CAD import in SimWorks

CAD import in SimWorks

SimWorks can import CAD files in IGES (.igs) and STEP (.step) formats. These are common import / export formats available in the majority of CAD software. Whichever CAD software you are using, it is likely that you will have either of the two or both available. 

SimWorks has the following main modes of importing geometries:

1) Standard import – default mode which automatically recognises and groups all the geometrical entities according to their names

2) Import with split – splits automatically the CAD into the individual faces in case the geometry is a single surface, it is also useful when the CAD does not import with the standard import functionality

3) Import error – if the imported CAD part has an error SimWorks will automatically import it as a single surface, in this case it is not possible to split it in SimWorks nor it is possible to carry out internal aerodynamics simulations. If you used the standard import try using Import with split. See the details below.

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Standard import in SimWorks

 

The standard geometry import functionality automatically recognises all the features of the imported geometries, this way the user can use certain areas of the imported geometry to both define boundary conditions or independently use them for defining mesh parameters.

Standard import functionality in SimWorks (step 2)

 

Ti import a geometry with this mode it is enough to select a geometry (step 1 in the image above), the geometry file path will also automatically appear in the Geometry entry and finally press Import geometry (step 2 in the image) to import it. This is the default methodology used in any of the SimWorks tutorials.

CAD representation of a glider plane, used in our SimWorks glider tutorial

We will analyse the way the CAD is defined and imported in our SimWorks Glider tutorial. You can download the CAD .igs file from here.

If you open the .igs with a common txt editor, like Notepad++, you will see the structure of the .igs file. The file contains names representing parts, e.g. collections of surface patches of the geometry.  For instance you can find a part called Fusolage in the file:

2_guide_cad_import
glider.igs file opened with a text editor

Once this file is imported in SimWorks (see the Glider tutorial to see how to do that), the software will automatically recognise and name those parts.

You can import the geometry and activate the Part names layering in the Geometry viewer window to check the recognised parts:

Part names recognised by SImWorks

Those part names are required to be able to create Part groups and therefore apply certain boundary conditions to some portions of the geometry. In a similar fashion, you can define Surface mesh groups to apply specific mesh settings on portions of the geometry. Moreover part groups are independent from mesh groups so you can define different collections of surfaces for the two cases.

It is therefore recommended that you group the base surfaces / patches representing each geometrical part you are planning to use in the CFD simulation and name those in your CAD system before exporting the geometry. You can then make sure that this is reflected in the .igs file you exported by opening it with a text editor like in the example above.

CAD file showing different part groups before exporting

What if the parts are not named in the .igs file?

If the surfaces/patches are not grouped and named before exporting the surface in the native CAD software or if the CAD software you are using does not write part names in the exported .igs files or in case the geometry is coming from a 3D scan, SimWorks will treat each surface/patch independently. In the example below, the same geometry of the glider has been saved without naming the parts. This time all the individual surfaces/patches are listed independently rather than grouped in meaningful portions of the geometry:

In this case every individual surface will be named in the exported .igs file, rather than belonging to a group. You can download an example of the glider CAD file without part names here:

This approach is not recommended because SimWorks will take much longer to open the file and defining new Part groups or Surface mesh groups will be more complicated. Moreover, naming the CAD with part names is a requirement in SimWorks Manager in order to be able to use the advanced duplicate simulation functionality. This would allow to automatically re-apply the original settings (both boundary conditions and meshing parameters) to a new design of the same geometry. 

By importing the file without part names in SimWorks,  we can see all the individual surfaces contained in the file using the Part names layering:

It is still possible to create Part Groups and Surface Mesh Groups containing multiple surfaces/patches and therefore applying different settings to those. For example, to create a new Part Group you just have to thick the boxes next to the surface patch name in the Regions tree in the Simulation editor window. You can then apply a specific boundary condition to the group you just defined, select Boundary conditions types layering menu.

Click on all the faces you are interested in while keeping the Ctrl key pressed. You will see that all the clicked faces will be highlighted in green and the relevant parts are automatically selected in the Parts dropdown menu. At this stage, it is possible to click on the Create Part Group button to create a new Part Group that can then be used to define new boundary conditions:

Import with split

When you have defined in your CAD a single part containing all the surfaces/patches. You can download an example CAD file from here.

In this case the Glider will be exported with a single surface called Surface.7

When you import this file in SimWorks you will be left with a single part. This situation will let you define a single boundary condition and apply a single mesh setting on the whole geometry. For instance, you can  define the glider surface as a fixed wall boundary conditions but you cannot define different mesh settings on different parts of this geometry. For example, you cannot set a finer mesh on the wings compared to the fusolage. 

SimWorks is recognising only a single entity in this case

The best way to fix this it to come back to the original CAD, split it in the desired entities, rename them according to the desired areas names and finally re-export a new .igs file. Once the file is imported in SimWorks the parts will be automatically recognised. 

Alternatively you can use the Import with split functionality, just click on the little arrow next to the Load geometry icon and select the Load geometry and split it command:

Load geometry and plsit it command

Once the command is clicked the original file will be automatically split into the base patches, in the same manner as when the parts are not actually named in the original CAD (see the case above). Also the Import with split functionality is much more stable then the standard import in case you are having issues with the default mode. 

From this point onwards you can still define bespoke boundary condition or mesh parameters on portion of the geometry:

The import with split funcitonality will split the CAD in the original faces

Import ERROR

Sometimes SimWorks is not able to load the imported geometry. In this case you will get the following warning message:

Warning message when the CAD geometry has issues

In this case SimWorks will import only a single entity like the image below, but in this case it will not be possible to split it down the line or to define any specific mesh settings or boundary conditions on individual portions of the geometry. For this reason when this happens it will only be possible to carry out external aerodynamics simulations because it would not be possible to define boundary conditions on the faces of a geometry used for internal aerodynamics simulations:

SimWorks is recognising only a single entity in this case

If you used the standard import functionality please try using the import with split functionality (see details above).  

The reason of this error is because the geometrical engine behind SimWorks is sometimes not able to recognise individual faces inside an imported igs or step file, most of the time there is nothing wrong with the geometry, but it is still worth checking in the original CAD and export subentities of the original geometry trying to understand which is the CAD part which is triggering the error. Sometimes this can be due to self-intersections issues, ill defined geometrical entities or poor geometries in general. Unfortunately there is not a single solution to fix this other then checking and fixing the original CAD.

If you want to carry out a conventional external aerodynamics simulation and the import with split did not work you can still do so splitting the geometry in CAD into independent igs files and load it up as multiple files either selecting them with ctrl clicked and then splitting the parts into independent part groups or importing them one after the other, in which case independent part groups for each geometry will automatically be created (from version 21.12).

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