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1 cfmesh v1.1 User Guide Document version: 1.1 Principal developer and document author: Dr. Franjo Jureti, M. Eng., Assist. Prof. Managing Director and Founding Partner Creative Fields, Ltd. Zagreb, May 2015

3 1. Introduction cfmesh is a cross-platform library for automatic mesh generation that is built on top of OpenFOAM 1. It is licensed under GPL, and compatible with all recent versions of OpenFOAM and foam-extend. cfmesh supports various 3D and 2D workflows, built by using components from the main library, which are extensible and can be combined into various meshing workflows. The core library utilises the concept of mesh modifiers, which allows for efficient parallelisation using both shared memory parallelisation (SMP) and distributed memory parallelisation using MPI. In addition, special care has been taken on memory usage, which is kept low by implementing data containers (lists, graphs, etc.) that do not require many dynamic memory allocation operations during the meshing process. 2. Available meshing workflows Meshing workflows implemented in cfmesh require input geometry in a form of surface triangulation, shown in Figure 1, and the user-specified settings, which will be explained in detail in subsequent sections. All workflows are parallelised for shared memory machines and use all available computer cores while running. The number of utilised cores can be controlled by the OMP_NUM_THREADS environment variable, which can be set to the desired number of cores. Figure 1 Surface mesh Currently available meshing workflows start the meshing process by creating a so-called mesh template from the input geometry and the user-specified settings. The template is later on adjusted to match the input geometry. The process of fitting the template to the input geometry is designed to be tolerant to poor quality input data, which does not need to be watertight. The available workflows differ by the type of cells generated in the template. 1 None of the OpenFOAM related offering by Creative Fields, Ltd., including cfmesh, is approved or endorsed by OpenCFD, Ltd. (ESI Group), producer of the OpenFOAM software. OpenFOAM and OpenCFD are registered trade marks of ESI Group. 2

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8 3. Input geometry cfmesh requires geometries in the form of a surface triangulation. For 2D cases, the geometry is given in a form of a ribbon of triangles with boundary edges in the x-y plane (other orientations are not supported). The geometry consists of the following entities: List of points contains all points in the surface triangulation. List of triangles contains all triangles in the surface mesh. Patches are entities that are transferred onto the volume mesh in the meshing process. Every triangle in the surface is assigned to a single patch, and cannot be assigned to more than one patch. Each patch is identified by its name and type. By default, all patch names and types are transferred on the volume mesh, and are readily available for definition of boundary conditions for the simulation. Facet subsets are entities, which are not transferred onto the volume mesh in the meshing process. They are used for definition of meshing settings. Each face subset contains indices of triangles in the surface mesh. Please note that a triangle in the surface mesh can be contained in more than one subset. Feature edges are treated as constraints in the meshing process. Surface points where three or more feature edges meet are treated as corners. Feature edges can be generated by the surfacefeatureedges utility. The user must define all sharp features transferred by the cfmesh prior to the meshing process. The edges at the border between the two patches and the feature edges are handled as sharp features in the meshing process. Other edges in the triangulation are not constrained. Figure 10 shows a surface mesh with a patch highlighted in green, a facet subset coloured in blue and user-selected feature edges coloured in red. Figure 10 Geometry with entities 7

9 The file formats suggested for meshing are: fms, ftr, and stl. In addition, the geometry can be imported in all formats supported by the surfaceconvert utility, which comes with OpenFOAM. In addition, cfmesh also provides additional utilities for conversion of surface mesh into fms and from fms to other formats. However, the three suggested formats support definition of patches, which are transferred onto the volume mesh by default. Other formats can also be used for meshing but they do not support definition of patches in the input geometry and all faces at the boundary of the resulting volume mesh end up in a single patch. The preferred format for cfmesh is fms, designed to hold all relevant information for setting up a meshing job. It stores patches, subsets, and feature edges in a single file. In addition, it is the only format, which can store all geometric entities into a single file, and the users are strongly encouraged to use it. An example of the surface mesh written in fms is depicted in Figure 11. Figure 11 Structure of the fms geometry file format 8

