F-DEMPack Tutorial 2: Annular pipe
Before starting with this tutorial, the user is strongly encouraged to follow the D-DEMPack Tutorial 2: Conveyor belt to get a feeling of how the problem type works, and in particular the DEM section. This tutorial will focus mainly in the Fluid section and its particularities.
The process should start by creating a new geometry from scratch, by modifying an existing one or by opening a finished one. It is assumed that the user already knows how to do this procedure so no details will be given about it.
In this tutorial, a simple geometry was created for the sake of simplicity. The idea is to understand all the steps involved in the problem type. The user must start by downloading the file File:F DEMPack2 Tutorial 6.gid.zip, which contains the geometry and mesh of the proposed geometry. This file has already created the groups that will be used in the simulation.
The geometry of study consists of an annular tube through which a flux of an intermediate viscosity mud passes. An inlet creating DEM particles with time is located in the base of the annular pipe.
The downloaded file has already created the groups that will be used in the simulation.
In this sample case, the following five groups were created: Dem_inlet, Fluid, Inlet, No_slip and Outlet. The lower circle (surface) would be the Inlet, the smaller one just on top of it was the DEM_Inlet, the circle at the top would represent the Outlet, the No_slip would be the walls of the cylinder and, finally, the volume named Fluid would constitute the mass of fluid.
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:
To begin inserting the fluid properties, we click on Fluid and then on the plus button at the bottom of the window to add a new Fluid material.
The next figure shows the details of the fluid material parameters that have been used in this example. To fill in or modify the value of the parameters, just unfold the General and Fluid submenus and double-click on the corresponding current data. The following data corresponds to an intermediate viscosity mud.
The same process can be followed, in this case, for the DEM material. We unfold the DEM-Defaultmaterial submenu at the bottom of the same window and fill the fields with the appropriate data. See Figure 6.
General Application Data
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 Properties window will show up. This menu can be accessed also by clicking on the icon showing in the next figure:
The first section of the menu is General Application Data. Within this section the user can specify the simulation parameters, the coupling parameters between both subdomains -fluid and DEM particles-, and some postprocess results options. A deeper explanation of those parameters can be found in F-DEMPack2 manual.
The previous screenshot shows the reference parameters that have been used in this particular case.
DEM-FEM wall group
The last section deals with the 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. To apply the DEM-FEM condition, the user must double-click on it and the tree in Figure 22 will show up.
Inlet DEM group
Finally, we have to assign the DEM boundary conditions. In practice they refer to the inlet of DEM particles in the domain and to the DEM-FEM entities. Apart from setting the value of the different parameters involved, the user must relate this inlet condition with its corresponding geometrical entity by right-clicking on Inlet > New and assigning the appropriate layer. See Figure 21 for details on the inlet section.
DEM Boundary Conditions
The last section deals with the 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. To apply the DEM-FEM condition, the user must double-click on it and the tree in Figure 22 will show up. The user must fill the Properties box at the bottom in order to set the motion of the corresponding DEM-FEM surface. This box has three different labels: Linear velocity, Angular velocity and Options. The next figure shows the second section. Both Linear velocity and Angular velocity properties can be set to be periodical, assuming that the group starts its movement at a certain point in the plane of symmetry of the complete motion path with a maximum speed equal to the imposed value. By default, motionless properties are imposed. In the 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. There is a third Options label where the user can specify the friction coefficient value in this domain. There is a last Results section where the user can define the dimensions box that will be used to measure the average velocity of the particles inside it as time progresses. See the next figure.
DEM General options
The second main step is to define the DEM settings. So we proceed by opening the DEM tree. We start by unfolding the General options menu. The first section is devoted to set the limits of the bounding box, the second to input the gravity vector and the last one to set activate or not some advanced features. The figure that follows shows this window.
The next section deals with the DEM basic time variables. The user must enter the desired time step, which should be the smaller the better. Taking into account that the program is using an explicit scheme, a too large delta time could lead to instabilities. The second parameter to be set is the neighbor search frequency, that should be again as small as possible to avoid neighbor detection problems. A good reference value is 10. It must be noted, though, that the smaller this value, the higher the computational cost. The next figure shows this section.
