For a flow problem involving moving boundaries, each sub-task defined in Ansys Polydata has its own remeshing rule. This is, in fact, the most common situation. There are some situations, however, where it is desirable to define more than one remeshing method for a single sub-task. You can accomplish this by defining several local remeshing regions within the domain of the sub-task.
For example, the flow may require the use of the method of spines in one part, while the Thompson transformation may be preferred for another part. The definition of more than one local remeshing region requires that there are multiple subdomains in the domain of the sub-task. Each local remeshing region will be identified by the subdomain(s) that comprise it.
A typical situation where several local remeshing rules are needed arises when a topological restriction of a particular remeshing rule makes it inappropriate over the entire domain of the sub-task, although it is perfectly acceptable for a portion of the domain. As an example, consider the 2D problem illustrated in Figure 16.14: Application of Local Remeshing.
This problem involves two free surfaces and an obstacle in the middle of the domain. A single material is used, so there is just one sub-task. However, the mesh topology prevents the use of a single spine technique applied on the entire domain (due to the presence of the obstacle in the middle). In this case, two independent spine techniques (one for each free surface) will be defined. With separate remeshing rules in subdomains 2 and 4, the presence of the obstacle no longer presents any difficulty.