RTM-Worx User Manual |
||||||||||||
3.2 Mesh Generation
3.2.1 FEM Mesh basics RTM-Worx subdivides the part defined with surfaces, curves and keypoints into so-called elements and nodes, which form the basis for the Finite Element Method. You don't need to be a FEM expert to use RTM-Worx. Everything (and probably even more than) you need to know is provided in this manual. 3.2.1 FEM Mesh basicsRTM-Worx uses the mesh for two purposes:
Because editing is interactive, and any changes to the geometry must be shown the mesh always needs to be defined for all geometry objects. This is taken care of by an incremental mesh generator that subdivides curves into edges and surfaces into triangles only when they have been modified. The mesh generator is invoked automatically by the geometry editors when you add or modify the model. However, to be able to control the size of the elements and set the meshing parameters, the Mesh generator Control panel is available. The relation between mesh size and calculation accuracy is easy to understand: using more and smaller elements increases the accuracy of the results but also increases the calculation time. The number of elements you need for a given accuracy depends on the geometry of your part and the way it is injected.
For example, in the neighborhood of single injection points, the pressure is a logarithmic function of the distance to the injection point which is difficult to capture with a piecewise linear approximation on a coarse mesh and you need small elements to get an accurate estimate of the pressure gradient, velocities and the filling time. On the other hand, if you use a line gate (several injection points at adjacent nodes on a curve, or with a connector element) the pressure gradient is linear and calculated exactly even on very coarse meshes. Each node in the mesh is assigned a volume, which is the sum of a part of the elements connected to the node (half the volume of a runner element and approximately one-third of the volume of a triangular shell element). You can view the subdivision in Nodal Control Volumes (complementary to the mesh) by toggling the Render|Control Volumes menu switch or the equivalent button on the Render toolbar. During filling of the mold, the filled fraction of each Nodal Control Volume is used to track the filled part of the mold (100%), the empty part of the mold (0%) and the location of the flow front (filled fraction in between 0% and 100%). The pressure calculation is only done for completely filled nodes and the pressure in the front nodes is set to zero. Therefore, the approximation of the location of the flow front becomes more accurate when a finer mesh is used that contains more nodes. In addition, the shape of the triangular shell elements influences the accuracy of the calculation. Ideally, the triangles are equilateral (all sides equal length with three internal angles of 60 degrees), and obtuse triangles (with one internal angle that is larger than 90 degrees) should be avoided whenever possible. The source of inaccuracy related to the element shape can be attributed to the following:
Two types of shaded plots are provided to visualize the quality of the mesh:
Because the accuracy of the simulation depends on many factors, accurate estimates of the error or rules that are easy to follow cannot be given. The best way to find out if the mesh is accurate enough is to repeat the calculation with a smaller mesh size and compare the results. 3.2.2 Controlling mesh generationThe mesh generator Control Panel contains four parameters to control the generation of the mesh and a set of buttons you can use to quickly change the mesh size and to remesh the model. The size of the elements generated is controlled by the global minimum and maximum limits 'Min' and 'Max' and locally by the length of the curve segments, e.g. the amount of detail. If you do not define bounds on the mesh size (set Min to zero and Max to anything larger than the model size) a graded mesh is generated with a mesh size that is defined in keypoints as the shortest curve connected to the keypoint and interpolated in between the keypoints on curves and the surface boundaries. Because the mesh size locally changes during editing of the model and the incremental mesh generator only partially (re-) builds the mesh, it is always recommended to remesh the whole model before running a simulation after you made changes to the geometry. Just use the [Apply] command in the Mesh generator Control panel. At the top of the Mesh generator Control panel, a few model statistics are provided to help you select working lower and upper bounds. The most common operations are implemented in the set of six buttons. Actually, each button modifies Min, Max or both and does a complete remesh of the model.
Use the [Coarse] command to generate a mesh with as few triangles and nodes as possible. The [10%] command is a good starting point. The mesh size is easily changed by using the [Larger] and [Smaller] buttons which double the lower bound and halve the upper bound respectively. Repeated use of those buttons will result in equal upper and lower bounds. For best results, all elements should have about the same size. The [Equal] button generates such a mesh by using an upper and lower bound both equal to the smallest edge in the geometry. This often generates a lot of elements however. The mesh is built in two stages. The first stage generates the triangles, edges and nodes. This 'raw' mesh is smoothed in the second stage where edges are swapped to remove as much obtuse triangles as possible and nodes are centered in between adjacent nodes. This is an iterative process, and you can control the maximum number of iterations and the tolerance in the 'Iter' and 'Tol' fields respectively. The default values work very well on flat and mildly curved surfaces, but you might want to increase the number of iterations and lower the tolerance for strongly curved surfaces. 3.2.3 Optimizing mesh and geometryIn the previous sections, the relation between mesh and accuracy was detailed and the methods to influence the mesh size were described. The most difficult part is to decide how much detail to include to get results that are accurate enough to be representative for the actual flow so that you can use the simulations to optimize injection of the product. Here are a few guidelines:
home |
contents |
about this manual |
quick start |
introduction |
||||||||||||
home > documentation > model and calculation > mesh generation |
User Manual |
contents |
about this manual |
quick start |
introduction |
installation |
features |
philosophy |
getting started |
interface |
workspace |
menu and commands |
viewing the model |
model and calculation |
geometry and properties |
mesh generation |
simulation |
Products and Services |
RTM-Worx Applications |
RTM-Worx Documentation |
POLYWORX Heemst 10 7443 EH Nijverdal The Netherlands +31 548 612217 |