COMSOL Multiphysics Mesh Import And Export Guide

Exporting to the COMSOL Native FormatsIf you export a mesh to a COMSOL native binary (.mphbin) or text (.mphtxt) file, as shown in the figure above,you specify the type of elements to export using the Domain elements, Boundary elements, Edge elements (availablein 3D), and Vertex elements (available in 2D and 3D) check boxes under Data to export. By selecting the Geometricentity information check box, the export operation also writes information on the corresponding geometric entityindex for each element. Select the Selections check box to also include selections that are defined on the currentgeometry in the mesh export. Such selections can then be included when importing a mesh. For a description ofthese options, see the section Geometric and Mesh Entities. Under Output settings, the check box for Export assecond-order elements will include second-order element node information if available.For the sectionwise (.txt) format, you specify the data to export using the Geometric entity level drop down list. Theoptions give you the possibility to choose to export Domain, Boundary, or Edge data.Exporting to the NASTRAN FormatIf you export a 2D or 3D mesh to a NASTRAN file, you specify the type of elements to export using the Domainelements and Boundary elements (available in 3D) check boxes. By selecting the Geometric entity information checkbox, the export operation also writes information on each element’s corresponding geometric entity index to thecorresponding property identification field of the resulting NASTRAN file.Under Output settings, use the Field format list to specify if the output NASTRAN file should be stored in the smallfield format (single precision), large field format (double precision), or free field format (comma separated). Theseoptions are specific to the NASTRAN format. The COMSOL native formats always store data with doubleprecision. For more information, see the various online resources for the NASTRAN format. Select the Export aslinear elements check box to export the linear, or first-order, element information only. By default, second-orderelements are exported if second-order elements are available. Second-order elements are available for meshesgenerated in COMSOL Multiphysics and for imported meshes already containing second-order elements; see alsoSecond-Order Element Data.Exporting to the STL, PLY, and 3MF FormatsYou can export the boundary elements of a 3D mesh to a STL, PLY, or 3MF file. For STL and PLY files, you canchoose binary or text format.Because the STL and 3MF formats only supports triangles, quad elements (four-sided elements) are exported as apair of triangles. STL an PLY mesh export has no additional settings.The 3MF format has an additional setting Data to export. From the Data to export list, select Entire boundary as asingle 3MF surface object (the default) to export the entire surface mesh as one 3MF object of Surface type, or selectDomains as separate 3MF model objects to export the surface mesh of each domain as a separate 3MF object of ModelIMPORTING AND EXPORTING MESH AND INTERPOLATION DATA 7

type and the remaining surface mesh, corresponding to the faces that are not adjacent to any domain, as a 3MFobject of Surface type.The PLY format supports both triangles and quadrilateral elements.For any of the STL, PLY, or 3MF file formats, second-order elements are converted to first-order elements.EXPORTING DATA FROM THE EXPORT NODEFrom the model tree Export node, you can export data in a number of ways, as described below.Exporting Mesh Data from Export MeshFrom an Export Mesh node, you can export to the COMSOL native file format or to an STL file (surface mesh partonly). No additional settings are available when exporting mesh data in this way. You can use this type of mesh dataexport as an alternative to exporting from the main Mesh node or if you, for example, have computed a solutionand want to export the mesh from a deformed configuration or a (geometric) parameter configuration from aparametric sweep.8 IMPORTING AND EXPORTING MESH AND INTERPOLATION DATA

Exporting Mesh and Simulation Data from Export DataFrom an Export Data node, you can export to the VTK format or to one of the sectionwise, spreadsheet, or gridinterpolation formats.Note that data sampled on a grid can be exported either to the spreadsheet format or the grid format. Thespreadsheet format is used for both unstructured mesh and structured grid export. Data can also be exported froma point set defined by a coordinate file. For more information on the interpolation file formats, see InterpolationData Formats.IMPORTING AND EXPORTING MESH AND INTERPOLATION DATA 9

Exporting Mesh and Interpolation Data from Export PlotFrom an Export Plot node, you can export the data contained in a plot to the STL, PLY, 3MF, VTK, sectionwise,or spreadsheet format.When exporting to the sectionwise format using Export Plot, the program will export triangular, edge, or vertexelement data, depending on the plot type. For example, an Isosurface plot will be exported as triangle elements anda Line plot will be exported as edge elements.Exporting Data from Export ImageFrom an Export Image node, you can export to the 3D glTF format, as well as to a number of 2D image formats.In addition, Image export allows you to export to a PowerPoint presentation.Overview of Data ImportDepending on the format used, a data file can be imported and used as a computational mesh, geometry, orinterpolation data.10 IMPORTING AND EXPORTING MESH AND INTERPOLATION DATA

