Troubleshooting

This section is intended to provide you with tips and strategies for avoiding and handling problems that may occur when using the Meshing application. Topics include:

Meshing Process

When you generate mesh, the mesh is generated as a separate process per part (unless using Assembly Meshing where all parts are meshed in one process).  If there are multiple parts, each part can be meshed in parallel. While meshing, the status window provides details on what is happening in the meshing process.  Other information, such as the amount of memory used can be found by using a task manager.

While the mesher is working at certain points it will highlight topology (edges, faces, bodies).  If the mesher gets stuck for a long time on a particular topology, you should inspect the highlighted topology and possibly merge it with another topology using virtual cells, or adjust the mesh sizes in that area.

Identifying Poor Quality Mesh

The following approaches are recommended to improve the mesh quality and obtain a valid mesh:

Recommended First Course of Action for Meshing Failures

If your mesh generation fails, it may be a partial or complete meshing failure. Your first course of action should be to examine any messages that the mesher returns to the Messages window. The messages include hints that explain why the meshing completely or partially failed. In some cases, you can right-click on the message and select Show Problematic Geometry to highlight any entities associated with the message in the Geometry window and see what the failed mesh looks like.

The mesher also provides visual cues to identify obsolete and/or failed meshes. As shown in the figures below, failed meshes are shaded in maroon and obsolete meshes are colored yellow. The approximate location of the cause of the meshing failure is identified by a convergence of white lines.

Figure 243:  Obsolete Mesh

Obsolete Mesh

Figure 244:  Failed Mesh

Failed Mesh

If the entities are very small, you can refer to the status bar at the bottom of the window to view statistics related to the entities. Then create a Named Selection to retain information about the problematic entities. Continue reading below for more information about messaging.

Understanding Messaging

The Messages window prompts you with feedback concerning meshing operations. Every message returned from the mesher is not necessarily an error. Messages come in three forms:

Once messages are displayed, you can:

Understanding States

Sometimes the mesher returns an invalid mesh. Refer to the state of the body in the Tree Outline to determine whether a body was meshed:

When your model contains an active unmeshed body, the Mesh object in the Tree Outline is preceded by a lightning bolt to indicate a body is out-of-date and requires action:

When your model is fully meshed (i.e., all bodies are in a meshed state), the Mesh object is preceded by a check mark to indicate that the meshing data is fully defined and ready for the next stage of your analysis (i.e., an update in the Meshing application or a solve in the Mechanical application):

Understanding Mesh Connection and Contact Match States

Mesh connections and contact matches use the following symbols to denote mesh state:

Shape Checks and Meshing Failures

Meshing may fail if the mesh quality does not meet the criterion of the defined shape checks. The following approaches are recommended to improve the mesh quality and obtain a valid mesh:

  1. To identify faces that do not meet the shape checking criteria, right-click the warning message and select Show Problematic Geometry.

  2. Use a different shape check setting.

    Some shape checks have a stricter set of criterion than others. By using a different shape check setting a mesh might be generated, and the mesh metrics bar graph can be used to find the mesh violating the stricter shape checks. In this way, locating the problem is the first step to fixing it.


    Note:  You can turn off most shape checks altogether by setting Shape Checking to None.


  3. Use the methods described in Identifying Poor Quality Mesh to determine the quality of the mesh.

  4. Use the Preview Surface Mesh and/or Preview Inflation features.

    With this approach the boundary mesh is generated even if the mesh would violate the defined shape checks. Once the previewed mesh is generated, use the mesh metrics bar graph to determine the location of bad quality elements. Generally, fixing the bad quality surface mesh is the best way to fix the volume mesh because bad quality mesh is usually a result of the geometry over-constraining the mesh topology. Using defeaturing controls (such as Loop Removal and Automatic Mesh Based Defeaturing), pinch controls, and virtual topologies are all good strategies to remove geometry features that may cause problems for the meshing algorithms.


    Note:  Not all mesh methods support the use of Preview Surface Mesh and Preview Inflation.


For additional information about the shape checking acceptance criterion used by ANSYS Workbench, refer to ANSYS Workbench and Mechanical APDL Application Meshing Differences.

