G-DEMPack Tutorial 1: Conveyor belt
This tutorial assumes that both the D-DEMPack and the GiD Pre and PostProcessor are installed, otherwise please follow the instructions in D-DEMPack installation. Please check the D-DEMPack manual page for a detailed explanation of all the fields and concepts present in this tutorial as well as for how to load the Problem Type.
Creating the Geometry of the Domain and the DEM Objects
In general, the process should start by creating a new geometry from scratch or by modifying or using an already existing one. For this tutorial, a group of simple entities will be created which try to represent a simplified example of most of the capabilities of the program. The main objective of this document is to understand all the steps needed for creating and running a DEM simulation using the D-DEMPack package.
- Succinct explanation of the creation of geometries
We will start then by assigning the corresponding group properties. In this tutorial, a very easy geometry was created for the sake of simplicity. Figure A2-1 shows the geometry chosen, very simple but representative enough.
We follow by assigning groups to the geometry. To do this, we must open the Group Editor window by clicking on the icon showing in the next figure:
Figure A2-2. Group Editor icon
Once the window is opened, we have to create the groups representing our domain. In this sample case, the following groups were created: box, inlet, outlet and fluid.
We must start assigning entities to the groups. To do this, just right-click on the corresponding layer, go to assign, choose the geometry type (in general surfaces or volumes) and choose the appropriate geometry. In this case, the lower circle (surface) would be the inlet, the upper one would represent the outlet, the box would be the walls of the cylinder and, finally, the volume would constitute the mass of fluid.
Figure 1. The geometry of study.
Setting the DEM Materials
We will continue by defining the properties of the fluid in the simulation. We just have to click on the icon showing in the next figure to open the menus and in particular the Materials tab:
Figure A2-4. Opening the Materials menu
To begin inserting the fluid properties, we go to Fluid>Water and unfold the Fluid submenu by clicking on the plus sign on the right of Water. A list of parameters to fill in will unfold as. The next figure shows the details.
Figure A2-5. Fluid properties in the Materials tab
To fill in or modify the value of the parameters, just double-click on the corresponding current data. The data above corresponds to an intermediate viscosity mud. The same process can be followed, in this case, for the DEM material. We unfold the DEM-Default material submenu at the bottom of the same window and fill the fields with the appropriate data. See Figure A2-6.
Figure A2-6. DEM material properties
Assigning the DEM Element Entities
A2.6. Adding initial DEM volumes in the simulation
The user can also include initial volumes composed of DEM spheres. To assign this volumes, the user must first double-click the DEM > Elements > DEM_Element section inside the Model tab. A 'Properties' window will show up open where the user will tell the program the group associated with this DEM domain and its corresponding material. See Figure A2-18 for details.
Figure A2-18. DEM Elements window
This volume must be discretized using an specialized sphere mesher, different from the tetraedra-based one used to handle the fluid part. This particular mesher is not used by default, so the user must tell the preprocessor that this mesher is going to be used.
To use the sphere mesher, the user must first select the DEM volume. This is done by going to Mesh > Element_type > Sphere, selecting the desired volume and pressing Esc when done. Secondly, an average radius must be chosen for the mesh by clicking on Mesh > Unstructured > Assign_sizes_on_volumes, inserting the desired size in the window that will open, clicking on Assign to select the volume and pressing Esc and Close to finish assignation.
Additional meshing settings can be set on the Preferences window. This window is accessible by clicking on Utilities>Preferences. The options are available clicking on the tree option ‘Meshing’, tree sub-option ‘Sphere Mesher’.
We set the fluid properties by assigning them to the Water material previously created. To do so, right-click on Properties and choose New. A New Property window will appear below. We choose Water on the label next to Material and click on OK. The properties of the fluid in the problem are those of the Water material. See next figure for details.
Figure A2-9. Linking the fluid properties to the Water material
The next step is to tell the program which are the fluid elements amongst all the geometric entities. We can do so by going to Elements and right-click on Fluid. As before, choose New and a window will open up below. On the Group tab, choose fluid and click OK. The elements are properly assigned. See Figure A2-10 as a guide.
