There are two general methods for performing mesh adaption. Incremental adaption takes an existing mesh and modifies it to meet the adaption criteria. The alternative is remeshing, in which the whole geometry is remeshed at every adaption step according to the adaption criteria. In Ansys CFX, incremental adaption is used because this is much faster; however, this imposes the limitation that the resulting mesh quality is limited by the quality of the initial mesh.
The particular implementation of incremental adaption that is adopted in Ansys CFX is known as hierarchical refinement or h-refinement. Each adaption step consists of a structured refinement of an existing mesh. A sequence of refinements form a set of hierarchical levels.
In each mesh adaption step, each mesh edge that is marked for adaption (see the previous section) has an extra node placed half-way along it. The mesh elements that share this edge are then divided to use the new node, subject to the following:
Neighboring elements must only differ by one refinement level. Hence, one mesh element cannot be divided twice if its neighbor has not been divided at all.
Where possible, regular refinement of an element takes place. Regular refinement means that all the edges of an element are divided into two, and the element split accordingly. To make this possible, extra nodes may be added.
No "hanging" nodes are allowed. This means that if an extra node is added to an edge, all the mesh elements that share that edge must be refined.
Only certain types of elements are allowed in the refined mesh: tetrahedron, prism, pyramid and hexahedron.
In regions where inflation has taken place (so that there are prisms and some pyramid elements near wall boundary conditions), the mesh adaption avoids refining these elements in the direction perpendicular to the wall. Only edges on the interface between the inflated elements and the rest of the tetrahedral mesh are allowed to be marked for adaption. When the refinement of these edges takes place, the refinement propagates through the layers of prismatic elements to the wall boundary condition itself.
Mesh adaption in Ansys CFX does not have the capability of refining the surface mesh back to the original geometry. This means that nodes that are added to the surface of the problem are added onto the edges of the existing mesh rather than to the surfaces of the original geometry. An illustration of these two alternatives is shown below. Note that only the option shown on the left is available in Ansys CFX.
Note: Support for adaption of the mesh to the original surface geometry is expected in future releases.