Handling Selective Meshing Failures

Selective meshing may lead to unexpected results in cases where a mesh control change affects only one body. This may in turn lead to sweep mesh failure because the source and target meshes no longer align or the resultant change makes a body unsweepable. If desired, you can set the Allow Selective Meshing option to No to disable selective meshing and allow the mesh control changes to ripple through the entire part.

Selective meshing is not persistent for a geometry update or re-mesh operation. However, you can use the Mesh worksheet to create a selective meshing history so that your meshing steps can be repeated in the desired sequence. Otherwise, you may need to go through your body meshing steps manually if the single mesh update does not satisfy your meshing requirements. Refer to Using the Mesh Worksheet to Create a Selective Meshing History for details.

Handling Patch Independent Meshing Failures

If there are gaps in your geometry and Patch Independent meshing fails, the mesh size may be set smaller than the size of the gaps in the geometry. In such cases, try adjusting the size of the mesh in those regions so they are larger than the geometry gap size.

Figure 245:  Patch Independent Mesh Failure Due to Geometry Gap

Patch Independent Mesh Failure Due to Geometry Gap

Figure 246:  Patch Independent Mesh Failure Corrected with Larger Mesh

Patch Independent Mesh Failure Corrected with Larger Mesh

Handling Patch Conforming Tetrahedral, Quad Dominant, and All Triangle Meshing Failures

Some mesh failures are due to an inappropriate defeaturing tolerance (either default or user input ) or dirty geometry. Use the following guidelines to determine which issue is the cause of the failure:

With Size Function turned off:

  1. Determine whether the model is a multiple part assembly, a multibody part, or a single body part.

  2. If a message provides “Problematic Geometry” information, use it to determine which portions of the model fail. Quite often, one or more faces fail to mesh.

  3. If a face fails to mesh, check whether the face is “regular”; that is, make sure that it is not too skinny or has skinny sections with misaligned edge spacings which would make it difficult to get a good mesh. Use of virtual topologies, pinch controls, etc. may help in these situations.

  4. If a face fails to mesh, check to see if the edges attached to that face may be problematic. For example:

    • Turn on the Show Vertices and Close Vertices options to see if any edge is significantly faceted (i.e., has many edge splits in comparison to mesh size), or if there are any unexpected clusters of vertices. The mesher will try to place a node on each vertex, so unnecessary vertices can lead to complications in meshing that may be avoidable. Use of virtual topologies, pinch controls, and so on may help in these situations.

    • Turn on the Edge Coloring > By Connection option to see if the edge connectivity is unusual. In some cases, the geometry connectivity may not be as expected, and this may create problems during meshing. These problems could be fixed in the DesignModeler application, the CAD package, or possibly through the use of virtual topologies or pinch controls.

    • For a multibody part, turn on the Edge Coloring > By Body Connection option to see if the edge connectivity is unusual between bodies.

Handling General Sweep Meshing Failures

In the event of a sweep mesh failure, the following approaches are recommended:

  1. Check for contradicting edge sizings.

  2. For Src/Trg Selection, use a manual setting instead of automatic.

  3. Check side faces to see if they are mappable. Use of virtual topologies can help make bodies sweepable:

  4. Turn on the Edge Coloring > By Connection option to see if the edge connectivity is unusual. In some cases, the geometry connectivity may not be as expected, and this may create problems during meshing. These problems could be fixed in the DesignModeler application, the CAD package, or possibly through the use of virtual topologies.

  5. For a multibody part, turn on the Edge Coloring > By Body Connection option to see if the edge connectivity is unusual between bodies.

For detailed information about the requirements and characteristics of sweep meshing, refer to Mesh Sweeping.

For additional information, refer to Figure 7.36: Strategies for Avoiding Stretched Elements in the Mechanical APDL help.

Handling Thin Sweep Meshing Failures

In the event of a thin sweep mesh failure, first refer to Thin Model Sweeping for detailed information about the requirements and characteristics of thin sweep meshing.

The Preview Source and Target Mesh and Preview Surface Mesh features do not support the thin model sweeper. Thus, if a failure occurs, you must use the feedback in the Messages window to determine the problem:

Handling General MultiZone Meshing Failures

In the event of a MultiZone mesh failure, the following approaches are recommended:

  1. If using automatic source face selection, try using manual source face selection (or vice versa). For manual source face selection, ensure that all sources and targets are selected. Refer to Using MultiZone for more information.

