G-DEMPack Tutorial 3: DEM Meshers

Revision as of 15:20, 25 June 2018

Introduction

This tutorial gives a practical view of the meshers explained in the G-DEMPack2 manual. To follow this tutorial, you must first download the GiD file File:D DEMPack2 Tutorial 3.zip, which includes the geometries that will be meshed and the corresponding assignation of groups. To learn how to create geometries and how to assign them to groups please check here and G-DEMPack Tutorial 2: DemPack 2.0 respectively.

Point

In this simplest case, we only have to give the mesher a diameter for the sphere in point. To enter the mesher settings for the point, go to the DEM Element subtree, double-click on 'Use Advanced Meshing Features' and Choose Yes by using the inverted triangle that sits on the right of the default value of No. The fields corresponding to the different meshers will open after selecting Yes. The program now lets you choose which algorithm type you would like to use. In this case we must use the FEMtoDEM, the other one does not make any sense. About the criterion type, in this case it does not matter which one you choose because in this particular case the three methods coincide, so we just choose Node based. The mesher now asks you for a diameter and for the characteristics of the distribution. We can for example assign a diameter of size of 0.4m, a normal distribution and a standard deviation of 0.

Line

When meshing the line group, we first have to choose the size of the FEM elements (line elements in this case), which will determine the distribution of spheres in space. We can do this by going to Mesh > Unstructured > Assign sizes on lines. In the window that shows up we can enter, for example, a value of 0.25 and click Assign. We then click on the line and press Esc. The previous window will show up again in case we would like to add more line entities. In this case is not necessary, so we just press Close to finish with this assignation. To enter the mesher settings for the line, we simply go again to the DEM Element subtree, double-click on 'Use Advanced Meshing Features' and Choose Yes. In this case we also choose the FEMtoDEM but choose the Element Centroid Base criterion instead. We enter a value of 0.15m, choose for example a Log distribution and a standard deviation of 0.15.

Triangles

We can mesh a surface with spheres by using a FEM mesh as a starting point. In this case we will mesh the given surface with a mesh size of 0.5m. To assign this mesh size to the triangles group, we must assign this mesh size to the 3 types of entities in the group, that is, to every surface, line and point in it. To do so, we follow the same procedure as in the line group, we start by going to Mesh > Unstructured > Assign sizes on surfaces, we enter 0.5 in the window that pops up, click on Assign, select the surface in the triangles group and press Esc. The previous window will show up again. Press Close to close it. We must repeat the same process, in this case for lines and points, so we must go to Mesh > Unstructured > Assign sizes on lines and Mesh > Unstructured > Assign sizes on points respectively and choose again the 0.5m value for both cases. Following the same procedure as in the line group, we go to the DEM Elements section and choose the FEMtoDEM algorithm type and the Both Nodes and Centroid criterion. We can also set a diameter of 0.15m, a normal distribution and a standard deviation of 0.15.

Quadrilaterals

We will now mesh the quadrilaterals group with spheres using a background of a FEM of quadrilaterals. We will divide the surface of the group in 2 elements vertically by 3 horizontally and mesh in the nodes of the resulting FEM mesh. To do so, we go to Mesh > Structured > Surfaces > Assign number of cells, click on the quadrilaterals surface and hit Esc. In the window that opens, we enter 2, press Assign, select the line at the bottom of the surface (the line at the top will be automatically assigned) and hit Esc. The previous window will show up again. We enter 3, click Assign, select one of the vertical lines, hit Esc and close the window. We finish by going to DEM Elements and choosing FEMtoDEM, Node Based, a diameter of 0.2m, a log distribution and a standard deviation of 0.1.

Tetrahedra

We will mesh the tetrahedra group with spheres by situating them in the centroids of a mesh of tetrahedra. Following a procedure similar to the previous groups, we go to Mesh > Unstructured and assign a size of 0.5m to the volume and all surfaces, lines and points in the group. In this case, we can for example go to the DEM Element section and use a settings of Element Centroid Based, a diameter of 0.15m, a log distribution and a null standard deviation.

Hexahedra

The hexahedra group will be meshed using spheres situated in the nodes and centroids of a mesh of hexahedra. We start by assigning the group an hexahedra element type by going to Mesh > Element Type > Hexahedra, selecting the volume in the group and pressing Esc. We will then divide the volume of the group in 2 elements vertically by 2 horizontally. To do so, we go to Mesh > Structured > Volumes > Assign number of cells, click on the volume in hexahedra and hit Esc. In the window that opens, we enter 2, press Assign, select all the lines in the group and hit Esc. The previous window will show up again so we close it to finish with the size assignation. In th DEM Element section we will choose Node Based criterion type, a diameter of 0.2m, normal distribution and a 0.2 standard deviation.

Circles

Finally, we will mesh the circles group by transforming a mesh of circles into spheres. In this case we will use an aleatoric circle mesher, unlike we have done in previous group, where we used a cartesian distribution of spheres. Please take into account that in this case the Rball circle mesher must be used or there might be errors. Make sure that that mesher is set by going to Utilities >Preferences > Sphere/Circle > Selected mesher. So we start by assigning a circle element type to the group by going to Mesh > Element Type > Circle, selecting the surface in the group and hitting Esc. We then go to the DEM Element tree and choose the 2Dto3D algorithm type. We can assign a circle size of 0.2m by going to Mesh > Unstructured > Assign sizes on surfaces, inserting 0.2, clicking on Assign and selecting the surface in the group, pressing Esc and Close.

Plane (FEM)

We can mesh the FEM plane with a very coarse mesh, in this case we can use a mesh size of 9m. To tell GiD that we would like to use this mesh size, we proceed as in previous groups, by going to Mesh > Unstructured and assigning a size of 9m to points, lines and surfaces of the group in the usual fashion.

To mesh, we simply press Ctrl-G and enter a size of 0.2. We can now save the file and launch the simulation. After a few seconds, we can shift to the Postprocess to load the results. The figures that follows show the expected solution.