Geometry importation and CAD cleaning operations

Description

This course is focused on the geometry importation and cad cleaning operations which are needed in most cases for prepare the imported model to be meshed

Final model obtained when following all the steps of the course.

We will import the geometry of half an aircraft (shown in the figure) in IGES format and we are going to follow the whole process from the importation to the meshing process of a control volume surrounding the aircraft.

Note: the model originally was a "NASA Common Research Model" (commonresearchmodel.larc.nasa.gov)

Import of the model

GiD import and export preferences

The user can customize some parameters for the import and export of geometry. This parameters are located in the Import and Export part of Preferences window.

Click on Utilities->Preferences and go to the Import and Export part. The window shown in figure should appear.

Import and Export preferences window.

When GiD imports a geometry (independently on the format), it automatically perform some cad cleaning and repairing operations such as check entities orientations, collapse small gaps, etc. User can avoid this automatic operations by unchecking the Automatic collapse after import.
If the automatic collapse is set, GiD can use a distance tolerance entered by the user, or can compute an automatic tolerance based on the representative size of the model. With the option Automatic import tolerance value user can choose whether the tolerance is automatic or the manually specified.
We are going to set all the options shown in the figure and click Apply.

Import model

The IGES file to be imported is named half_aircraft.igs and it should be found at Material location. .

Select Files->Import->IGES... and select the file half_aircraft.igs.

A progress bar appears showing the state of the automatic CAD cleaning operations GiD does when imports the IGES file.

On the lower messages bar you could see the tolerance used for the automatic collapse operations, and the result of the automatic collapse on import.

Import tolerances

After importing a model with automatic tolerances, it is very useful to check how the model looks like, and get a minimum characteristic distance, to ensure the automatic tolerance computed by GiD does not collapse any geometric detail interesting for simulation. In this case, it should be useful to measure the distance of the ending part of the tail, as it is shown in figure.

Size of the end part of the tail.

You can check it by using Utilities ->Dimension ->Create ->Distance, selecting the two points and placing the dimension label.

As it can be seen in the preferences window, now the automatic tolerance for the collapse computed by GiD is 0.126698.

Depending on the version of GiD used, this tolerance can be different, so the result of the import of the model may differ in some details.
To ensure that all the participants of the course have the same model resulting from the import, we will use a tolerance specified by the user. Taking into account the distance shown in the previous figure, we can try for example with an Import tolerance of 0.025 that is smaller than the detail to preserve. For this purpose we need to import again the model:

Select Files->New, and select No for the question Save changes to this project.
Open the Preferences window, and go to the Import and Export part.
Select from the combobox Import tolerance: Used define
Enter 0.025 in the Value of import tolerance field.
Click on Apply and close the Preferences window.

Now import again the IGES file half_aircraft.igs (Files->Import->IGES...).
Once the model is imported, save the model (this will be the model we will work with during this course). For saving the model just select Files->Save and choose a proper location in your computer to save the model in.

CAD cleaning operations

Visual check of geometry

A useful step when a file is imported is to check visually how it looks like. Rotate the model and set the Render mode to Flat (from the View menu or mouse right button menu). With this initial visual scan some possible big geometry imperfections or import errors may be detected.
If some surface is not well rendered (like strange shadows or holes in the view) or no render is done, it means that GiD had some problem to generate the render mesh of the surface. This should point a possible error in that surface (or may be not).

Trick: the GiD-Tcl procedure GiD_Geometry list -unrendered surface provides the list of surfaces that were not able to be rendered.
You can write this in the command line to know this list:

-np- W [GiD_Geometry list -unrendered surface]

A message window will appear, but in this case it is empty, as all surfaces were rendered.
As it can be seen in the figure, the render of the surface which connects the wing of the aircraft with the main fuselage looks a bit off.

Render view of the model imported, with the surface which render is not so good highlighted.

Let's check if this surface is well defined or not. A good idea is to isolate this surface in another layer to visualize it more clearly.

Open the Layer window ( Ctrl-l , (lower L) or Utilities->Layers and groups, or the corresponding icon
in the Toolbar)
Create a new Layer (using the New Layer option in the mouse right button menu, or using the corresponding icon in the upper icon bar of the Layers window).
Send the surface to the new layer by using the Sent to->Surfaces option of the mouse right button menu after the target layer is selected (the corresponding icon in the upper part of Layers window can be also used).

'New layer' and 'Send to' icons.

Now we can switch off all the layers except the one in which the surface is. We should be able to see just this surface, as shown in the figure.

View of the problematic surface isolated.

We can change the render mode between Normal and Flat to understand better the definition of the surface:

'Render normal' view

Generate automatic test mesh

Another useful check that can be done when importing the geometry is to generate a first mesh, using the default meshing preferences of GiD. Often, some detail of the model which can be skipped during the visual scan is detected when looking at the mesh, for instance, possible concentration of elements, entities which cannot be meshed, etc.

Open the Preferences window select Meshing and with right mouse click select Default values on selection from the pop-ip menu, ensure that Also selection children is checked, and press Apply.

Set default values to selection and its chidren

Then generate the mesh by pressing Ctrl-g or by selecting Mesh->Generate mesh... and click OK using a mesh size of 140 and unchecking the Get meshing parameters from model option:

General mesh size window

As it can be seen in the figure GiD can generate a mesh on all the surfaces.

Coarse mesh

In this step is not important to check the mesh quality, but to check where the elements are concentrated, and if there are surfaces which cannot be meshed.