CST2013: Mesh Problem Handling

This page contains a list of the most important warning and error messages in the meshing context together with a detailed explanation of the meaning and proposal for handling and resolution.

Hexahedral Mesh

Mesh representation

The radius of wire "xxx" is greater than some of the adjacent mesh lines.
The wire radius of discrete port "xxx" is greater than some of the adjacent mesh lines.
The wire radius of lumped element "xxx" is greater than some of the adjacent mesh lines.

When a wire (a bondwire or a wire part of discrete edge ports or lumped edge elements) is defined with a finite radius (r>0), but not modeled as a solid, this warning can appear. In that case the hexahedral solvers simulate the wire as a chain of segments along hexahedral mesh edges and try to consider the finite radius using a so-called thin-wire model. This semi-analytical technique locally modifies the algorithm along this chain. However, the thin-wire model can only be applied, as long as the size of the adjacent mesh cell is bigger than the given wire-radius (meaning the complete wire radius is located in the first neighbored mesh cell). The warning now indicates, that in some regions the adjacent mesh lines are smaller than the wire radius, which no longer allows the internal usage of the thin-wire model. In any case the connectivity of the wire structure is not affected.

Solutions:

In very complex geometries (e.g. PCB structures with many bondwires), this warning might be negligible, since the effect only happens locally.

For bondwires in less complex models (e.g. simple transitions between microstrip- or cpw-lines) the radius of a wire has a more significant effect on the results and should therefore be taken into account. In that case the flag "Solid wire model" on the Make Wire from Curve dialog page could be activated, which allows the more accurate 3D-wire handling by PBA and TST.

In order to have easy switches between the different wire models, you also find a button "Simplify model..." in the Solver dialogues, where wire models can be switched for all existing wires at a central place rather than editing each wire separately.

Not enough longitudinal points for port number "xxx". The space in front of the port needs to be at least 3 cells long.
Waveguide for port number "xxx" is too short. The waveguide must be homogeneous for at least three mesh planes along propagation direction.

The transient solver needs a small homogeneous structure and mesh region in front of the port (in propagation direction) in order to apply the port mode extraction and imprint in a correct way. These warnings are shown when the geometry in front of the port is inhomogeneous or smaller than three mesh cells. In this case please check the model setup.

Not enough transversal points for port number "xxx" to calculate the desired number of modes.
Port number "xxx" has very few transversal points to calculate the desired number of modes. This may lead to inaccurate mode patterns.

This warning usually occurs when the mesh in the port region is very coarse. During the calculation of port modes the number of unknowns are compared with the number of desired modes. In principle one degree of freedom is needed to represent one mode, if this is not given the solver stops with the first message as an error. However, also with just a few more unknowns the accuracy of the modes might be affected and the second message is shown as a warning. In both cases the solution would be to refine the mesh in the port area or to check whether some unwanted metal object is located in the port region.

Port number "xxx" and "yyy" have overlapping areas in mesh representation. Consider to decrease the port dimension or to refine the mesh distribution.
Port number "xxx" and "yyy" have adjacent areas in mesh representation. Consider to decrease the port dimension, to activate electric port shielding or to refine the mesh distribution.

In case of overlapping or adjacent port areas in mesh representation the mode extraction and imprint of the ports cannot work correctly anymore. In order to resolve the problem, the mesh should be refined in this region to allow a disjunct setup of the ports. In principle it is also possible to reduce the dimension of the corresponding port areas, but only when the mode patterns are not strongly affected by this change. In case of adjacent and not overlapping ports it is also possible to activate electric shielding on the port dialog to realize some kind of decoupling between the ports.

Matrix generation

Shape "xxx" was internally patched to maintain watertightness.

It might happen that the surface of a solid body is not completely closed. This might be caused by a CAD model imported from a different vendor.
During the process of matrix generation the surface of the respective solid body is repaired automatically in order to run the hexahedral based solver correctly. The repaired part of the shape is shown in the mesh view of the project. The original CAD model remains unchanged.

Some shapes of the CAD model overlap each other.

This warning provides a hint that some parts of the geometric model overlap each other. A numerical simulation of such a model is still possible if mesh type "FPBA" is used. However, in the overlap region of the overlapping shapes one shape is preferred and therefore used for the simulation setup. More detailed information about the overlapping shapes can be found in the solver log file.