IMPORTING MESH DATA AS A GEOMETRY MODELImported mesh data can be used in two different ways: directly as a mesh used for analysis or indirectly as ageometry object.Using a Surface Mesh as a GeometryA surface mesh (triangle elements) on the STL, PLY, 3MF, or VRML file format can be used as a geometry objectby using Import from the Geometry node. In this case, a Mesh Part is automatically created under Global Definitionsand used as the basis for the geometry object.Using a Volumetric Mesh as a GeometryYou can import a volumetric mesh, (for example, consisting of tetrahedral elements) as a geometry object by:1 Importing the mesh data to a Mesh Part.2 Referencing it from an Import node under Geometry by selecting it under Source. In such a case, only the surfacemesh information of the volumetric mesh is used. After a geometry object has been defined in this way, a mesh,independent from the original mesh data, can be generated by the automatic meshing tools under the Mesh node.Converting a Mesh to a Geometry as a New ComponentFor both surface and volumetric mesh data, you can create a geometry from a mesh that is defined or imported tothe Mesh node. To do so:1 Select the mesh part or meshing sequence with the imported mesh.2 In the Mesh toolbar, click Create Geometry from Mesh or right-click and select it from the context menu.In this case, a new model Component is created of the same dimension as the current mesh. The geometry sequencein the new component has an Import node that imports the original mesh as a geometry.You can use the original, imported volumetric mesh, or parts of it, as the computational mesh in the newComponent:1 Add a Copy feature to the meshing sequence.2 Select the original import sequence as Source Mesh.3 Select source and destination entities.Click the Copy button to copy the mesh. For more information on copying mesh data, see the COMSOLMultiphysics Reference Manual.IMPORTING AND EXPORTING MESH AND INTERPOLATION DATA 11

Boundary PartitioningThe Mesh Parts Import node provides settings for partitioning the boundary of an imported mesh. Boundarypartitioning makes it convenient to set boundary conditions on desired portions of the boundary.12 IMPORTING AND EXPORTING MESH AND INTERPOLATION DATA

The default setting for Boundary Partitioning is Automatic. The other options are Minimal and Detect boundaries. TheMinimal setting creates a partitioning with a minimum number of boundaries. The Detect boundaries settingprovides a number of options for detailed control.For more information, see the COMSOL Multiphysics Reference Manual.Mixing CAD and Surface Mesh DataIf the mesh data is used to define a geometry object, COMSOL Multiphysics allows for solid operations on theimported mesh using a mix of surface mesh and CAD representations. For example, a surface mesh of a mechanicalpart can be imported as an STL file to the Geometry node and subsequently wrapped in a larger sphere or box torepresent a portion of air surrounding the part for further CFD or acoustics analysis. Other examples of solidoperations are drilling holes, adding geometry objects, or cutting a geometry in half. The figures below show aIMPORTING AND EXPORTING MESH AND INTERPOLATION DATA 13

vertebra model imported as an STL file that has been modified with solid operations: subtraction of a cylinder tocreate a hole, solid union with a cylinder to add material, and partitioning and cutting the object in half.14 IMPORTING AND EXPORTING MESH AND INTERPOLATION DATA

The STL geometry was provided courtesy of Mark Yeoman, Continuum Blue, UK. The original STL surface meshcan be visualized as a Mesh Part, as shown below.The modified solid object can be meshed using the automatic meshing tools under the Mesh node. The figure belowshows an automatically generated volumetric mesh. Note that the surface mesh is modified and in this case madecoarser than the original STL mesh by the automatic mesher to accommodate the user-defined Mesh Size settings.IMPORTING MESH DATA AS A COMPUTATIONAL MESHIf you wish to use an external volumetric mesh directly as the computational mesh in COMSOL Multiphysics youcan import mesh data from a file on the following formats: COMSOL native binary, COMSOL native text,IMPORTING AND EXPORTING MESH AND INTERPOLATION DATA 15

