External Flow Meshes

Our developers, as the original creators of ICEMCFD, have many years of experience in crafting grids for external or aerodynamic flows. It is well known that for high quality aerodynamic solutions, structured grids or multi-block structured grids preform well in most solvers. However, structured or multi-block structured grids are not easy or fast to make on anything but the simplest conceptual geometry, and they often require hours of manual input to make a grid. As the geometry becomes more complicated when the meshing process requires manual effort, then the time to create the mesh becomes proportional to the amount of manual effort. Hence if the model complexity is doubled then the manual effort is at least doubled too. So there becomes a limit to how complex the model can be in being meshed in a fixed amount of time. Complicating the situation is if the meshing process is to ultimately be part of an optimization process. In these cases, Ennova can produce a high quality hybrid mesh automatically. Ennova can identify structured regions in the model itself or can be guided to force geometric regions to be multi-block structured as required for the CFD solution. Such regions may include stretched quads on airfoil leading or trailing edges, for example, Wall high gradients in the flow field can automatically be accounted for using automatic prism growth. In addition, often in aerodynamics a flow aligned structured or anisotropic mesh in the wake is needed to correctly resolve the induced drag and to compute CLMAX. Ennova deploys a special prism algorithm that grows the trailing edge wake region in a preferred direction. For Aerodynamic Mesh generation our method proceeds automatically as follows: The imported CAD data is assembled into a virtual distributed assembly. This allows a path to full parallelization without the restriction of single processor geometry operations. Using many tests, the geometry is interrogated and reduced to N closed volumes with (usually) the largest volume used for the aerodynamic analysis. Once the geometry has been automatically cleaned and made watertight, the topology is again tested and divided into isotropic and anisotropic regions and surface meshing rules are applied. Mesh sizing is applied simultaneously based on these tests. For example, for a prescribed y+ and N layers, the surface size of the element can be deduced based on local curvature, growth rates, and solver dependent aspect ratio constraints. Once a surface mesh is obtained, viscous boundary layer prisms are grown in an advancing front manner until they become isotropic. From here, an advancing front mesh method can be used to fill in the isotropic background mesh. Often when using a tetrahedral volume mesh, it can be difficult obtaining a fine enough resolution in the mesh to resolve wakes, especially those from upstream high lift elements. In our method, anisotropic wakes can be directly modeled by the addition of topological regions.

Finally, this meshing process is designed to be flexible and iterative. First, an automatic mesh of lower quality can be obtained very quickly allowing an initial CFD solution. Once this solution is available, any part of the automatic process can be manually over-ridden to improve the mesh. For example, unstructured regions can be forced to be structured and anisotropic. Extra refinement can be added or removed as necessary. Often in meshing complex geometries, meshing can take weeks, if not months, before the first mesh is available for solving. In our approach, an initial mesh is available almost immediately, which can be incrementally improved by using the solver results as a guide.