A CFD mesh can be created using one of the three main cell types—hexahedral, tetrahedral or polyhedral—or a hybrid mesh can be created, which uses a combination of these cells. For instance, a structured hexahedral mesh could be used in a near-wall region where a very fine wall-normal spacing is required, due to the presence of large gradients in the flow and then a coarser tetrahedral or polyhedral mesh could be used in other regions with smaller gradients.
Optimally, the different mesh types should be connected in a conformal manner, meaning that the cell faces of the adjoining, but different, cell types match perfectly, creating a continuous mesh. It is possible to create non-conformal (mismatching) mesh interfaces, however this needs to be done carefully as it necessarily results in an interpolation of the fluxes that are passed between adjacent cells. This interpolation can lead to inaccuracies in the simulation or even introduce completely fictitious effects, especially if there is a large discrepancy in the size of the different adjoining cell types. As such, if non-conformal mesh interfaces cannot be avoided, the size of the adjacent cells should be relatively similar.