So we first start by setting the values of the main time variables by going to Fluid > Problem_parameters. See Figure 9 for details on this tree.
We can then set the fluid properties by assigning them to the Water material previously created. To do this, 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 Ok. The properties of the fluid in the problem are those of the Water material. See the next figure for details.
The next step is to identify the fluid elements amongst all the existing geometric entities. We can do so by going to Elements and right-click on Fluid. As before, we must choose New and a window will open at the bottom. On the Group tab, choose Fluid and click Ok. The elements are properly assigned. Figure 11 shows the process.
We follow the same procedure to assign the initial conditions to the problem. In this case, we give the mass of fluid an initial vertical velocity field. The picture that follows shows the process.
Now we can proceed to assign the boundary conditions. We begin by setting the inlet velocity of the mass of fluid. To do so, we right-click on Boundary_conditions > Inlet_velocity and create the new inlet condition. We fill in the fields with the desired values, relate the Group with the corresponding entity and click Ok. See Figure 13 for details. We proceed in the same for imposing the pressure boundary condition. In this example we assigned a null pressure value at the top surface. The process is the same as before and it is sketched in the figure that follows. The same process applies for the slip conditions in the domain. In this case, we chose no-slip condition on the cylinder walls. As shown in the figure that follows, we assigned this property to the appropriate layer, in this case the box.
DEM-Fluid Interaction Settings
Meshing and Running
The last step before launching the calculation is to mesh the domain. The user must take into account that two different meshers should be used when using the DEM Element section. Firstly, a triangle-based mesher for should be employed for the surfaces, that will be discretized in a standard FEM fashion and, secondly, a sphere mesher for the groups made up of DEM particles. By default, GiD uses a tetrahedra/triangles based mesher, so the user should manually select the geometrical entities that will be meshed using the sphere mesher. These entities will be volumes that will be set as meshed with spheres by going to Mesh > Element_type > Sphere, selecting the desired group and pressing Esc to finish. The user can modify or tweak the sphere mesher settings by going to Utilities > Preferences > Meshing > Sphere_mesher. The next step is to set the mesh size of the different parts of the geometry. For the entities meshed with spheres, the assigned number would correspond with the average diameter of the particles. To assign these sizes, the user should go to Mesh > Unstructured > Assign_size_on_volumes for the particles entities and to Mesh > Unstructured > Assign_size_on_points (lines, surfaces) for the inlets or DEM/FEM groups. As a general rule, after selecting the corresponding layers, the user must press Esc to confirm and exit the selection process. It is important to note that the purpose of this size-setting procedure is to endow some particular entities with a specific mesh size. Nevertheless, when most of the domain has a certain mesh size, no previous size-setting procedure should be applied on it. GiD will obtain this mesh size when the user enters a default value at the beginning of the meshing process. This meshing operation can be accessed by clicking on Mesh > Generate_mesh, entering the desired size and pressing OK. Finally, to confirm if all sizes were correctly assigned, the user can go to Mesh > Draw > Sizes > All_types to check graphically the assignation. The following figure shows the obtained mesh for the sample case.
So after the geometry was successfully meshed and the case saved, the user is now ready to launch the calculation. To do this, the user must go to Calculate > Calculate or press the Run the simulation button in the interface. Figure 26 shows the buttons in the Process Management section of the interface. The button inside the black square runs the simulation, the one inside the red one gives information about the calculations and the one in green stops the process.
Once the program starts writing results, the user can shift to the Postprocess in order to analyze the obtained results. After clicking on Open multiple files and selecting the desired group of files to be opened, the user can observe the sequence of results with time by using the Animate utility. The Results animation window is available when pressing Ctrl-m. Figure 27 shows a succession of results after running the sample case. It is also possible to print several physical results as, for example, the velocity field. To do this, the user must click on View_results > Display_vectors > VELOCITY > |VELOCITY|. A result like the one showing in Figure 28 should be obtained. More information can be obtained on the use of GiD by pressing F1.