COMSOL sectionwise, or NASTRAN. For modeling in 2D, the same formats are supported when importing 2Dmesh data. The third coordinate must be the same for all mesh vertices when importing 2D NASTRAN mesh data.Importing a mesh clears the geometry defined in the corresponding geometry sequence. In order touse an imported mesh together with an existing geometry, import it as a Mesh Part or to a separatecomponent. Then, use the Copy or geometry Import features to transfer the mesh to the geometrysequence.The figure below shows a structural analysis of a wrench based on an imported mesh. The original CAD-basedmodel is available in the COMSOL Multiphysics Application Library with a description in the documentIntroduction to COMSOL Multiphysics, available in the installed documentation set or vely, you can import a surface mesh on any of the formats STL, PLY, or 3MF. After import, you can usesuch a surface mesh to directly create a volumetric mesh without first converting it to a solid geometry object. Whenstarting from a surface mesh, directly creating a volumetric mesh, under the Mesh node, is often more robust thatwhen first converting it to a geometry, that becomes available under the Geometry node. Note that before creatinga volumetric mesh you may first need to use an Adapt operation to improve the quality of the imported surfacemesh. For an example of using an Adapt operation, see timization-results-as-model-geometries.Mesh PartitioningWhen a mesh is imported into COMSOL Multiphysics, the Import node automatically determines a partitioning ofthe mesh into domains, boundaries, edges, and vertices. If the automatically performed partitioning does not match16 IMPORTING AND EXPORTING MESH AND INTERPOLATION DATA

the requirements, you can modify the boundary partitioning by manually adjusting the corresponding parameters.The figure below shows the Settings window for the Import node under Mesh.In addition, you can perform manual partitioning operations by using Ball, Box, and Cylinder operations as well aslogical expressions in terms of the coordinate variables x, y, and z. The figures below show the imported mesh andimport settings for the wrench example, where the center part of the object has been partitioned using a ballIMPORTING AND EXPORTING MESH AND INTERPOLATION DATA 17

operation. The additional domains and boundaries formed by this type of operation can be used, for example, todefine other material properties or loads.To import additional mesh files, add another Import node. Then, COMSOL Multiphysics adds the elements andvertices of the newly imported mesh data to the existing mesh. Mesh data from different Import nodes form anassembly with potentially overlapping elements. Mesh data with overlapping elements has very limited use incomputations. However, for disjoint sets of mesh data that are overlapping only at touching surfaces, you can, forexample, define pair continuity conditions and proceed with computations as usual.Remeshing Imported MeshesYou can remesh an imported mesh to create a new mesh that is suited to solving the problem at hand. For moreinformation, see Mixing CAD and Surface Mesh Data and the COMSOL Multiphysics Reference Manual.18 IMPORTING AND EXPORTING MESH AND INTERPOLATION DATA

Modifying Imported MeshesFor 3D and 2D imported mesh sequences, the Refine, Adapt, Convert, Create Vertices, Create Edges, Delete Entities,and Join Entities operations are available for modifying imported meshes. For 1D imported mesh sequences, theRefine and Adapt operations are available. In 3D, the Create Entities menu include: Create Vertices, Create Edges,Create Faces, Create Domains, and Fill Holes.SelectionsSelections are used for grouping geometric entities in order to better organize the model setup. They are used forassigning materials, volume forces and sources, boundary conditions, edge conditions, and point conditions todomains, boundaries, edges, and points. User-defined selections are defined under Component Definitions, as shownin the figure below, where three domain selections have been defined: Ti Bolts, Copper Piece, and Single Bolt.In this example, the selection Ti Bolts is used to assign the material Titanium beta 21S to domains 2–7.When imported mesh data contains selection information and the Import selections check box is selected, a list ofselections is displayed in the sections below the Import section in the Settings window for Mesh Import. The figureIMPORTING AND EXPORTING MESH AND INTERPOLATION DATA 19

below shows an example of imported mesh data containing three different Domain Selections corresponding to thesame selections as in the previous example (which did not make use of an imported mesh).Depending on the contents, there will be sections for Domain Selections, Boundary Selections, Edge Selections, andPoint Selections. These selections can then be used in the same way as the user-defined selections defined underComponent Definitions. For convenience, since imported mesh data may contain a large number of selections, suchselections are listed in table format in the Import window instead of as separate nodes in the model tree.IMPORTING INTERPOLATION DATAData on the sectionwise, spreadsheet, and grid formats can be imported to an existing model for comparing aCOMSOL simulation with a simulation done in another software or for use as input; for example, a source term orboundary load in a new COMSOL Multiphysics simulation. In addition, the sectionwise and spreadsheet formatscan be imported and used to create Interpolation Curve and Polygon geometry objects in 2D and 3D. You can, forexample, export a set of contour curves or streamlines on the sectionwise format and then import the set of curvesas an Interpolation Curve geometry object.20 IMPORTING AND EXPORTING MESH AND INTERPOLATION DATA

