Type of Mesh

The shape of the control volume depends on the solver’s capabilities; structured-grid codes utilize quadrilaterals in 2D flow and hexahedrons in 3D flow.

Meanwhile, unstructured grids use triangles in 2D flow and tetrahedrons in 3D flow

Figure 3.2. Type of mesh

In general, meshes composed of hexahedra offer advantages in terms of efficiency in the creation of element count (cells).

Imagine a 2D box requiring only one element to form a 1×1 meter square, while a triangular shape needs two elements to form the same box. This discrepancy becomes even more significant when dealing with 3D models.

Moreover, in numerical methods (to be discussed in the solver theory), data transfer is faster with a higher number of ‘neighbors’ or surfaces from these elements.

In this case, one hexahedral element has six neighbors, whereas one tetrahedral element has four neighbors, indicating that hexahedra has more neighbors.

Advancements in polyhedral mesh types aim to accommodate this advantage by increasing the number of surfaces.

Structured mesh configurations, also known as structured mesh, as illustrated in Figure 2.3, can be employed to achieve optimal data transfer. In this mesh type, data transfer speed and plotting can be performed effectively, quickly, and accurately, supporting good visualization.

Figure 3.3. Structured grid of NACA 0012 with OpenFOAM

However, a primary drawback of hexahedral meshes is the difficulty in shaping complex geometries (although achievable, as shown in Figure 3.3, the time required to create them is not feasible for fast daily analysis).

Consequently, tetrahedral meshes have become more commonly used due to their advantage in creating complex geometries

Figure 3.4. Hexa (top) versus Tetra (bottom) mesh

Besides using a single type of mesh within a domain, we can also mix different types, for instance, combining hexahedral and tetrahedral or polyhedral meshes simultaneously, known as a hybrid mesh, as illustrated in Figure 3.5.

Figure 3.5. Hybrid mesh of Hexa and Polyhedron constructed with Cradle CFD

There’s also a type of mesh based on hexahedra that is quite adaptive for detailed models. This model combines a simple hexahedral block mesh ‘cut’ by the object. Then, one hexahedral block is subdivided into smaller sizes in the areas around the walls to form the required geometric details.

In software like OpenFOAM, this type of mesh is known as snappyHexMesh.

Figure 3.6 illustrates the snappyHexMesh created using openFOAM software.

Figure 3.6. SnappyHexMesh generated with OpenFOAM