The March 2018 issue of Digital Engineering came across my desk the other day and one article in particular caught my attention. Written by Tony Abbey, the Siemens FEMAP with NX NASTRAN Overview, Part 2 article focused on meshing. In particular, Tony walked through the process of meshing a tie rod for an FEA analysis. Two things stood out while reading this article:
- It took 30 minutes to mesh using FEMAP.
- The geometry is available for download.
I thought it would be a fun exercise to attempt generating a similar mesh using Pointwise. I downloaded the geometry, imported into Pointwise, and meshed it to similar specifications in five minutes. Here’s how I did it.
Geometry Import & Preparation
The geometry was made available in STEP, Parasolid, and SolidWorks formats. I opted for the STEP file, but confirmed all three formats imported into Pointwise cleanly. The geometry consisted of a single watertight model of a tie rod. In Part 1 of Tony’s series, the fitting at the end was isolated for analysis, so I did the same in Pointwise using our solid modeling toolset. In addition, I trimmed the model such that appropriate boundaries were available when setting up constraints and loading within FEMAP. And lastly, meshing regions defined by quilts, were assembled to further prepare the geometry for meshing.
The tie rod geometry prepared for meshing. Each color represents a different quilt, or logical meshing region, within the model.
Global Surface Meshing
Tony’s meshing process was quite similar to the approach I decided to take whereby an isotropic, baseline surface mesh is first created, followed by local refinement where high stress concentrations are anticipated, ensuring that sufficient grid quality is maintained. One difference however, which may explain the difference in meshing time, was that Tony explored a variety of his mesher’s capabilities.
With the geometry prepared for meshing, I used an average grid spacing of 0.03 inches and automatically generated a watertight advancing front surface mesh consisting of triangular elements. Each quilt within the model received a single domain, or surface mesh patch, as part of this operation. This one-to-one associativity between geometry and surface mesh is advantageous as the solid modeling operations conducted can greatly accelerate the meshing process.
The baseline surface mesh consists of a uniform advancing front surface mesh while remaining fully constrained to the original underlying geometry.
Local Refinement & Volume Meshing
Pointwise employs a bottom-up meshing approach. Once the surface mesh is of sufficient resolution and quality, the volume mesh is then assembled. Therefore, the surface mesh was first refined and interrogated, particularly around the blend region that joins the tie rod shaft with the end fitting. The surface mesh in this vicinity was refined to an average grid spacing of 0.015 inches in an effort to more appropriately resolve this area of curvature. This resolution was then propagated away and into the interior of any adjacent domains.
Additional surface refinement was used in the blend region to more accurately represent this local area of high curvature.
The surface mesh quality was examined prior to assembling the volume mesh. The area ratio, or ratio of adjacent cell areas, indicates how smooth the transition in cell size is across the surface mesh and can indicate local areas of rapid cell size changes. Such rapid transitions can degrade volumetric cell quality, so it’s best to examine area ratio throughout the surface meshing process. In this case, the maximum reported area ratio was just over 2.5 with an average of 1.2.
Cell area ratio was examined prior to volume meshing to ensure a smooth transition in cell area across the entirety of the surface mesh.
With the surface mesh complete, the volume mesh was then constructed and initialized to populate the interior of the computational domain using tetrahedra. As with the surface mesh, the volume mesh was interrogated to ensure sufficient cell quality. With more than 30 quality metrics to pick from, 14 of which are accessible for unstructured volume meshes such as this, the metric selected for interrogation is often solver dependent. That said, one metric I use frequently is the maximum included angle which looks at the maximum angle between any two faces of an element and is a good indication of skewness. In this case, the maximum angle reported is only 145 degrees with an average of 97 degrees.
A slice through the volume mesh colored by maximum included angle for each element.
More Than a CFD Preprocessor
While Pointwise may be “The Choice for CFD Meshing,” the tools and the workflow can be applied to a variety of applications. From automatic high order finite element meshing for centrifugal impellers to supporting North American’s largest 3-D printed structure, Pointwise has been used as a preprocessor to prepare meshes for some fascinating applications. With native support for nearly 20 geometry formats and the ability to export to more than 70 solver mesh formats, Pointwise will more than likely integrate with your workflow. To see more solutions developed using Pointwise, visit the solutions gallery on our website: http://www.pointwise.com/solutions/
If you want to try Pointwise for yourself on this case using the geometry available from Digital Engineering (or any others), request your free trial license today and we can help you explore what Pointwise can do for you.
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