Using fields and expressions to define boundary conditions

Video: Assign a field to a boundary condition

You can use fields and expressions to define how boundary conditions such as loads and constraints vary with respect to:

• Spatial independent domains such as Cartesian, cylindrical, spherical, and parametric coordinates.

• Non-spatial independent domains such as time, temperature, and frequency.

• Independent domains that are combinations of spatial and non-spatial variables.

If you can describe how a boundary condition varies from a mathematical relationship, use a formula field or an expression. If you can describe how a boundary condition varies only from tabular data, use a table field, a table of fields, or a 4D field.

When deciding whether to use a formula field or an expression, consider the following:

• You can define spatial variables in an expression relative only to the global coordinate system. You can define spatial variables in a formula field relative to the global coordinate system and local coordinate systems.

• With formula fields, you are limited to a 4D independent domain. A 4D independent domain that consists of a set of coordinates and a single non-spatial variable. With expressions, there is no limit to the number of non-spatial variables that you can use in combination with a set of spatial variables.

• With formula fields, the dependent domain can be components of a vector. With expressions, the dependent domain is limited to a single quantity.

• You can create linked fields and table of fields from formula fields, but not from expressions.

From the dialog box of a boundary condition, you can create a field or expression, or you can select an existing field or expression.

• When you create a field from the dialog box of a boundary condition, the software predefines the dependent domain of the field to be the same as that of the boundary condition.

• When you create an expression from the dialog box of a boundary condition, you must define the dependent domain of the formula for the expression to be either the same as that of the boundary condition, or evaluate to be dimensionless. If the formula evaluates to be dimensionless, the software assigns the dependent domain of the boundary condition to the expression.

• When you select an existing field, you can select from only those fields that have a dependent domain that is the same as that of the boundary condition.

• When you select an existing expression, the software issues a dimension error if the dependent domain of the expression is not the same as that of the boundary condition.

For more information on using fields and expressions to define boundary conditions, see Using fields to define boundary conditions and Using expressions to define boundary conditions.

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