CAD import in SimWorks

CAD import in SimWorks

SimWorks can import CAD files in IGES (.igs) format and soon will support STEP (.step) files too. 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.

This tutorial explains how SimWorks recognises the imported geometries and how the user can make sure that the different parts in the geometry are identified and can be used.

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:

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.

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 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 to apply a specific boundary condition to a group of surface, 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:

What if I have a single part defined in CAD before exporting?

The final scenario is 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 only way to be able to define different boundary conditions and mesh settings is to come back to the original CAD, split it in the desired entities, export a new .igs file and finally reimport them in SimWorks.

How can I split a single entity in individual faces?

Most CAD software have the functionality to split surfaces into individual faces and the capability to group them or join them. Those groups can be named before exporting the file to IGES or STEP format.

If this is not possible in your CAD software, as already seen above, SimWorks will let you to work with individual geometrical faces and patches anyway. But if your geometry has a single entity and you want to split it into individual faces you could try the free and open source CAD software FreeCAD.

In FreeCAD it is possible to open the geometry and export it so that SimWorks will recognise independent faces.

Open your original igs file there:

Slider saved as a single surface opened in FreeCAD

Simply click on the imported part, click on the Part menu and then on the Export CAD… option and select the .igs format. This way you will create an .igs file which can be imported in SimWorks recognising a different patch for each face of the original CAD:

Export of Glider geometry from FreeCAD

If you are unsure about how to name your CAD or experience issues importing your geometry in SimWorks, do not hesitate to contact us and we will be able to help.

Try our free CFD software SimWorks, no registration required or contact us to find out more

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