This installment of the Cart3D tutorial will walk through the process of generating an unintersected component wireframe in VSP and then using Cart3D's intersect utility to process the geometry into a single watertight body. If you haven't built the Onera M6 wing, or you don't have your own geometry to analyze, you should go back to the geometry generation phase of the tutorial or you can download a ready-to-go Onera M6 geometry from the VSP Hangar.

Although Cart3D's intersect tool is very robust, there are some cases it does not handle. One of these cases is when surfaces of two parts are perfectly coincident. This situation occurs at the wing root of the Onera M6 model created in the tutorial; the root surface of the left and right wing halves are perfectly coincident.

One common fix for this situation would be to move the parts slightly (say 10^-6) in the spanwise direction so the root surfaces are no longer coincident. Since the wing roots are normally buried inside a fuselage component, the gap between the wing halves would be insignificant. However, the Onera M6 geometry has no fuselage and any positive offset would result in a gap between the wings and any negative offset would result in intersection artifacts at the wing root. Furthermore, demonstrating the intersect tool with two parts that don't really intersect is not very useful.

Consequently, for this installment of the tutorial, a pod component will be added to the geometry to play the role of a fuselage in the model. This obviously would not be appropriate for matching the Onera M6 wind tunnel results, but it should do well for the purposes of this tutorial.

As you proceed through the tutorial to mesh the Onera M6, remember that images have been included as thumbnails, so any place you see a picture, click on it for a larger version.

1. Onera M6 start
   • Start with the Onera M6 model from the end of the model building tutorial.
     
2. Refine airfoil points
   • The triangulated wireframe is created by connecting the diagonals of the quadrilateral wireframe shown on screen. Consequently, the resolution of the triangulated wireframe is determined by the resolution of the quadrilateral wireframe shown on screen.
   • As discussed in the Mesh Generation discussion of the tutorial, Cart3D has unique mesh resolution and quality requirements which make it particularly well suited to VSP and this approach.
   1. In the 'Multi Sect Wing Geom' window, under the 'Gen' tab, enter 101 in the box to the right of 'Num Points:'
      
3. Refine spanwise wireframe
   1. Select the 'Sect' tab and then click one of the right-facing arrows under 'Num Interpolated XSecs' until there are 40 interpolated cross sections.
      
4. Add a pod
   • A pod component at the wing root will give the intersect routine something to do for this demonstration and will also allow the wing halves to be offset slightly in the spanwise direction.
   1. In the 'Geom Browser' window, select 'POD' from the pulldown next to the 'Add' button.
   2. Click the 'Add' button.
   3. In the 'Pod Geom' window, go to the 'Design' tab and change 'Length' to 1.0.
   4. Go to the 'XForm' tab and shift the pod forward by changing 'X Loc' to -0.1.
      
5. Refine the pod
   • Like the wing, the wireframe resolution of the pod will determine the resolution of the triangulation.
   1. In the 'Gen' tab of the 'Pod Geom' window, change 'Num XSecs:' and 'Num Points:' to 50.
      
6. Offset the wings
   • The wing components must be offset slightly in the spanwise direction so the root faces of the opposite wing halves are not perfectly coincident.
   1. In the 'Geom Browser', select the wing component to bring up the 'Multi Section Wing Geom'.
   2. Under the 'XForm' tab, change the 'Y Loc' to 0.0001.
      
7. Export unintersected file
   • An unintersected triangulated surface can be exported from VSP by attempting to export a triangulated format when no triangulated mesh exists. In this situation, VSP will create an unintersected triangulated mesh on-the-fly.
   1. From the 'File' menu, select 'Export…'.
      
   2. From the displayed list of export options, choose 'CART3d File (.tri)'. Unintersected triangulated meshes must be written in TRI format.
      
   3. In the 'Select File' dialog, navigate to the save location and choose a file name. When you are satisfied, click 'Accept'.
      
   • After the export has completed, VSP will display the unintersected mesh that was created on-the-fly. The mesh shown on screen exactly corresponds to the information written to the file. Note that the original geometry components are still present, but they have automatically been hidden.
     
   • Zooming in on the nose-region of the model demonstrates that no intersection or trimming operation has been performed.
     

The VSP interface is the best way to set up and adjust the wireframe resolution, but once that is established, you may want to generate the wireframe from the command prompt.

To generate an unintersected triangulated wireframe from the command line, enter the following command:

vsp -batch infile.vsp -tri

This command will generate the unintersected wireframe and write 'infile.tri'.

Cart3D's intersect tool is used to intersect the triangulated components and create a single watertight body. This tool is more thoroughly documented in the Cart3D documentation.

If you copy the unintersected triangulated wireframe file from VSP to 'Components.tri' you can run Cart3D's intersect utility with no command-line options. Otherwise, execute intersect with the following command:

intersect -i infile.tri

This command will intersect the components in 'infile.tri' and will write the result out to 'Components.i.tri' ready for analysis.

Once you have successfully intersected the model, continue to the Cart3D Setup Tutorial.

Or, feel free to jump to your topic of choice.

1. Geometry Generation
2. Mesh Generation
   • Isotropic Generated Mesh
   • Unintersected Component Wireframe (This Page)
   • Intersected Component Wireframe
3. STL to TRI Conversion
4. Cart3D Setup
5. Cart3D Execution