10 4. Required dictionaries and available settings 4.1. Dictionaries The meshing process is steered by the settings provided in a meshdict dictionary located in the system directory of the case. For parallel meshing using MPI, a decomposepardict located in the system directory of a case is required, and the number of nodes used for the parallel run must match the numberofsubdomains entry in decomposepardict. Other entries in decomposepardict are not required. The resulting volume mesh is written in the polymesh directory inside the constant directory. The settings available in meshdict will be handled in the remainder of this section Mandatory settings in meshdict cfmesh requires only two mandatory settings (see Figure 12) to start a meshing process: surfacefile points to a geometry file. The path to the geometry file is relative to the path of the case directory. maxcellsize represent the default cell size used for the meshing job. It is the maximum cell size generated in the domain. Figure 12 Mandatory settings 4.3. Refinement settings in meshdict Quite often a uniform cell size is not satisfactory, and there are many options for local refinement sources in cfmesh. boundarycellsize option is used for refinement of cells at the boundary. It is a global option and the requested cell size is applied everywhere at the boundary. An example is given in Figure 13. boundarycellsizerefinementthickness specifies the distance from the boundary at which the boundarycellsize is still applied. mincellsize is a global option which activates automatic refinement of the mesh template. This option performs refinement in regions where the cells are larger than the estimated feature size. The scalar value provided with this setting, specifies the smallest cell size allowed by this procedure, see Figure 13. This option is useful for quick simulation because it can generate meshes in complex geometry with low user effort. However, if high mesh quality is required, it provides hints where some mesh refinement is needed. Figure 13 boundarycellsize and mincellsize 9

15 Figure 21 - edgemeshrefinement 4.4. Keeping/removing of cells in user-defined regions Meshing workflows implemented in cfmesh are based on the inside-out meshing, and the meshing process starts by generating the so-called mesh template based on the user-specified cell size. However, if the cell size is locally larger than the geometry feature size it may result with gaps in the geometry being filled by the mesh. On the contrary, the mesh in thin parts of the geometry can be lost if the specified cell size is larger than the local feature size. keepcellsintersectingboundary is a global option which ensures that all cells in the template which are intersected by the boundary remain part of the template. By default, all meshing workflows keep only cells in the template which are completely inside the geometry. keepcellsintersectingboundary keyword must be followed by either 1 (active) or 0 (inactive), see Figure 22. Activation of this option can cause locally connected mesh over a gap, and the problem can be remedied by the checkforgluedmesh option which also must be followed by either 1 (active) or 0 (inactive). Figure 22 keepcellsintersectingboundary and checkforgluedmesh keepcellsintersectingpatches is an option which preserves cells in the template in the regions specified by the user. It is a dictionary of dictionaries, and each dictionary inside the main keepcellsintersectingpatches dictionary is named by patch or facet subset, see Figure 23. This option is not active when the keepcellsintersectingboundary option is switched on. Patches can be specified using regular expressions. 14

17 4.5. Boundary layers Boundary layers in cfmesh are extruded from the boundary faces of the volume mesh towards the interior, and cannot be extruded prior to the meshing process. In addition, their thickness is controlled by the cell size specified at the boundary and the mesher tends to produce layers of similar thickness to the cell size. Layers in cfmesh can span over multiple patches if they share concave edges or corners with valence greater than three. All boundary layer settings are provided inside a boundarylayers dictionary, shown in Figure 25. The options are: nlayers specifies the number of layers which will be generated in the mesh. It is not mandatory. In case it is not specified the meshing workflow generates the default number of layers, which is either one or zero. thicknessratio is a ratio between the thickness of the two successive layer. It is not mandatory. The ratio must be larger than 1. The default value is 1. maxfirstlayerthickness ensures that the thickness of the first boundary layer never exceeds the specified value. It is not mandatory. patchboundarylayers setting is a dictionary which is used for specifying local properties of boundary layers for individual patches. It is possible to specify nlayers, thicknessratio and maxfirstlayerthickness for each patch individually within a dictionary with the name equal to the patch name. By default, the number of layers generated at a patch is governed by the global number of layers, or the maximum number of layers specified at any of the patches which form a continuous layer together with the existing patch. allowdiscontinuity option ensures that the number of layers required for a patch shall not spread to other patches in the same layer. Patches can be specified using regular expressions. Figure 25 boundarylayers subdictionary cfmesh provides additional controls of boundary layer quality, intended for situations when the large number of layers is required, and when the thickness shall vary smoothly. Activation of the feature is controlled via the 16 2351a5e196