Grid-Based Boundary Conditions

Grid-based boundary conditions are applied to nodes on the grid and can set velocity, temperature, and concentration.

Introduction

Grid-based boundary conditions are used for fixed edges, for applying displacements with moving edges, 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, it is better to create the symmetry plane boundary conditions automatically rather then to set them with boundary condition commands. The use of a moving edge to apply displacements, however, 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.

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 boundary condition 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. This 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.

Notes

1. You can apply more than one condition to a node and the resulting boundary condition will be a superposition of all assigned conditions. When applying multply velocity conditions, the combinations must either be in the same direction or in orthogonal directions. For example, when using skewed boundary conditions, you should not apply some in the x direction and others in the skewed x-y or x-z direction, because these two directions are not orthongal. You could apply some in x and some in y direction or some in two different skewed directions whose normals are othogonal.
2. Grid boundary conditions are typcially placed on grid elements, on, or outside the body defined by the material points. When using GIMP methods, a particle near the boundary will interact with the boundary conditions, but may also interact with nodes beyond a single row of boundary conditions. To prevent non-physical results from the extra nodes that are physcially outside the body, it may be necessary to set up two rows (or columns) of boundary conditions. For example, to have a rigid wall, set the velocity of two rows (or columns) or nodes to be zero. An alternative solution is to fill a boundary area with rigid particles. Rigid particle automatically create the required boundary conditions on two rows (or columns) of nodes.