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For a CFD solution, a CAD geometry goes in, and a CFD mesh comes out! Only if it was as simple as it sounded. Although file management in practice is more complex than that, it need not be complicated. Below are a few tips and tricks for reading and writing files from Fidelity Pointwise -
Most meshing projects begin with the geometry on and around which your mesh will be generated. Fidelity Pointwise refers to this geometry as the database and supports two general classes of database entities -
Pointwise supports native, standard, and proprietary formats for CAD geometry import, such as ACIS, CATIA, Creo, EGADS, IGES, NMB, etc. CAD interoperability wouldn't have been an issue if CAD import only had to deal with NURBS surfaces. Complications are introduced by the use of topological (as opposed to geometric) entities such as trimmed surfaces and solid models.
As you can imagine, intersections and related computations are sensitive to the tolerance used. Fidelity Pointwise uses a length scale called Model Size (default = 1,000) to represent the size of your grid. All of Pointwise's tolerances are derived from Model Size. Therefore, it is critically important that the Model Size be the same order of magnitude as the longest side of a box that fits around your database, excluding any outer boundaries.
Often the surfaces that comprise the geometry as imported may not be ready for meshing due to problems such as gaps, overlaps, and more. All or most of these problems can be avoided by having Pointwise automatically assemble your CAD geometry into a solid model (or models) during import.
What is a solid model?
A solid model is a topological assembly of adjacent trimmed surfaces that is “watertight” and ready for meshing (Solid models in the CAD file are imported directly).
The database in Figure 1. shows a CAD geometry imported as 10 solid models that are watertight and ready for meshing.
Figure 1. The CAD model of this football helmet imports 10 solid models: the helmet, face mask, 4 brackets, and 4 screws. The seams shown as black lines on the white helmet are watertight and can be meshed across.
To enable automatic model assembly in Pointwise, you only have to set one parameter: the value of the tolerance within which two surfaces are to be considered neighbors.
Even if your CAD geometry is watertight, more can be done to make it meshable. While Pointwise gives you the ability to generate meshes that are independent of the CAD topology, the highly automated methods rely on the CAD topology as a starting point. Therefore, a complex CAD topology leads to a complex mesh topology which is not necessarily good.
This is where Pointwise's quilts help. A quilt is simply a topological collection of trimmed surfaces in the CAD model that is discretized using a single mesh (a single domain in Pointwise's vernacular). Two adjacent surfaces that share an edge across which the relative angle of the surfaces' normal vectors is less than the designated value will be assembled into a quilt. Figure 4 illustrates the vast simplification that results from creating quilts within the solid model.
Figure 2. Simplification of surface topology with quilts for the DrivAer geometry.
Automatic quilt assembly during CAD import is triggered by a single parameter: Quilt Assemble Angle
Many issues associated with meshing analytic CAD geometry are related to topology. For this reason and many others, people prefer to work with faceted geometry that is imported from stereolithography (STL) files or existing grids. In Fidelity Pointwise, these entities are called shells. Shells can be imported using file types such as NASTRAN, PLOT3D, STL, UCD, VRML, and XPATCH.
Note: you can use both shells (faceted geometry) and NURBS (analytic geometry) in the same database within Fidelity Pointwise.
Faceted geometry is meshed data and, therefore, is perceived to be simpler to use. This simplicity comes at a price. Because an entire 3-D object can be imported as a single shell comprised of tens or hundreds of thousands of facets, there is no explicit representation of feature lines to which you want your mesh to conform.
The pink image in Figure 3. shows that the entire geometry is imported as a single shell. You will likely want to resolve the edges of the spiral teeth with a grid line, but that will be difficult for the pink surface because there isn't a boundary edge in the shell to use. However, during import, you can split a shell into smaller shells along feature lines by specifying a turning angle.
Figure 3. A faceted geometry model is imported as a single surface (top), and by automatically extracting features during import, the meshable geometry is recovered (bottom).
Of course, the result of your meshing effort is to run the mesh in your flow solver. You select your flow solver at the beginning of the meshing process from the menu of 60 or more solvers using the CAE Select Solver command. Flow solver compatibility also includes the computational fluid dynamics (CFD) boundary and volume conditions (e.g., no-slip wall, far-field boundary, porous media) and other parameters you have set in Fidelity Pointwise for your analysis.
The boundary conditions (BCs) and volume conditions (VCs) are the main difference between grid export (see below) and exporting your CAE data. The former only writes the grid; the latter writes the grid, BCs, VCs, and other flow solver attributes. The most interesting part of File, Export, CAE is the ability to have your mesh automatically mirrored about a plane during export. This lets you generate half of the mesh using a symmetry plane and have the other half created automatically for you.
If you are ready to import your CAD file and export a CFD-ready mesh, now is the time to request a demo of Cadence Fidelity CFD by clicking the button below.
To learn a few more tips on streamlining CAD import and mesh export on Fidelity Pointwise, download or read the article 'Tips and Tricks for Reading and Writing Files from Pointwise' by clicking the button below.