# 7. Boundary Conditions and Initial Conditions

Sierra/SM offers a variety of options for defining boundary and initial conditions. Typically, boundary and initial conditions are defined on some subset of mesh entities (node, element face, element) defining a model. Sierra/SM offers a flexible means to define subsets of mesh entities. Section 7.2 describes commands that will let the user define some subset of a mesh entity using a collection of commands that constitute a set of Boolean operators.

The remaining parts of this chapter discuss the following functionality:

- Section 7.3 presents methods for setting the initial values of variables in Sierra/SM. Sierra/SM has the flexibility to set a complex initial state for some variable such as nodal velocity or element stress.

- Kinematic boundary conditions typical of a solid mechanics code (fixed displacement, prescribed acceleration, etc.) are options in Sierra/SM and described in Section 7.4. Most of these boundary conditions allow the specification of a time history using a function, a user subroutine, or by reading values from a mesh file.

- Section 7.5 documents many initial velocity options available in Sierra/SM.

- Force boundary conditions typical of a solid mechanics code (prescribed force, traction, etc.) are options in Sierra/SM and described in Section 7.7. Most of these force boundary conditions allow the specification of a time history using a function or a user subroutine.

- Section 7.9 discusses the gravity load option. A gravity load is a body force boundary condition.

- Section 7.10 provides a description of centripetal force boundary conditions can be applied about a prescribed axis.

- Section 7.12 details many options available for describing a temperature field in Sierra/SM.

- Section 7.13 details the options available for describing a pore pressure field.

- Section 7.14 documents how fluid pressure boundary conditions can be applied as a hydrostatic pressure to each node pertaining to a set of prescribed surfaces.

- Section 7.15 documents how the inertia relief boundary condition can be applied to counterbalance the other force boundary conditions on the specified blocks, surfaces, and/or nodesets.

- Section 7.16 describes many specialized boundary conditions.