About Hex-dominant Meshes

The hexadedral-dominant mesher, or hex-dominant mesher, on the 3DEXPERIENCE platform is primarily used for computational fluid dynamics (CFD) applications.

To achieve the most accurate simulation results, three-dimensional models can be meshed with hexahedral elements. Hexahedral elements are six-sided "bricks" that transfer loads and reaction forces linearly since each element face has a parallel—or near parallel—face opposite it. All other element types used for 3D meshing involve triangular forms, where the forces must be split and redistributed at angles.

For relatively simple model shapes, a hex-dominant mesh may be composed almost entirely of hexahedral elements. As the model complexity increases, the percentage of hexahedral elements in the mesh typically decreases. Other 3D element types are added to accurately mesh the geometry without devoting excessive computational resources to creating the mesh. A hex-dominant mesh contains one or more of the following 3D element shapes along with hexahedral elements:

• Tetrahedral
• Pyramidal
• Prismatic

Although hex-dominant meshing is not required for computational fluid dynamics simulations, it offers one major benefit over using other mesh types: you can mesh 3D regions that do not contain solid geometry. These regions, called fluid domains, can be internal or external; they are the open areas through which fluid flows within or around model geometry. For example, when simulating flow through a pipe, the structural model is the pipe. You can define a fluid domain through the interior volume of the pipe and use the hex-dominant mesher to mesh that volume. Likewise, to simulate flow around an object, you define the fluid domain as a volume outside of the geometry that defines the shape of the object. In addition, you can define bounding planes to limit the fluid domain to investigate an interesting flow behavior that occurs in one area.

Other important advantages of hex-dominant meshing for CFD simulations include the following:

• Addition of boundary layers, useful for capturing transient effects in fluid flow near walls.
• Wrapping of small (compared to mesh size) gaps and holes in geometry.
• Meshing within or around geometry represented by an orphan mesh. The orphan mesh can be a surface mesh or a solid mesh if it completely encloses a volume.

Hex-dominant meshing can also be used to mesh three-dimensional structural regions. Hex-dominant meshes are associated with their supporting geometry. Therefore, modeling and scenario features (such as loads and boundary conditions) can be applied directly on geometry.

Hex-dominant meshing typically requires more time than other meshing methods to generate the mesh and to complete a simulation. Due to this extra expense, hex-dominant meshing may not be ideal for early design or proof of concept iterations or for noncritical components. However, the potential for improved solution accuracy may warrant the use of hex-dominant meshes even in early design stages. Good engineering judgment and experience are the best guides to help you decide whether a hex-dominant mesh is appropriate to meet your needs.

If you create or update any fluid simulation features that use geometry as supports, the app regenerates the hex-dominant mesh.

Boundary layers consist of mesh elements that are stacked normal to the walls. The elements are relatively thin in the stacking direction to provide a high mesh density in the direction normal to the fluid flow. This specialized mesh configuration is useful for capturing transition effects such as the significant velocity and temperature gradients that may occur near pipe walls and other structures due to viscous and heat transfer effects. The layers decrease in density as they get further from the boundary.

Because boundary layers can be useful for capturing these gradients near walls, adding more boundary layers can provide even more detail about these areas in your simulation data. If you are not concerned about the behavior around walls or openings in your analysis, removing boundary layers can provide better performance.

The first boundary layer is the one closest to the wall. You can customize its thickness to suit your analysis: a thin boundary layer is denser and provides more detail about the behavior close to the wall, while a thick boundary layer covers more volume and can capture behavior farther away from the wall.

If your fluid simulation includes solid thermal parts, you can change the mesh density of the solid parts independently of the rest of the mesh. The solid mesh scale factor specifies the relative mesh density for the solid parts: values lower than 1 provide better mesh quality, while values higher than 1 provide a coarser mesh that can improve analysis performance.

For conjugate heat transfer (CHT) simulations like heat distribution on a circuit board, you can generate the mesh using the trimmed mesh approach. In a trimmed mesh, the mesher cuts elements based on the intersections between dual mesh elements and geometry tessellation. The main difference between a trimmed mesh and a traditional hex-dominant mesh is that the trimmed mesh does not perform geometry projection after the trim. This approach allows the mesher to generate the mesh quickly and efficiently; however, the geometry representation in the mesh can have reduced accuracy. You can recover some of the accuracy by meshing with finer elements in local regions with sharp corners or edges.

The following image illustrates how elements in a trimmed mesh and a traditional hex-dominant mesh might differ when you generate a mesh along a curve:

The trimmed elements approach is available only when your fluid simulation includes solid thermal parts.