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The Connector, the newsletter for CFD Mesh Generation from Pointwise

September / October 2011

New Meshing Capabilities Coming in Pointwise Version 17

Internal development of new features for Pointwise Version 17.0 is complete. While we test, document, and otherwise prepare the release for you, here is a preview of what is coming in this major release.

T-Rex and Other Extrusion Techniques

The surface mesh formulation of anisotropic tetrahedral extrusion, T-Rex, has been in Pointwise for a while now (see the article in the July/August 2011 issue of The Connector), but V17.0 includes the full volume mesh formulation of this highly automated hybrid meshing technique. For example, consider the fan geometry shown in Figure 1. The underlying T-Rex algorithm is identical to what you are used to using in Gridgen and generates meshes like the one shown in Figure 2 for a single blade passage. Each passage was meshed with approximately 2 million cells with about 75 percent of those cells generated by T-Rex and the remainder coming from the isotropic, Delaunay-based tet mesher.

fan geometry

Figure 1: Fan geometry for a T-Rex mesh generated in Pointwise. Only one blade passage was meshed and then it was copied and rotated to the other positions. +

fan mesh

Figure 2: The T-Rex mesh in each passage was clustered to both blades, the hub, and the shroud. The inset shows a magnified view of the mesh in the blade-hub corner. The mesh is shown as a crinkle cut with cells colored by maximum included angle. +

New to T-Rex: Push Attributes

Pointwise's implementation of T-Rex has a time saving feature relative to what you are used to in Gridgen. Instead of having to decide in advance how to distribute grid points on connectors (curves) and domains (surfaces) so they will match the eventual T-Rex extrusion, a simple checkbox called Push Attributes (Figure 3) lets the block "push" its mesh spacings onto domains and connectors that have been tagged with the new Match boundary condition (Figure 4).

The Push Attributes control in the menu

Figure 3: The Push Attributes checkbox (circled) is on the T-Rex tab of the Solve panel.

The Match BC control in the menu

Figure 4: The new Match boundary condition for T-Rex indicates where attributes are to be pushed.

To illustrate this, a surface mesh for a simple projectile is shown in Figure 5. The goal is to generate a symmetry plane mesh (blue) using T-Rex that clusters around the perimeter of the projectile (green). In order to do this, the distribution of points along the axis of symmetry of the projectile will have to match the surface extruded by T-Rex. The symmetry plane comprises two separate surface meshes separated by the vertical line you can see toward the left side of the mesh.

unstructured mesh before applying T-Rex

Figure 5: A simple Delaunay-based mesh before applying T-Rex with Push Attributes enabled +

If you just generate the T-Rex mesh directly without doing any advance work on those connectors (and without using Push Attributes), the surface mesh is inconsistent with its edges and the result is a poor quality grid (Figure 6).

T-Rex mesh without push attributes

Figure 6: An unusable mesh results if the surface mesh and curve meshes are inconsistent. (Note the bad mesh at the leading and trailing edges and along the top around part of the mesh topology.) +

In Gridgen, you would fix this by first manually changing the distribution of points on the three connectors to match what T-Rex would generate for the surface mesh and then reapplying T-Rex. However, in Pointwise, this all is handled automatically by the algorithm. You just set Match boundary conditions on the three connectors and check the box Push Attributes. The resulting mesh in Figure 7 is ready for use.

T-Rex mesh with push attributes

Figure 7: Using Push Attributes and Match BCs in T-Rex ensures a consistent mesh. +

T-Rex and Baffles

Hybrid mesh generation is even more automated using Pointwise's implementation of T-Rex and the addition of Push Attributes and Match boundary conditions. You can do even more with the addition of baffles, thin surfaces that can model geometry and/or set clustering on a block's interior. As an example, consider the mesh for the trapezoidal wing, the subject of AIAA's High Lift Prediction Workshop, shown in Figure 8.

