Grid Generation and Grid Independence
Situations arise in which a structured grid is preferred (e.g., the CFD code requires structured grids, boundary layer zones need high resolution, or the simulation is pushing the limits of available computer memory). Generation of a structured grid is straightforward for geometries with straight edges. All we need to do is divide the computational domain into four-sided (2-D) or six-sided (3-D) blocks or zones. Inside each block, we generate a structured grid. Such an analysis is called multiblock analysis. For more complicated geometries with curved surfaces, we need to determine how the computational domain can be divided into individual blocks that may or may not have flat edges (2-D) or faces (3-D)
Some CFD codes accept only elementary blocks, namely, blocks whose edges or faces cannot be split. Multiblock grids are necessary for structured grids of complex geometry

A hybrid grid is one that combines regions or blocks of structured and unstructured grids. For example, you can mate a structured grid block close to a wall with an unstructured grid block outside of the region of influence of the boundary layer.

A hybrid grid is often used to enable high resolution near a wall without requiring high resolution away from the wall

HybridGrid
3DCell

If a four-sided 2-D face with structured cells is swept in the third dimension, a fully structured 3-D mesh is produced, consisting of hexahedral cells (n = 6 faces per cell). When a 2-D face with unstructured triangular cells is swept in the third direction, the 3-D mesh can consist of prism cells (n = 5 faces per cell) or tetrahedral cells (n = 4 faces per cell—like a pyramid).

The most recent enhancement in grid generation is the use of polyhedral meshes, such a mesh consists of cells of many faces, called polyhedral cells.

Time spent generating a good grid is time well spent, since the CFD results will be more reliable and may converge more rapidly. A high-quality grid is critical to an accurate CFD solution; a poorly resolved or low-quality grid may even lead to an incorrect solution. It is important for users of CFD to test if their solution is grid independent. The standard method to test for grid independence is to increase the resolution (by a factor of 2 in all directions if feasible) and repeat the simulation.

1Fluid Mechanics: Fundamentals and Applications Fourth Edition. Çengel and J. M. Cimbala, McGraw-Hill, New York (2018)

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