Importing Numerical Data and Comparing with Simulated DataData stored on the sectionwise, spreadsheet, and grid formats can be imported as interpolation data, available underGlobal Definitions Interpolation, as shown in the figure below in the case of the sectionwise format.The example in this section is a modified version of the example in the document Introduction to COMSOLMultiphysics, available in the installed documentation set or from https://www.comsol.com/documentation.Files corresponding to the example below can be downloaded from:https://www.comsol.com/model/72351The filenames are:busbar box fully coupled export from normal mesh.mphbusbar box fully coupled compare.mphbusbar box T data grid.txtbusbar box T data spreadsheet.txtbusbar box T data sectionwise.txtIn this case, temperature data in the computational volume of a fluid flow with heat transfer simulation is beingimported. The data is computed on a coarser mesh than that of the current simulation.Two interpolation methods are available in 2D and 3D: Nearest neighbor or Linear. For regions where nointerpolation data is available, the data will be extrapolated using (in 3D) one of the following Extrapolationmethods: Constant, Linear, Nearest function, and Specific value. The available extrapolation methods depend on theData format and the Interpolation method.Imported data can be visualized on an existing mesh, automatically applying data interpolation where needed, oragainst a regular grid using a Grid Dataset. The figure below shows the temperature data visualized on a Grid 3DIMPORTING AND EXPORTING MESH AND INTERPOLATION DATA 21

Dataset. In this case, the dataset is created automatically when clicking Create Plot in the Interpolation Settingswindow.The figure below shows the same data visualized on the unstructured tetrahedral mesh of the current simulation.The two visualizations are very similar in appearance.22 IMPORTING AND EXPORTING MESH AND INTERPOLATION DATA

The figure below shows a comparison between the temperature field of the current simulation, using a mesh withthe Element Size set to Finer, and the imported data, previously simulated on a mesh with the Element Size settingNormal. The maximum temperature difference between the two simulations is about 5 kelvins.Note that the same type of comparison can be done by using two different meshes within the user interface withoutusing file export and import. This example illustrates the use of file export and import.The mesh in the example above is generated in COMSOL Multiphysics, but an externally created mesh could alsobe used with the functionality provided when using Mesh Import.The figure below shows a similar comparison, but where the imported data is originally defined on a regular gridwith 50-by-50-by-50 grid points.IMPORTING AND EXPORTING MESH AND INTERPOLATION DATA 23

In this case, the discrepancy is a bit higher due to the mismatch between the regular grid and the unstructuredmesh.Importing Data on SurfacesWhen interpolating data sampled on a surface, only the sectionwise and spreadsheet formats are available. Thesectionwise format is faster, but the spreadsheet format is recommended, since it gives higher accuracy ininterpolation.Files corresponding to the example below can be downloaded from:https://www.comsol.com/model/72351The filenames are:non newtonian flow compare surface.mphnon newtonian flow export from normal mesh surface.mphnon newtonian flow normal sectionwise w.txtnon newtonian flow normal spreadsheet w.txtThe figure below shows a flow field in the z direction visualized by slices.24 IMPORTING AND EXPORTING MESH AND INTERPOLATION DATA

This simulation uses Element size set to Normal. The figure below shows the same flow field in the z directionvisualized on the surface. In this visualization, Manual color range is used with a minimum value of 0 and maximumvalue of 0.1 (m/s).The following figure shows the corresponding mesh.IMPORTING AND EXPORTING MESH AND INTERPOLATION DATA 25

The figure below shows the simulation where the Element size is set to Finer. The same manual color range is used.The following figure shows the corresponding mesh.The figure below shows the z-directional flow data imported as an interpolation function in the spreadsheet format.In this example, the data is first exported from a simulation based on the Normal mesh element size and thenimported to a model where the Finer mesh element size is used. The simulation data based on the coarser mesh is26 IMPORTING AND EXPORTING MESH AND INTERPOLATION DATA

automatically interpolated to the finer mesh and can be used, for example, as input to subsequent simulati

import and export functionality available in COMSOL Multiphysics and a detailed description of the import, export, and native COMSOL formats. For more detailed information on how to use COMSOL Multiphysics for import and export of data, see the COMSOL Multiphysics Reference Manual in the installed documentation set. Overview of Data Formats