civilfem_gridgen.pdf

FLAC3D Grid Generation with ANSYSCivilFEM Prepared by Bharath Mukundakrishnan and Thomas L. Ruen Itasca Consulting Group, Inc. 708 South Third Street, Suite 310 Minneapolis, Minnesota 55415 USA November 1, 2002 Ref 8506 -1- 1.0 GRID GENERATION................................................................................................2 1.1 GENERAL COMMENTS...........................................................................................2 1.2 SOLID MODEL GENERATION..................................................................................3 1.3 FLAC3D MESH FROM ANSYSCIVILFEM MESH...................................................4 1.4 GEOMETRY TESTS IN FLAC3D..............................................................................7 1.4.1 Geometric Parameters Orthogonality, Aspect Ratio, Face Planarity .......7 1.5 GRID GENERATION USING RATIOS........................................................................11 1.6 EXAMPLE OF A GRADED GRID..............................................................................14 1.7 ANSYSCIVILFEM MESH EXPORTED AND ATTACHED IN FLAC3D .....................16 1.7.1 Several sub-grids attached together...........................................................16 1.7.2 Circular tunnels embedded in a grid..........................................................19 1.8 CREATION AND MESHING OF TUNNEL INTERSECTION GEOMETRIES.......................24 1.8.1 Building and meshing an intersection between two perpendicular tunnels24 1.8.2 Building and meshing three tunnels perpendicular to each other..............27 1.8.3 Building and meshing tunnel intersection with ratios................................30 1.8.4 Building and meshing multiple, intersecting tunnels and shafts................33 1.8.5 Building and meshing tunnel intersection at 45○ angle..............................36 1.8.6 Building and meshing tunnel intersection at 45○ with different radii ........41 1.9 MISCELLANEOUS EXAMPLES................................................................................43 1.9.1 Importing and using an IGES surface to extrude and mesh a volume .......43 1.9.2 Import of *.SAT at files ASIC at from AutoCAD ......................46 1.9.3 A quarter ellipse model...............................................................................48 1.9.4 A cubic volume cut by an arbitrary surface................................................51 1.9.5 Building and meshing a tunnel extruded along a given path .....................54 -2- 1.0 GRID GENERATION 1.1 General Comments Grid generation in FLAC3D is accomplished using five basic primitives brick, wedge, pyramid, cylinder and tetrahedron that are built in as a part of the software. In addition, special geometries, such as tunnel intersections, are built into the software based on the five basic primitives. The built-in models, although useful, do not cover the entire spectrum of problems. Sometimes it is easier to create the model geometry defining curves, surfaces and volumes and then use a meshing process to convert the geometrical entities into discrete points and elements in space for analysis. A solid modeler can be used to accomplish the building of geometry, which can then be used as an for a meshing utility to mesh the geometry. ANSYSCivilFEM1 is one such software program that can help in both building and meshing the geometry. It is relatively easy to create solid models, but meshing the created model is a complex process. Meshing a 3D model depends on the type of element used. It is possible to mesh almost any complicated solid model with a basic three-dimensional element, namely tetrahedron. But if the solution analysis requires the model to be meshed with a certain type of element, then meshing can become an involved process. FLAC3D requires that all elements be hexahedrons in order to provide accurate solutions for plasticity. ANSYSCivilFEM provides capabilities to build solid model geometry using two different approaches and also provides tools to mesh these geometries. The user can choose the eight noded SOLID45 element from the database of built-in elements to mesh the created solid model into hexahedral zones. This element can automatically degenerate into a wedge type element or a tetrahedron. The resultant mesh from ANSYSCivilFEM can be exported into a FLAC3D at data file that translates the nodal positions into FLAC3D gridpoint positions and ANSYSCivilFEM primitives to FLAC3D primitives. ANSYSCivilFEM detaches the problem of meshing from the problem of building the solid model. With any numerical , the accuracy of the result depends on the grid used to represent the physical system. In general, finer meshes more zones per unit volume lead to more accurate results. Furthermore, the aspect ratio ratio of the smallest length to largest length in a zone also