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Unstructured Quad/Hex Meshes and Mesh Sizing Sources Coming to Pointwise
The new meshing capabilities coming in the next major release of the Pointwise software are perhaps the most significant and impactful since unstructured meshing was added in 1998. New algorithms provide the rapid generation of quad-dominant unstructured surface meshes. These surface meshes can be used as the basis of a T-Rex (anisotropic tetrahedral extrusion) hybrid volume mesh resulting in boundary layers resolved by layers of unstructured hexahedral cells. Off-body flow features can be resolved using new shape-driven clustering sources. And unlike Pointwise's current capability, each hybrid mesh can be maintained in the software in its hybrid form for mesh quality metric examination and any other function.
Unstructured Quads and Hexes
A new quadrilateral -dominant meshing algorithm generates surface meshes consisting of mostly well aligned quadrilateral cells and does so with the same speed with which you are used to generating all triangular meshes. Figure 1 shows a simple, quad-dominant mesh for a golf club.
An ONERA M6 wing was also meshed using the quad-dominant algorithm as can be seen in Figure 2.
The surface meshes in Figure 2 were then used as input to Pointwise’s T-Rex algorithm to generate fully hybrid meshes with layers of unstructured hexahedral cells in the near-wall region. Relative to prismatic/tetrahedral hybrid meshes, hexahedral/tetrahedral meshes have a lower cell count, improved convergence, and more accurate results (see In Pursuit of the Ideal Mesh on our Another Fine Mesh blog). Unlike T-Rex meshes in previous Pointwise releases, the full hybrid mesh is available for you to Examine in Pointwise V18 (see Figure 3) where the cells are colored by type (hexahedra, tetrahedra, etc.).
Sources and Shapes
Local control over the cell sizes in a tetrahedral mesh away from the boundary layer regions are traditionally handled in Pointwise using a concept called baffles. A baffle is a piece of topology (for example, a surface mesh) that is embedded inside a volume mesh. The surface mesh of the baffle becomes part of the volume mesh such that small triangle cells on the baffle result in small tetrahedra in the volume mesh.
In addition to baffles, Pointwise V18 now includes a much more flexible means of obtaining local mesh sizing control using sources. A source is a geometric entity (point, curve, surface, or volume) but is neither topological nor meshed. After defining the source's geometry, it is assigned a cell size and an optional cell size variation. The source influences the tetrahedral mesh size but does not constrain it precisely (compared to a baffle where points, edges, and faces become part of the tetrahedral mesh). Furthermore, a source need not be strictly contained within the interior of a block volume.
An example is shown in Figure 4 where a box source behind the DrivAer vehicle is used to ensure clustering of cells in the wake region. For more details on sources, see the article Controlling Localized Element Size Gradation in an Unstructured Mesh in the Nov/Dec 2015 issue of The Connector.
To facilitate use of sources for local tetrahedral mesh sizing control, Pointwise V18 includes a new entity type called a shape. A shape is simply a geometric entity (polyhedron, cylinder, box, and sphere). The user interface for creating shapes (see Figure 5) uses a new sketching mode where handles on the shape (the green arrows) are used to adjust the size, position and orientation of the geometry.
Other Improvements and Additions
Pointwise V18 includes new capabilities that go beyond meshing algorithms and techniques. Several of the more interesting ones are described below.
Entity Selection by Message Links
As you know, actions that result in new entities are announced in the Messages window (see Figure 6). These references to new or modified entities are now links (note the blue color and underlining) that select those entities when clicked. This is certain to be a handy tool.
Figure 6: Entity references in the Messages window are now clickable links for selecting those entities.
Expanded Mesh Examination
With Pointwise V18's expanded support for quadrilateral surface meshes (now both structured and unstructured), a metric for quad cell warpage became more important. The new Warp metric reports the difference in degrees of the two normal vectors that result from diagonalizing the quadrilateral cell (see Figure 7). Because a quadrilateral can be diagonalized in either of two directions, the maximum difference, or worst case is reported.
Figure 7: You can now compute and visualize the degree of warp of quad cells in the Examine command.
When it comes to understanding how and where your mesh is constrained to the geometry model, the expanded Database Associativity function in Examine now provides even more insight. You can display a table of point counts and percentages by whether points are on or off the database (geometry model). Another technique, shown in Figure 8, colors each point by the database entity to which it is attached, and individual entities can be turned on and off to make it easier to find outliers.
Simplified Faceted Geometry
With the expanded cell-type support for meshes, it seemed perfectly reasonable to similarly upgrade Pointwise's support for faceted geometry as imported from STL and other files (also known as shells). During import you have the option (see Figure 9) to merge coplanar cells into polyhedral or set your desired mix of triangles and quads. This is in addition to the current feature that allows shells to be split along feature lines. The result can be a shell entity that is substantially less dense and therefore easier to work with as shown in Figure 10.
Figure 9: In addition to splitting faceted geometry at feature lines, the shells can be simplified by joining those that are coplanar.
Figure 10: By merging coplanar cells during faceted geometry import you can vastly reduce the size of your shell entities. The magenta shell on the left imported all the triangular facets directly. The cyan shell on the right shows the result of merging coplanar cells.
There are a few changes in Pointwise V18 that deserve special mention.
We would like to take this opportunity to thank our Pointwise Advisory Team, a select group of clients and partners, for spending time reviewing, commenting on, and guiding the implementation of features in Pointwise V18.
Start Meshing with Pointwise Today
These features and more are coming soon to Pointwise V18 but if you would like to get ahead of the curve on using Pointwise for your CFD meshing, request a no-cost evaluation license today.