trapezoidal wing geometry

Figure 8: The trap wing geometry includes a wing with slat and flap. +

Three meshes were generated for the trap wing using the workshop's resolution requirements: coarse (approximately 17,000,000 cells), medium (approximately 50,000,000 cells) and fine (approximately 151,000,000 cells). A cut through the medium mesh at 80 percent span is shown in Figure 9. Optional combination of extruded tets into prisms is available and reduced the overall cell count by 48 percent in the case of the medium density trap wing mesh.

trapezoidal wing mesh

Figure 9: A cut through the trap wing medium density T-Rex mesh at 80 percent span with cells colored by maximum included angle +

Next, we add baffles to cluster points in the wake region. The resulting coarse mesh for the trap wing is shown in Figure 10.

trapezoidal wing mesh with baffle

Figure 10: This constant-span cut through a coarse mesh for the trap wing illustrates the use of baffles to resolve flow features like wakes. +

Re-extrude

For those of you using the more traditional extrusion methods, Re-Extrude will be available in Pointwise V17. This command lets you select surface or volume meshes that you created using extrusion and re-enter the extrusion command to go farther or back up or change attributes and try again. Re-extrusion is illustrated in Figure 11.

re-extruding a structured grid

Figure 11: Two of four extruded blocks are being Re-Extruded. +

Other Grid Generation Enhancements

In the theme of migrating tools from Gridgen to Pointwise, several enhancements have been made to the structured solver, the unstructured solver, and the ability to set spacing constraints.

Pointwise simplifies the process of copying a spacing constraint value (aka Δs) to other locations in your mesh. In Gridgen, you do this by entering a -1 for the spacing value and then selecting a constraint that shares a node with the one you are setting. (In other words, Gridgen restricts you to copying spacings among constraints that share a node.) In Pointwise, you select a Δs, copy (Ctrl+C), select where you want to copy it to, paste (Ctrl+Shift+V), and you are done. Pointwise's implementation goes beyond what Gridgen could do in other ways:

  • Copy one spacing value to many other constraint locations
  • Copy many spacing values to one or more other constraint locations (in this case, the average value is pasted.)

Another spacing constraint upgrade is that the spacing constraints now are available in the List (Figure 12). You can select spacing constraints from the List and sort the List by spacing value, but you also can set the spacing value directly simply by double clicking and typing the new value.

spacing constraints in the List

Figure 12: Spacing constraints now can be set, selected, and sorted via the List.

Other additions to the Solve command for structured grids include a Project button so your grids can be projected onto the database while in the solver and changes to the panel's layout so you can see residuals per block.

Also added were structured subgrids, the ability to define a subset of a domain or block so the solver can be applied locally. Subgrids are created in the Solve command as illustrated in Figure 13 with a block.

creating subgrids in a structured block

Figure 13: Subblocks are defined using constant index planes, similar to how the Examine command works. +

Once defined, subgrids appear in the main Solve panel, where you select which grids to run, as shown in Figure 14 with a domain with two subgrids.

selecting subgrids in a structured domain

Figure 14: Subgrids are selected to be run in the main Solve panel. +

Utility Commands

What else does Pointwise have waiting for you?

  • The addition of Groups gives you another organizational tool to complement Layers. You can Create, Edit, and Ungroup sets of homogeneous entities (in other words, entities all of the same type: connectors, domains, blocks) and then conveniently select those groups with a single mouse click.
  • The Layer Manager now lets you select a layer and have all its entities added to the current selection.
  • Graphics in the Display window now optionally may be drawn in double precision for accurate views of very fine mesh clustering.
  • The script commands may be made to appear in the Messages window when journaling an interactive session to a file.
  • The Examine U,V diagnostic functions gives you the ability to query parametric coordinates on your database surfaces by using the probe or a highlighter.
  • Database boundary entities now can be used for the creation of offset curves.

Learn More at the Pointwise UGM 2011

If this preview has piqued your curiosity about Pointwise V17.0, you can spend some quality time with the software and our technical staff at the Pointwise User Group Meeting 2011 to be held 8-9 November in Fort Worth. The first day will consist entirely of hands-on training seminars on various feature suites in Pointwise. For more information about training at the Pointwise UGM, go to www.pointwise.com/ugm. Register today.


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