Structured Grids Are Here For An Eternity

With unstructured meshes delivering promising results in resolving boundary layers, wakes, and other flow features for complex geometries, one might arrive at the conclusion that structured grids will soon be out of the markets because of their reputation for taking a long time for generation. On the contrary, structured grids give you two things that unstructured meshes may lack, i.e., quality and control, and since the choice of grid plays a significant role in the accuracy of the solution, it is evident that the structured grids are here to stay!

Fig. 1: Structured grid generated for a multi-element airfoil designed specifically for noise computations.

Advantages of using structured grids

Time and memory: Using structured grids, one can fill the same volume with fewer hexes than tets, thereby lowering the cell count, which consequently reduces the CFD computation time and memory usage. Structured grids generally have a different topology than unstructured grids, so it is difficult to make a direct cell count comparison. At its simplest, each hexahedron can be decomposed into five tetrahedra that share its edges, giving a 5:1 reduction in cell count for the same flowfield resolution. The benefit of reducing cell count becomes very apparent when generating a mesh with a wide variation in resolved length scales.

Resolution: The flow of fluid will often exhibit strong gradients in one direction with milder gradients in the transverse directions (e.g., boundary layers, shear layers, wakes). In these instances, high-quality cells are easily generated on a hex grid with a high aspect ratio (on the order of one thousand or more). It is much more difficult to generate accurate CFD solutions on highly stretched tetrahedra. (Plus, not all stretched tets are equal depending on the maximum included angles.)

Alignment: CFD solvers converge better and can produce more accurate results when the grid is aligned with the predominant flow direction. Alignment in a structured grid is achieved almost implicitly because grid lines follow the contours of the geometry (as does the flow), whereas there’s no such alignment in an unstructured mesh.

Definable normal: Application of boundary conditions and turbulence models work well when there is a well-defined computational direction normal to a feature such as a wall or a wake. Transverse normals are easily defined in a structured grid.

Fig. 2: Structured grids are well-suited for turbomachinery applications.

Fidelity Pointwise provides a great breadth of capability in its structured grid generation methods, enabling the use of structured grids for various simulations. One point to remember is that by using grid generation methods that are sufficiently broad in their applicability, the use of grid topology as a crutch to achieve your desired results is eliminated.

Structured grids will continue to be used in CFD for a long time to come, and we plan to continue adding new capabilities to Fidelity Pointwise to make them even more amenable to your work.

For more information about structured grid generation, click here.