Grid-Based Boundary Conditions

Introduction

Grid-based boundary conditions are used to create fixed edges, to create moving edges that displace the object, and for setting boundary temperature or concentration. Fixed edges are created be setting velocity to zero. If the edge is a symmetry plane in the object, however, it is better to create the symmetry plane boundary conditions automatically rather then to set them manually with boundary condition commands. The use of a moving edge to apply displacements is limited to small displacements because in large-displacement problems, the particles may move away from the boundary conditions and the conditions will no longer have the correct influence. The alternative method for creating a moving edge is to use rigid material particles set to use the moving boundary condition mode. These particles create grid-based boundary conditions that move with the particle position. They can set velocity or moving temperature and concentration values. Besides grid-based conditions, simulations can set Particle-Based Boundary Conditions.

Grid-Based Boundary Condition in Scripted Files

All grid-based boundary conditions are created by a series of shape commands that select nodes on the grids. The shape commands define shapes (lines or arcs) and all nodes near those shapes are selected.. In scripted 2D simulations, the commands are

• MoveLine - select nodes along aline
• MoveArc - select nodes along and arc

In scripted 3D simulations, the command is:

• MoveBox - select nodes within a box or a cylinder

The commands within these shape commands are used to set velocity, temperature and/or concentration conditions.

Grid-Based Boundary Condition in XML Files

All grid-based boundary conditions must be set up within a single <GridBCs> element. The format is

<GridBCs>
   (one or more grid BC shape commands)
     ...
   <DisplacementBCs>
      (one or more explicit boundary conditions)
   </DisplacementBCs>
</GridBCs>

There are two ways to specify grid boundary conditions. The most common approach is to generate boundary conditions using one or more shape commands to select nodes and assign specified velocity, temperature, and/or concentration conditions to those nodes. The other way is to explicitly list each grid point. The explicit method is shown in the <DisplacementBCs> section above; it is limited to velocity conditions, and is usually generated with other software. You can use both shape commands and a <DisplacementBCs> section in the same input file.