  2. Ensure that all side faces are mappable. Refer to MultiZone Face Mappability Guidelines for more information. Use of virtual topologies can help make bodies sweepable:

  3. If MultiZone doesn’t respect edge biasing, as shown in Figure 247: Edge Biasing Not Respected by MultiZone below, it may be because the opposite edge is split. To work around this, perform the edge biasing on the opposite edges to get a better edge distribution, as shown in Figure 248: Edge Biasing Respected by MultiZone.

    Figure 247:  Edge Biasing Not Respected by MultiZone

    Edge Biasing Not Respected by MultiZone


    Figure 248:  Edge Biasing Respected by MultiZone

    Edge Biasing Respected by MultiZone



    Note:  When a curve with bigeometric distribution is split, the curve is split into GEO1 and GEO2 starting at the split point.


  4. Turn on the Edge Coloring > By Connection option to see if the edge connectivity is unusual. In some cases, the geometry connectivity may not be as expected, and this may create problems during meshing. These problems could be fixed in the DesignModeler application, the CAD package, or possibly through the use of virtual topologies.

  5. For a multibody part, turn on the Edge Coloring > By Body Connection option to see if the edge connectivity is unusual between bodies.

For detailed information about the requirements and characteristics of MultiZone, refer to MultiZone Meshing.

Handling Failed Mesh Connections

In the event of a mesh connection failure, refer to Diagnosing Failed Mesh Connections.

Handling Failed Contact Matches

In the event of a contact match failure, refer to Troubleshooting Failed Contact Matches.

Avoiding Bad Feature Capturing in Assembly Meshing

In some cases, you may encounter bad feature capturing when using assembly meshing in the Meshing application, even though the faceting of the same model looks fine in ANSYS Fluent . The bad faceting may be apparent in ANSYS Fluent only if you turn off the viewing of edges and view surfaces only. The following approaches are recommended to avoid bad feature capture:

Handling Assembly Meshing Failures Due to Min Size

Failure in the assembly meshing algorithms is almost always related to faceting issues in relation to minimum size. Make sure that the values of the Min Size and Proximity Min Size options truly represent the smallest sizes that you want the curvature and proximity size functions to capture. By default, many meshing features operate based on the smaller of these two minimum size values. Consider the following:

  1. It is strongly recommended that you ALWAYS adjust the value of Min Size/Proximity Min Size as appropriate for your problem. Make sure that the minimum size is 1/2 of any small feature or gap that you need to capture. Similarly, the minimum size should be about 1/10 of the diameter of the smallest pipe. For very simple cases, make sure to increase the minimum size appropriately. Failure to do so may result in an over-refined mesh with a huge number of facets.

  2. Use local (scoped) size controls to ensure two layers of elements in any gap/thickness. If the scoped sizing is smaller than the minimum size, you must adjust the Tessellation Refinement accordingly. If you add a hard size that is smaller than the minimum size, make sure that the tolerance specified by the Tessellation Refinement control is now 10 times smaller than the specified hard size.

  3. If you receive a warning about missing tessellations, it may help to lower the tessellation tolerance by 50%.

  4. In some cases, small defects in the faceting may lead to bad quality meshes. In many of these cases, a minor modification of the minimum size or tessellation tolerance can rectify the problem.

Handling Assembly Meshing Failures Due to Flow Volume Leaks

Virtual bodies are used with assembly meshing to represent flow volumes in a model so that you can mesh flow regions without having to model geometry to represent them. These flow volumes are extracted during meshing; however, an extraction failure may occur if there are gaps between bodies and/or faces such that the extracted flow volume would not be watertight and therefore would leak. If a leak is present, the flow volume mesh will contain only elements from the leak path (that is, surface and line elements will be returned but volume elements will not). As a result, assembly mesh generation will be unsuccessful and an error message will be issued. In such cases, the assembly meshing algorithms detect and trace the leaks and display their leak paths graphically as follows:

If a leak is detected, the leak path should clearly indicate its location, in which case you should return the model to the DesignModeler application or your CAD system to close the gap that is causing the leak (for example, by adding a face or moving a body).


Note:
  • If you suppress a virtual body, any leak path associated with it will be hidden from view in the Geometry window.

  • In some assembly meshing cases, contact sizing can also be used for closing leaks discovered during meshing. Refer to Applying Contact Sizing for details.


The figure below shows the leak path for a failed assembly mesh.

Figure 249:  Leak Path for a Failed Assembly Mesh

Leak Path for a Failed Assembly Mesh

The figure below shows a closer view of the leak.

Figure 250:  Closer View of Leak Path

Closer View of Leak Path

Handling Assembly Meshing Inflation Problems

If a high aspect ratio is obtained for cells in the inflation layer when an assembly meshing algorithm is being used, reducing the value of Gap Factor may help. Refer to the discussion of inflation controls in Selecting an Assembly Mesh Method for more information about specifying Gap Factor for assembly meshing algorithms.

The CutCell algorithm does not support very thick inflation layers, so instead of using an Inflation Option of First Layer Thickness or Total Thickness, use aspect ratio-based inflation.

Tips for Using Virtual Topology

Virtual topology surfaces made up of two loops are not automatically mappable. For a faceted surface made up of two loops to be map meshed, a mapped Face Meshing control must be scoped to it or it must be a side area of a general sweep body.

Meshing Complicated Models

Meshing a complicated model may require special attention and experimentation. In such cases, the following strategies and guidelines are recommended for obtaining a successful mesh:

  1. Analyze the model to determine its complexity:

    • Identify the small features that you do (and do not) want to retain.

    • Consider the model's size and its relationship to the element size transitions that are appropriate for the mesh. A smoother transition from the fine element size to the coarse element size will result in a larger number of elements, which should be considered (especially if the model is quite large). Coarser transitions will result in a smaller number of elements. You must determine what is acceptable.

    • Refer to the value in the Minimum Edge Length field. This field provides a read-only indication of the minimum edge length in the model.

    • Think about the element size that you expect to obtain, especially the desired minimum element size. To help you determine the desired size, in the Geometry window, select the edges of small features that you want to retain and refer to the status bar for feedback about the selections.

  2. Perform a low-effort mesh evaluation by using appropriate sizes as determined from #1, but without controls such as inflation, match mesh controls, etc. that add constraints to the mesher. Also, try to start with a coarser mesh size and refinement in later steps.

    • If the mesh is successful, examine it to see whether the mesh size and transition rates are acceptable. In most cases, you will need to make some adjustments to obtain the desired results.

    • If the mesh fails, examine any messages that the mesher returned to the Messages window, as described elsewhere in this Troubleshooting section.

  3. Adjust settings to retain desired small features:

    • In many cases, small features are either small holes or channels in the model and are associated with high curvature. For this reason, using the Size Function with Curvature is a good strategy for retaining these features.

    • Be careful when using the Size Function with Proximity. If the value of Minimum Edge Length is too small, using Proximity may lead to meshing problems.

  4. Adjust settings to defeature (remove) unimportant small features:

    • The Meshing application automatically defeatures small features according to the specified Defeaturing Tolerance. Refer to the Minimum Edge Length value to help determine which small features will be defeatured automatically.

    • For solid models, Defeaturing Tolerance is set to 50% of the value of Min Size by default. If you set a larger Defeaturing Tolerance, you must also set a larger Min Size because the defeaturing tolerance cannot be as large as the minimum element size.

  5. Adjust the mesh settings to achieve the desired quality.

    Continue making adjustments until your results are satisfactory. Try adjusting controls such as face sizing, edge sizing, transition rate, and smoothing. You may also want to experiment with virtual topology.

Using a Localized Operating System on Linux

If you are using a localized operating system (such as French or German) on Linux, you must perform additional steps in order for the Meshing application to recognize the correct numerical format. Refer to the platform details section of the ANSYS, Inc. Linux Installation Guide for details.

Using Lustre Parallel File Systems on Linux

Meshing application projects created prior to Release 16.0 need to be updated before they can be used on Lustre parallel file systems on Linux. To do this:

  1. Load the project into Release 16 software on a system that does not use a Lustre parallel file system.

  2. Perform an operation that changes each model in the Meshing application (for example, hide and then show a part). If systems share the same model, the change needs to be done for only one of the systems.

  3. Save the project.


Release 17.0 - © SAS IP, Inc. All rights reserved.