Detected regions with short-circuited PEC/PTC material. (This may happen if the mesh is too coarse in regions where small gaps are embedded in PEC/PTC material).

The internally used representation of the geometric model is limited regarding the resolution of geometric details if the mesh type "FPBA" is used. If the mesh is very coarse compared to the geometric detail within one mesh cell it might happen that parts of the geometry touch each other although they are actually not connected. If this situation is detected the warning above appears.

Solution:

To avoid unintended short-circuits it might help to increase the number of mesh lines around the short-circuited shapes.

Port "XXX" is inhomogeneously filled within the first 3 mesh cells. Cannot start solver.

The first 3 mesh cells in front of a waveguide port must be filled homogeneously regarding the orientation of the port.

Solution:

Please modify the mesh line distribution or the geometric model to meet this requirement.

Parts of the CAD model referring to the sensitivity parameters "xxx" and "yyy" are located within TST cells and will be ignored during the sensitivity analysis. This might reduce the accuracy of the sensitivity results.

TST cells are currently not supported for the time domain sensitivity analysis. It might help to increase the mesh line distribution at locations where TST cells appear.

"xxx" mesh cells are represented in staircase mode and have been filled with PEC/PTC.
"yyy" mesh faces are represented in staircase mode and have been filled with PEC/PTC.

Depending on the chosen hexahedral solver this warning might appear. It means that within a mesh cell or a mesh face a certain geometrical configuration cannot be used to run the solver correctly. Therefore these cells or cell faces are filled with PEC or PTC depending on the chosen problem type. However, in many cases these filled cells do not influence the simulation results significantly. The staircase based cells can be seen in the mesh view.

Solution:

Please make sure that the mesh is not too coarse at locations where the staircase based cells appear.
Depending on the chosen problem type, please activate the "Use TST cells" checkbox if provided in the "Mesh Properties - Hexahedral->Specials...->Advanced" dialog box.

The point accuracy setting for the PBA matrix calculation seems to be low. In some rare cases this might lead to inaccurate simulation results.
To increase the point accuracy enter the Home: Mesh Global Properties Specials... Advanced dialog box and set the "Point accuracy enhancement" value to a higher number.

During the process of matrix generation with mesh type "PBA" the internally used geometric accuracy limitations might not be suitable to run the chosen hexahedral solver.

Solution:

Setting the point accuracy to a higher value will increase the internally used accuracy.
Please note, the default point accuracy setting will provide good results in most cases. Increasing the point accuracy setting might increase memory usage and solver run time.

Parts of the geometry cannot be processed properly with mesh type "PBA". This might cause inaccuracies during simulation. Please consider using "FPBA" (Home: Mesh Global Properties Specials... Mesh type).
For high accuracy requirements consider activating "Enhance FPBA accuracy" as well.

The "PBA" mesh type provides results of very high accuracy. However, in some rare situations this meshing technique might not reach the desired accuracy, e.g. due to an improper geometric setup caused by importing CAD models of a different vendor.

Solution:

If this warning appears the "FPBA" mesh type provides a solution which is much less sensitive to improper geometric data.
It is recommended to also activate "Enhance FPBA accuracy" to get the same level of accuracy as with mesh type "PBA".

Matrix calculation failed with mesh type "PBA". Please consider using "FPBA".
(Mesh Properties->Specials...->Mesh type). For high accuracy requirements consider activating "Enhance FPBA accuracy" as well.

The "PBA" mesh type provides results of very high accuracy. However, in some rare situations this meshing technique might fail, e.g. due to an improper geometric setup caused by importing CAD models of a different vendor.

Solution:

If this error appears the "FPBA" mesh type provides a solution which is much less sensitive to improper geometric data.
It is recommended to also activate "Enhance FPBA accuracy" to get the same level of accuracy as with mesh type "PBA".

Port x contains n decoupled regions. Please consider defining one port for each of these regions.

This warning may occur when multiple decoupled regions, e.g. multiple coaxial waveguides, are present in one port definition.

Solution:

A typical cause for this problem is the case where the port is both shielded by "electric shielding" or an electric boundary condition and by geometry. Here a simple solution would be to deactivate "electric shielding" or to set another boundary condition.