affects accuracy. When creating model geometry with FLAC3D, it should be kept in mind that the greatest accuracy is obtained for a model with equal, square zones. If 1. ANSYSCivilFEM is available from Ingeciber, S.A. For purchasing ination visit -3- the model must contain different zone sizes, then a gradual variation in size should be used for maximum accuracy; this factor is important enough that a special option is provided in the GENERATE command in FLAC3D whereby zone sizes can be arranged to increase or decrease by a constant ratio along any grid line. ANSYSCivilFEM has the capability to specify ratios while meshing a solid model and thus control the size of elements generated. As a general rule, the aspect ratio of a zone should be kept as close to unity as possible. Anything above 51 is potentially inaccurate. The purpose of this document is to show the potential application of ANSYSCivilFEM as a grid generator for FLAC3D. Several example grids are given to illustrate the types of geometries that can be considered for grid generation. All save files *.db and several script files *.log from ANSYSCivilFEM v6.1 are provided for the examples in this document. Also, FLAC3D data files *.dat exported from the ANSYSCivilFEM models are provided. All files are compressed in the file “CIVILFEM_DAT. ZIP.” 1.2 Solid Model Generation There are two approaches available in ANSYSCivilFEM to model any given geometry. One approach is called the “bottom-up” approach, and the other is called the “top-down” approach also called the constructive solid-geometry approach. The bottom-up approach can be used for very complex geometries that cannot be defined using conic sections or by simple Boolean operations of basic solid primitives. Points, lines, areas and volumes can be created to describe the model geometry using this approach. The constructive solid-geometry approach, on the other hand, is used to create models using basic primitives provided by ANSYSCivilFEM and applying Boolean operations on these primitives to construct a model. The approach that should be used is dependent on the complexity of the model and perhaps the resourcefulness of the user. Users should refer to ANSYSCivilFEM manuals for guidance on solid modeling. Once a solid model has been created, it should be meshed to generate nodes and elements to fill in the solid. Hexahedral meshing of the solid geometry may require extra effort with sub- division, addition and subtraction of volumes. It should be noted that ANSYSCivilFEM can generate tetrahedral meshing very easily with the least from the user. There are two types of meshing mapped meshing, and sweep meshing for creating zones in 3D volumes. Mapped meshing involves filling a model volume with the chosen element. Any geometry that needs to be map-meshed must be made topologically equivalent to certain basic shapes, which can be trivially meshed with the chosen element. ANSYSCivilFEM has tools that allow volumes, areas and lines to be manipulated so that they con to some basic topological entity that can then be easily meshed. Mapped meshing guarantees the -4- same type of element throughout the solid model. Sweep meshing is a technique in which a particular area called a source area, which can be automatically determined or manually specified of a volume is meshed and the mesh pattern is swept through the whole volume up to a target surface automatically determined or manually specified interpolating the pattern within the volume. Internally the meshing algorithm meshes the source area such that the resulting 3D element ed by sweeping corresponds to the element chosen by the user. For FLAC3D grid generation, the 3D element chosen can only be a SOLID45 element. Sweep meshing can be used effectively to mesh complicated geometries. However, in a complex model, sweep meshing may have to be done one volume at a time, specifying the source and target areas manually. The user should take care to ensure that resulting elements at the interface share the same nodes in order that the model properly transfers forces and other quantities across the meshed volume in FLAC3D. Also, see the ATTACH command in the FLAC3D command reference manual. 