions on the solver sequence, but for good convergence, proper scaling of the extra degrees of freedom can be important.

The default value for the kp depends on the physics interface, and on what type of entity the node is added. In the expressions below, <phys> is the tag of the physics interface, and <pd> is the tag of the node.

Solid Mechanics, Multibody Dynamics, Layered Shell, and Membrane

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For points, the default expression is 100*<phys>.Eequ*<phys>.<pd>.charLen

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For edges, the default expression is 100*<phys>.Eequ

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For boundaries, the default expression is 100*<phys>.Eequ/<phys>.<pd>.charLen

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For domains, the default expression is 100*<phys>.Eequ/<phys>.<pd>.charLen^2

Above, the terms point, edge, boundary, and domain refer to the actual physical entities represented by the chosen geometric entities. For instance, in a 2D solid the out-of-plane thickness is only implicitly modeled, and thus the physical entity level is interpreted as one level higher than the chosen geometric entity level.

The variable <phys>.<pd>.charLen is by default equal to the length of the mesh element for the Solid Mechanics and Multibody Dynamics interfaces. For the Layered Shell and Membrane interfaces, <phys>.<pd>.charLen is the thickness.

Truss

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For points, the default expression is 100*truss.Eequ*truss.area/truss.<pd>.charLen

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For edges, the default expression is 100*truss.Eequ*truss.area/truss.<pd>.charLen^2

The variable truss.<pd>.charLen is by default equal to the length of the mesh element.

Wire

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For points, the default expression is 100*wire.k_A/wire.<pd>.charLen

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For edges, the default expression is 100*wire.k_A/wire.<pd>.charLen^2

The variable wire.<pd>.charLen is by default equal to the length of the mesh element.


	For details about setting maximum and minimum limits for the displacements, see Limited Displacement


	In versions prior to 6.2, you could chose to formulate the constraint in two ways: standard notation or general notation. Since it was rarely used, the general notation option is no longer supported. There are more natural ways of specifying identical constraints. If you have a model created in version 6.1 or earlier, where general notation was used, the settings for this option are visible, and are still functional. You should, however, consider to reformulate the constraints, since the general notation may no longer be supported in future versions. For reference, the documentation for the general notation is provided below: You can specify the displacements using a general notation that includes any linear combination of displacement components. For example, for 2D components, use the relationship For H matrix H select Isotropic, Diagonal, Symmetric, or Full and then enter values as needed in the field or matrix. Enter values or expressions for the R vector R. For example, to achieve the condition u = v, use the settings , which force the domain to move only diagonally in the xy-plane.

Constraint Settings

To display this section, click the button (

) and select in the dialog box.

In the COMSOL Multiphysics Reference Manual:

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Constraint Reaction Terms

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Weak Constraints

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Constraint Settings

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Excluded Surfaces, Excluded Edges, and Excluded Points


	When Individual dependent variables is selected in the Apply reaction terms on list, the constraint forces are applied directly on the degrees of freedom, which are the displacements along the global coordinate axes. If you use this setting together with a local coordinate system, the results will be inconsistent since the constraint forces will not match the constraint orientation.

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You can add a Harmonic Perturbation subnode for specifying a harmonic variation of the values of the prescribed displacements in a frequency domain analysis of perturbation type.

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You can activate and deactivate this boundary condition by assigning it to a constraint group.

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You can assign the value of the prescribed displacement to a load group. See Load Cases in the Structural Mechanics Modeling chapter.

Location in User Interface

Context Menus

Ribbon

Physics tab with selected:

Physics tab with node selected in the model tree: