Using Restraints

Displacements/Accelerations/Velocities

A CUBIT user has the ability to create displacement boundary conditions on most geometric entities found within Cubit.

Create Displacement [id] [Name <'name'>] [{Add|On} {Nodeset|Volume|Surface|Curve|Vertex|Hex|Tet|Face|Tri|Edge|Node} <entity_list>] [DOF {All|{[1][2][3][4][5][6]}} Fix <value>] [SmallestCombine|Average|LargestCombine|OVERWRITE]

Modify Displacement {id_list|'name'|all} [name <'name'>] [{Add|Remove} {Nodeset|Volume|Surface|Curve|Vertex|Hex|Tet|Face|Tri|Edge|Node} <entity_list>] [DOF {All|{[1][2][3][4][5][6]}} {Fix <value>|Free}] [SmallestCombine|Average|LargestCombine|OVERWRITE]

Create Acceleration [id] [Name <'name'>] [{Add|On} {Nodeset|Volume|Surface|Curve|Vertex|Hex|Tet|Face|Tri|Edge|Node} <entity_list>] [DOF {All|{[1][2][3][4][5][6]}} Fix <value>] [SmallestCombine|Average|LargestCombine|OVERWRITE]

Modify Acceleration {id_list|'name'|all} [name <'name'>] [{Add|Remove} {Nodeset|Volume|Surface|Curve|Vertex|Hex|Tet|Face|Tri|Edge|Node} <entity_list>] [DOF {All|{[1][2][3][4][5][6]}} {Fix <value>|Free}] [SmallestCombine|Average|LargestCombine|OVERWRITE]

Create Velocity [id] [Name <'name'>] [{Add|On} {Nodeset|Volume|Surface|Curve|Vertex|Hex|Tet|Face|Tri|Edge|Node} <entity_list>] [DOF {All|{[1][2][3][4][5][6]}} Fix <value>] [SmallestCombine|Average|LargestCombine|OVERWRITE]

Modify Velocity {id_list|'name'|all} [name <'name'>] [{Add|Remove} {Nodeset|Volume|Surface|Curve|Vertex|Hex|Tet|Face|Tri|Edge|Node} <entity_list>] [DOF {All|{[1][2][3][4][5][6]}} {Fix <value>|Free}] [SmallestCombine|Average|LargestCombine|OVERWRITE]

A number of required and optional keywords make the BC create displacement command one of the more complicated of the boundary condition commands. These keywords will be examined individually in detail.

Degrees of Freedom

The dof keyword is the heart of this command. It specifies how to constrain the entity in question. The keyword is an abbreviation for ‘degree of freedom’. Typing the optional keyword all tells CUBIT that the entered command will encompass all six degrees of freedom. The degrees of freedom (1 - 6) are defined below in Table 2.

Table 2: CUBIT definitions of the six degrees of freedom.

DOF Physical analog
1 x-translation
2 y-translation
3 z-translation
4 x-rotation
5 y-rotation
6 z-rotation

CUBIT will allow displacement commands to be applied upon between one and all six of the degrees of freedom. The degrees of freedom do not need to be entered in any order. The command strings ‘ 1 2 3 4 5 6 ‘ ‘2 6 1 4 3 5’ and ‘all’ will end with the same result.

Fixed or Free

The fix and free keywords tell CUBIT whether an entity’s displacement defined by the dof keyword is to be enforced with a finite value or not. If the displacement is fixed, the entity will be constrained in the pre-specified degrees of freedom. A decimal number entered after the fix keyword will be the value of the enforced degree(s) of freedom. CUBIT allows the user to leave this value blank if the enforced displacement is to be zero, for convenience. However, entering ‘0’ is still permitted. If a user wishes to remove a displacement from an entity, he or she should just delete it rather than trying to set all of the degrees of freedom to free.

Displacement Combinations

The SmallestCombine, Average and LargestCombine keywords deal with displacement combinations. These keywords only apply when a user is modifying an existing displacement boundary condition.

The SmallestCombine keyword will compare the existing displacement values with the current (residing on the command line) displacement values. The keyword will modify the displacement to the match the displacements dictated by the boundary condition that has the smallest absolute value. If the boundary condition with the smallest absolute value is the existing value, the displacement boundary condition will be unchanged. If the current boundary condition has a smaller absolute value than the existing displacement, the displacement boundary condition will be changed to incorporate the new values.

The Average keyword will average the existing displacement values with the current (residing on the command line) displacement values. Note that these averages are not continually updated (i.e., they are not weighted). If a user created a displacement boundary condition and constrained a degree of freedom to 10.0 and then constrained the same degree of freedom to 20.0 with the Average keyword, the new displacement value would be 15.0. But if a user constrained the same degree of freedom to 30.0, while using the Average keyword, the new displacement value would be 22.5 ([15+30]/2), not 20.0 ([10+20+30]/3).

The LargestCombine keyword will compare the existing displacement values with the current (residing on the command line) displacement values. The keyword will modify the displacement to the match the displacements dictated by the boundary condition that has the largest absolute value. If the boundary condition with the largest absolute value is the existing value, the displacement boundary condition will be unchanged. If the current boundary condition has a larger absolute value than the existing displacement, the displacement boundary condition will be changed to incorporate the new values.

When none of these keywords are specified, CUBIT will combine displacements in its default mode, Overwrite. The Overwrite keyword overwrites the entity’s previous displacement boundary condition(s) with the displacement values specified in the command.

Temperature

CUBIT can create temperature boundary conditions on most geometric and mesh entities. The temperature boundary condition can also be applied to thin-shell elements.

Create Temperature [id] [Name <'name'>] [{Add|On} {Nodeset|Volume|Surface|Curve|Vertex|Hex|Tet|Face|Tri|Edge|Node} <entity_list>] [Value <val>]

Create Temperature [id] [Name <'name'>] [{Add|On} {Nodeset|Volume|Surface|Curve|Vertex|Hex|Tet|Face|Tri|Edge|Node} <entity_list>] [{ Top <val> Bottom <val> | [Middle <val>] [Gradient <val>] } ]

Modify Temperature {id_list|'name'|all} [name <'name'>] [{Add|Remove} {Nodeset|Volume|Surface|Curve|Vertex|Hex|Tet|Face|Tri|Edge|Node} <entity_list>] [Value <val>]

Modify Temperature {id_list|'name'|all} [name <'name'>] [{Add|Remove} {Nodeset|Volume|Surface|Curve|Vertex|Hex|Tet|Face|Tri|Edge|Node} <entity_list>] [{ Top <val> Bottom <val> | [Middle <val>] [Gradient <val>] } ]

The value keyword defines the amplitude (temperature). The other command options are discussed below

Top, Gradient, Middle, Bottom

The above keywords are only used for thin-shell elements (i.e., 2D entities). The valid combinations are limited to: top and bottom, middle and gradient, only gradient or only middle. It should be noted that temperature boundary conditions cannot contain regular and thin-shell temperature values.