SMS:Unstructured Grid Generation from a Conceptual Model

Traditionally, the most time consuming component of using a multi-dimensional hydrodynamic numerical model has been the generation of unstructured grids (also called meshes). This effort has given models based on Cartesian grids (structured grids) a decided simplifying advantage. Digitizing node points and connecting them into elements, while seemingly not a complicated process, becomes overwhelming when you consider the number of nodes and elements that compose a numeric simulation (thousands to even millions and the number is still growing).

The SMS interface includes the capability to define a 2D mesh using the feature objects in a 2D Mesh Coverage. When the Map → 2D Mesh command is selected, the 2D Mesh Options dialog opens. A meshing polygon must have been defined prior to issuing this command. The attributes of the meshing polygon(s) are used to generate the 2D mesh.

The meshing options are used with coverages that generate meshes for specific numeric engines. Some of the options may not be available for all coverage types since some models have specific requirements such as a limited number of supported element types.

Feature Polygon attributes

The process of generating a mesh involves filling the polygons in the coverage with elements. These elements can be triangular or quadrilateral depending on the numeric engine they will be used with. The user specifies how the polygons will be filled choosing from the following "Mesh Type" options:

• None – there will be no elements in this polygon. This will represent an island in the domain.
• Patch – the mesh is topologically a triangle or rectangle that will be filled with elements that conform to its sides.
• Paving – the mesh will be filled with elements by offsetting from the boundaries. The distribution of the vertices on the arcs comprising the polygon control the mesh density.
• Scalar Paving Density – the mesh will be filled with elements using the paving approach, but the distribution of vertices along the arcs, and throughout the interior, will be controlled by a scalar dataset specifying a target edge length.
• Existing Nodes – the polygon will be used as a stencil to keep the mesh nodes and elements already in that region.

In addition to specifying the method to fill the polygon, the user also specifies the source of bathymetric or topographic elevation data for the newly constructed mesh using the "Bathymetry Type" controls. The user can choose from:

• Constant – all newly created nodes will be assigned a single specified value.
• Scatter Set – all newly created nodes will be assigned a value based on interpolation from a scatter set or TIN.
• Raster – all newly created nodes will be assigned a value based on interpolation from a Raster object.
• Exiting Mesh – all newly created nodes will be assigned a value based on interpolation from the previously existing mesh.

The user also has control to specify the material type that will be assigned to newly created elements.

Feature Arc attributes

The feature arcs in the conceptual model serve three purposes.

1. They carry boundary condition attributes for the specific model or engine.
2. They control final mesh density for the paving or patch options.
3. They control detailed feature maintenance when they lie inside of paved polygons. An arc representing a thalweg (channel), crest, ridge, or shoulder will be incorporated into the meshing pattern to ensure that these features are maintained.

Feature Point attributes

Feature points can be included in polygons to carry a boundary condition such as a source or sink, or they can be used to control resolution in a specific area. The feature point can be assigned an attribute to be a refine point. In this case, the user specifies the size of element around that location. The mesh generation process will generate an element or cluster of elements at that location matching the specified size. These are then incorporated into the surrounding mesh using the advancing front paving method.

Feature points can also be assigned an attribute to control whether a mesh node will be incorporated into the mesh at the exact feature point location.