Computational Grid

It is not possible to overemphasize the importance of using a well-designed grid if a highly accurate numerical simulation is required. Most CAA problems include solid surfaces and bodies. In these cases, a body-fitted grid is most desirable. If a single set of body-fitted grids cannot be found for the entire domain, one may use local body-fitted grids and then transfer numerical data from one local grid to another through the overset grids method.

As mentioned previously, many aeroacoustic problems involve multiple scales. For problems of this kind, the use of multisize mesh is most appropriate. In order to design the mesh properly so as to offer adequate spatial resolution, one must have some idea of the governing physics in different parts of the computation domain. The mesh size is dictated by the dominant physics of the flow and acoustics. Once the spatial resolution required in different parts of the computational domain is known approximately, the domain may then be divided into subdomains. The mesh size of adjacent subdomains is allowed to change by a factor of 2. Overall, the mesh sizes of the entire computation are determined by the subdomain requiring the highest resolution.

In certain types of fluid flows, such as boundary layers or strongly sheared mixing layers, the flow field is characterized by two disparate length scales. The spatial rate of change of the flow field in the flow direction is mild compared with that in the per­pendicular direction. Because of this disparity, it is possible to use computation grids with a fairly large aspect ratio for mean flow calculation. However, for aeroacoustic problems, sound waves usually propagate without a preferred direction. Therefore, it is recommended that the meshes, in physical domain, should have an aspect ratio close to unity. This will avoid any mesh-related anisotropy being introduced into the computation.