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Mesh Quality

The purpose of mesh quality metrics are to assess the suitability of a discrete computational domain (Mesh) for simulations. A mesh is usually considered to be of higher quality compared to another mesh if it improves at least one of the most important simulation properties; Time to convergence, stability or accuracy without affecting the others negatively. This article gives more insights into what a mesh is.

There are mainly two applications for this type of quality assessment:

  1. Prior to running the simulation:
    This is to ensure the mesh is a good fit for the upcoming simulation run. Evaluating at this level is more for CFD-experts, less experienced users can usually rely on the default settings.
  2. After facing an issue in simulation:
    If your solution diverges and the simulation fails, quality assessment gives you powerful tools at hand to identify problematic areas that can be, for example, fixed by adjusting the CAD model or refining certain areas in the mesh.

The following picture gives a preview of what the result of checking the mesh quality could look like:

vizualisation of aspect ratio in a mesh for cfd analysis
Figure 1: Mesh metric aspect ratio for an example CAD model.

1. Mesh Quality Metrics

The quality of a mesh can be assessed by different metrics that are usually based upon the geometrical properties of the mesh cells (e.g. Aspect ratio) or upon the relation between neighboring cells (e.g. non-orthogonality). As the suitability of a mesh for a simulation is also strongly influenced by the simulated physics, the simulation solver, and the geometrical domain, the quality metrics can only give an indication if a mesh is a good fit for a simulation setup. As an example, you can find that simple incompressible CFD simulations are usually much less sensitive towards poor quality cells than compressible flow with high-density changes.

Available mesh quality metrics:

Mesh Quality MetricRecommended Range
Aspect ratio<<100
Cell volume
Non-orthogonality*<<75
Edge ratio<<100
Volume ratio*<<100

* Volume ratio and non-orthogonality are defined on each face of the mesh. SimScale displays the maximum value for all faces which belong to a cell.

2. Mesh Visualization

Mesh quality metrics can be accessed in the navigation tree under ‘Mesh’ after the meshing finished.

simscale simulation tree part for mesh visualization
Figure 2: Finding mesh quality in the simulation tree.

The post-processing viewer loads the mesh. and computes the mesh quality on the fly. It might, therefore, take up to a few minutes for big meshes (> 10 Mio cells). You can display the different mesh quality metrics the same way result fields are displayed for simulation results.

global distribution of non orthogonality degrees
Figure 3: Displaying a value over the surface of the whole domain.

3. Isolate Cells Based on a Metric

While all display filters are available for mesh fields, the isovolume filter is especially handy. This filter allows you to isolate bad quality cells by adjusting the minimum or maximum threshold value to the recommended limits. The following picture shows how to create an isovolume:

demonstration about how to create a insovolume to isolate the elements based on a limit and a metric to be shown
Figure 4: Isolating bad elements using isovolumes.
  1. Open the mesh quality feature.
  2. Create a new isovolume by clicking the ‘+’ next to it.
  3. Select the metric you want to analyze.
  4. Use the sliders to adjust the limits of what you want to be displayed.

Sometimes the worst elements in the mesh will be very small, and when you try to isolate them, they will be hard or near enough impossible to spot. In such a case, it is helpful to hide the Parts completely and use the Home-button next to the orientation cube to zoom into the current extents of the visualization. In case there are tiny bad elements in several parts of the model, you may find yourself needing to restrict the Isovolume thresholds further. Once you have zoomed into the bad elements, you can then enable the Part visualization again to locate the cells within the model.

demonstration about how to utilize the home button for finding tiny elements in a mesh
Figure 5: Finding single elements in the whole domain by using the home button.

If you are wondering how to create good meshes, check out the meshing tutorial for the standard mesher.

Note

If you have questions or suggestions, please reach out either via the forum or contact us directly.

Last updated: October 8th, 2020

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