In this section, we create cylindrical objects with four holes to be used as catalyst piled inside the reactor, instead of the short circular fibers. This object is not included in the predefined object types in GrainGeo and it will be generated here using GadGeo.

If you worked through the previous sections of this tutorial, discard the current structure through File → Discard Structure and Volume Files. If there is a message box asking if the material with ID 01 should be set to visible, click Yes.

Make sure to select the Results-User folder as the project folder (File → Choose Project Folder → Select Project Folder).

1. Select Model → GadGeo in the menu bar.
   
2. In the GadGeo module section, choose Create / Add GAD Objects from the pull-down menu and click Edit… to open the Create / Add GAD Options dialog.
   
3. Set the Result File Name to Cylinder-4-holes.gdr. On the top right, choose mm as length unit.
4. The list of objects to be generated shows the short circular fiber from Creating a Reactor if you worked through the tutorial or the default objects, a sphere and a general short circular fiber. We don't need them in this step, so, unfold and remove all objects by clicking on their names and then clicking Remove for all of them.
   
5. At the bottom of the dialog, more object types can be added to the list. From the Object Type pull-down menu, scroll down and select Combined Object and then click Add.
   
6. A sphere is automatically added as first object (visible in the Ob. 1: Sphere (add) tab), but we do not need it throughout this tutorial. In the General 1 tab, click the Delete button to remove Object No. 1.
   
7. Afterwards, in the General 1 tab, choose ShortCircularFiber from the left pull-down menu and choose Add from the one on the right side. Click Create to insert a cylinder in the currently (still) empty structure. Then, a new tab appears to the right of General 2 tab.
   
8. In the General 2 tab, check that the combined object is defined in the correct coordinate system. This means that the main Direction is set to (1,0,0) and that the Perpendicular is (0,1,0)
   

9. Now go to the new tab Ob. 1: ShortCircularFiber (add) in the General tab. To define a cylinder of height 3 mm and diameter 1.6 mm, where the center of the domain is equivalent to the mass center of the cylinder, enter the following values:
   1. Point1 / (mm): (0, 0, -1.5)
   2. Point2 / (mm): (0, 0, 1.5)
   3. Diameter / (mm): 1.6
      
10. Next, we subtract four cylinders (with the same height but a much smaller diameter) from the first object.
    For this purpose, go back to the General 1 tab and choose ShortCircularFiber again. From the pull-down menu on the right side select this time Subtract.
    
11. Click four times on Create, once for each of the holes that we want to generate.
12. In the new tabs for each subtracted cylinder, set the points as given in the images below. The diameter is set to 0.3 mm for all cases. Use the buttons with the black arrows to navigate through the object tabs.
    
13. Go to the General Options tab to specify the settings for the domain.
    1. Set NX, NY, NZ to 400, respectively, and the Voxel Length to 0.01 mm.
    2. Make sure, that the boxes Periodic X/Y/Z are ticked.
    3. Click Center Domain. Leave the remaining settings as they are.
       
14. Then, click OK in the dialog and Run in the GadGeo module section.
    
15. After the creation the resulting structure is shown in the visualization area in 3D.
    
16. Click File → Save Structure as... in the menu bar and enter Cylinder-4-holes.gad as file name. Then choose GeoDict Analytic Data – ASCII (*.gad) as file type. This changes the way in which the structure is saved. Click Save to export the current structure as GAD data.