1.3 FLAC3D mesh from ANSYSCivilFEM mesh Figure 1 shows the ANSYSCivilFEM v 6.1 screen that has the FLAC3D export option. Figure 1 ANSYSCivilFEM to FLAC3D export option. CivilFEM Preprocessor menu has to be invoked, for this option to work in ANSYSCivilFEM v 6.1 Any model geometry meshed with a SOLID45 element in ANSYSCivilFEM can be -5- imported into FLAC3D. ANSYSCivilFEM export option does not recognize other types of elements. SOLID45 can automatically degenerate into a wedge or a tetrahedron. ANSYSCivilFEM creates a script file that generates gridpoints from the nodal positions of the meshed geometry. This is done by issuing a number of GENERATE point commands to create gridpoints. ANSYSCivilFEM then generates FLAC3D primitives using these newly created gridpoints. This is done by issuing a series of GENERATE zone commands for FLAC3D zone generation. Since a SOLID45 element in ANSYSCivilFEM has eight nodes and can degenerate into a wedge or a tetrahedron, only the GENERATE zone brick, GENERATE zone wedge or GENERATE zone tet commands are used in the script file. The at of the output data file created by ANSYSCivilFEM is shown in Figure 2. Figure 2 Data file generated by ANSYSCivilFEM to export mesh into FLAC3D The last line in Figure 2 is necessary to establish links to the grid when structural elements are generated. It is possible to create cables, beams and shells in ANSYSCivilFEM and export them into FLAC3D. This option has not been tested extensively and has not been included here. Structural elements can be easily created on the faces of the FLAC3D grid after importing the grid from ANSYSCivilFEM. The data files provided in “CIVILFEM_DAT.ZIP” have additional commands for saving and turning off inational messages echoed to the screen. These commands have been added externally for convenience and do not a part of original FLAC3D script file generated by ANSYSCivilFEM. “CFTOFL3D.DAT” is the default name of the data file exported by ANSYSCivilFEM that contains grid generation data. The other data file “FL3DRES.DAT” created by ANSYSCivilFEM is for exporting FLAC3D structural element results into an ASCII at file which can be into ANSYSCivilFEM for post-processing. GEN POINT ID id xpos ypos zpos .. .. GEN ZONE primitive p0 POINT id p1 POINT id SIZE x y z GROUP material number .. .. SEL NODE INIT XPOS ADD 0.0 -6- Figure 3 ANSYSCivilFEM to FLAC3D export dialog window Figure 3 shows the ANSYSCivilFEM to FLAC3D export dialog window when “Export Model” option in ANSYSCivilFEM window Figure 1 is invoked. The size variable that determines the number of elements generated can be specified at the time of exporting of ANSYSCivilFEM mesh into FLAC3D. When the ANSYSCivilFEM model needs to be exported into FLAC3D, the user is presented with the dialog box shown in Figure 3. The first three edit boxes can be used to specify the size of zones along x, y and z directions “X Number of divisions” corresponds to division of y size “Y Number of divisions” corresponds to division of z size “Z Number of divisions” corresponds to division of x size Thus, it is possible to have a coarsely meshed geometry in ANSYSCivilFEM and finely meshed geometry in FLAC3D. Volumes that need to be grouped in FLAC3D can be meshed with different material names in ANSYSCivilFEM. These different material names assigned to different meshed volumes in ANSYSCivilFEM translate into different group names during the export process. When an exported mesh is plotted in FLAC3D and compared with the ANSYSCivilFEM default plot, the plots are rotated because FLAC3D by convention plots the geometry on the X-Z plane while ANSYSCivilFEM plots the geometry on the X-Y plane, even though both use a right-handed coordinate system. The radio buttons provided in the dialog box can be used to create irregular hexahedral zones out of tetrahedral elements generated by ANSYSCivilFEM. The model should be meshed completely with tetrahedral elements for this option to work. -7- 1.4 Geometry tests in FLAC3D FLAC3D has three basic tests built in to check the integrity of meshed models to make sure that the model is adequate for simulation purposes. For example, these tests can be used to check if there is improper mapping of node points during export of the model into FLAC3D, resulting in some zones being inside out and hence not suitable for simulation. It can also be used to check if a zone is degenerate or not. Degeneracy can occur if a primitive is created without satisfying the requirements of geometry conditions such as the number of vertices, edges and faces for that particular primitive. The geometry tests in FLAC3D are designed for hexahedral elements only. 1.4.1 Geometric Parameters Orthogonality, Aspect Ratio, Face Planarity The geometric aspects of a hexahedral element are uated using three quantities, orthogonality, and aspect ratio and face planarity. The quantities compare the hexahedrons to a perfect cube, which is the ideal shape for hexahedral meshes. The GEOM_TEST command invokes the test for all three of these geometric quantities. Orthogonality. – For each grid point in each zone, the determinant of the matrix defined by the three edge vectors is computed and divided by the produce of their lengths. This gives 1.0 for a cube, and approaches zero as pairs of edges approach being coplanar or all three approach being coplanar. Each zone is measured by the worst orthogonality value of all grid points. Figure 4 Orthogonality Test -8- Aspect ratio – For each grid point, the ratio of the shortest edge leng