Figure A2-10. Assigning the appropriate entity to the fluid elements
Setting the Boundary Conditions
Figure A2-3. Group Editor window
We now need to tell the program the model characteristics: boundary and initial conditions, inlet options, time considerations, etc. To begin filling all this data, we just click on the Model menu located at the left of the Materials tab and the Model window will show up. This menu can be accessed also by clicking on the icon showing in the next figure:
Figure A2-7. Icon for the Model menu
Creating Inlet Objects
A2.7.1. DEM Inlets
Apart from setting the value of the different parameters that will show up, again we have to relate this inlet condition with its corresponding geometrical entity by right-clicking on Inlet>New and assigning the appropriate layer. See Figure A2-19 for details.
Figure A2-19. Inlet boundary conditions
Creating DEM-FEM Wall Entities
A2.7. DEM boundary conditions
We follow by assigning the DEM boundary conditions. In practice they refer to the inlet of DEM particles in the domain and the walls.
A2.7.2. DEM contact with walls
By default, the DEM elements do not experiment any force from any surface of the meshed domain. If this is desired, there is an extra condition to mark the surfaces that will be impenetrable. This condition has been called ‘DEM-FEM wall’. To apply this condition, just double click on the condition as shown in Figure A2-20.
Figure A2-20. Condition to set walls for the DEM and embedded structures in the fluid
A properties box is opened to fix the motion of this surface. This box has 3 labels to set the properties for ‘linear velocity’, ‘angular velocity’ and ‘Embed’.
Figure A2-21. Properties of the ‘DEM-FEM wall’ condition
Both ‘linear velocity’ and ‘angular velocity’ parameters can be set to be periodical, assuming that the group starts at the center of the motion with maximum speed equal to the imposed value. By default, motionless properties are imposed. I case of imposing both linear and angular velocities, the linear velocity only affects the center of rotation, while the rotation is around the updated position of the center of rotation. Under the label ‘Embed’, the option ‘Embed this Wall in the fluid’ can be activated. When this option is active, the group marked with this condition is seen by the fluid as impenetrable also. It is actually set as a Slip boundary condition with a wall law. Important note: the groups marked with the ‘DEM-FEM wall’ condition do not need to be formed by walls being boundary of the fluid tetrahedrical mesh. They can be independent sets of surfaces intersecting volumes of the fluid mesh, and the actual intersection of every triangle and the tetrahedra is calculated internally to apply the boundary condition on the fluid. Also, the group of surfaces must be closed (or near to closed allowing small gaps) if an empty body is wanted, otherwise it will be considered as an embedded membrane.
Setting up the General Options
A2.5. DEM general options
The next step is to define the DEM parameters. Still in the same menu, we go to DEM and unfold both DEM-General Options,DEM-Fluid interaction and Solution strategy submenus. We fill in the fields with the desired values. Figure A2-16 shows the menus.
Figure A2-16. DEM options menu
We follow by setting the limits of the bounding box:
Figure A2-17. Bounding box options
Choosing the Solution Strategy
This item has two subsections. On the first subsection, the user can set the type of parallel computing to use as well as the number of threads. The second subsection is devoted basically to time parameters, where the calculation delta time and the total time of the simulation can be entered. The user can also set the DEM neighbours search frequency and how often he gets information output on screen.
A2.9. Results options
In the tree branch called Results we can set also the size of the trap box that will serve to measure the averaged velocity of the particles inside it.
Figure A2-23. Velocity trap options
A2.2. Fluid general options
Figure A2-8. Global time parameters
We continue by setting the time properties by going to Fluid>Problem_parameters. See Figure A2-8.
Meshing and Running the Simulation
A2.8. Meshing (both fluid and DEM)
Finally, the last step before launching the calculation is to mesh the domain. One good way to save number of elements –and that way reducing the computational cost of the simulation- is to mesh using the semistructured option.
When it is expected that the solution flux of our simulation is going to have a preferred direction, deforming the tetrahedra in that direction will introduce very small error.
So it is an interesting option to strain the elements following the expected flow in order to obtain lighter and faster simulations.
To mesh following this criterion, we must go to Mesh>Semi-Structured>Volumes. A window will show up asking for the number of divisions we want to apply on the fluid volume following such direction. In the present case, we chose 2 just as an example. After filling in the desired number, we click on Assign and select the volume. To finish the operation, we hit Esc and close the window. Now we can mesh and we will obtain a mesh similar to the one in the figure:
Figure 2. The meshed domain.
A2.10. Running the case
Finally, we save the model and launch the calculation by clicking on Calculate>Calculate. The user can follow the state of the simulation by going to Calculate>View_process_info. Once the calculation is finished, we can examine the results by shifting to the GiD Postprocess.