The topology of port xhas changed after adaptive port meshing. This could influence the 3D simulationas well as the port modes.

This case occurs when the port is inaccurately meshed.

Solution:

If mesh type "PBA" is selected,please consider using "FPBA" with "Enhance FPBA accuracy"(Mesh Properties->Specials...->Mesh type). The problem may also appearwhen the mesh density in the port region is insufficient to capture all geometric detail.In this case, a higher mesh resolution could solve the problem(Mesh Properties->Specials).

Tetrahedral Mesh

Following regions contain degenerated elements...  Generating a finer mesh for listed regions or removing small features may help.
The quality of some tetrahedrons is low.
Please consider improving the initial mesh.

The so-called "Low quality elements" may occur during the tetrahedral mesh generation in the CST MWS frequency domain solver. Very "flat" or very "thin and high" tetrahedra are typical examples.

The presence of low quality elements can influence the solution in two ways:

• Influence on the mesh adaptation process: Low quality elements in the initial mesh can negatively affect the mesh adaption process. During the mesh refinement in the mesh adaptation, more refinement steps are typically needed in order to obtain a good final solution.

• Influence on the solution's accuracy: Low quality elements which also have large dihedral angles worsen the condition number of the matrix. This typically leads to larger numbers of necessary iterations in the iterative solver, and may also lead to less accurate solution.

What to do: you can try to improve the quality of the initial mesh by adjusting the mesh parameters:

• Local mesh parameters (select object, then right mouse button -> Mesh properties): in the areas in which low quality elements occur, you can choose a smaller mesh step for the individual objects concerned: this typically leads to better mesh quality.

• Global mesh parameters (Mesh -> Mesh properties): if you choose a larger number of steps per wavelength, as well as "Smooth density transitions" in the "Specials - Volume mesh" dialog, the overall mesh will be finer and of better quality.

• Other techniques: very thin metallic sheets can be modeled, in a first step, as having zero thickness. To do this, first select the relevant faces, then use Objects -> Face healing -> Shape from picked faces to create a sheet object. This technique results in a better quality of the mesh, which can often compensate the small geometric inaccuracy thus introduced.

The length ratio between the shortest and the longest model edges is 1: xxxx. This is often caused by modelling thin metallic layers by using solids. If this is the case here, it may be advantageous to model the metallizations as infinitely thin sheets.

This warning occurs only, when tetrahedral meshing is used. The tetrahedral mesher is much more sensitive to the initial mesh quality. The effects of this will be apparent in the adaptive meshing process and solve time (poorly conditioned matrix due to "low quality elements"). To avoid "Low quality elements" we typically recommend modeling thin conductors as zero-thickness sheets. This can easily be accomplished by picking the face of the microstrip where it meets the substrate. Then in the face healing tools, the option to make shape from picked faces will create the sheet. If the thickness is a critical feature (edge coupled microstrip for example), you can set the maximum mesh step by right-clicking on the object and entering the mesh properties dialog. This helps keep the tetrahedrons from having a very long aspect ratio.

If any curved surface exists, please consider increasing the "Curved element order" in the "Specials..." of the tetrahedral mesh properties to obtain more accurate results.

This message is a hint that curved elements are not yet enabled for your model. It is displayed even if no curved surface or model edge exists, in which case the following recommendation to use curved elements does not apply. Curved elements provide a better approximation of the geometry than linear elements. The latter are a special case of the former: linear elements are "curved" with a "curved element order" of One. This "curved element order" can be increased in the special tetrahedral mesh properties. For the default "Second" order solver elements, a "curved element order" equal to Two or Three is recommended. Further increase the value for higher solver order.

"xxx"% of all mesh cells do not resolve the wavelength properly. They are larger than "yyy" times the wavelength.

The solver has found that the size of some mesh cells with respect to the wavelength in their material seems to be too large. As the "worst case" wavelength in media usually is considered for the generation of the initial mesh even for dispersive materials, most of the mesh cells should be sufficiently small, already before any adaptive mesh refinement is applied. Exceptions may exist for coupled simulations, where the material parameters are not known at the time of mesh generation.
If there is wave propagation in the concerned materials, results may become inaccurate. Furthermore, the iterative solver may need more time and memory. Please check the mesh settings and the material properties. The mesh may need manual refinement, or settings which result in a finer initial mesh.