Play by the Rules – Identify and Fix Mesh Quality Issues Right Away!

The Golden Rule of mesh generation is to produce a mesh that provides a foundation for an accurate and converged computational fluid dynamics (CFD) solution. One might say that meshing experts know how to enforce the Golden Rule. On the contrary, only a CFD practitioner can tell whether the mesh is suitable for analysis. The physical models, the solver algorithm, the grid type, the available computer resources, desired outputs, desired level of accuracy, organization's best practices determine a grid's acceptability. Using Fidelity Pointwise’s Rules command, one can monitor and verify the specified grid requirements at any time during the meshing process.

Assessing Mesh Quality Metrics

Mesh quality metrics are typically evaluated once the mesh is complete. Most CFD solvers have some built-in mesh diagnostics that will identify issues. But the practical problem with using the CFD solver is that your mesh assessment is performed after the entire mesh generation process is completed.

Most mesh generators, like Fidelity Pointwise, also include mesh diagnostics that can evaluate any part of the mesh at any point during the meshing process. Also, it quantifies mesh quality using various metrics. This is a more logical technique to evaluate mesh quality because any issues with the mesh can be fixed right away.

Set Precise Mesh Criteria

The Rules in Fidelity Pointwise's Examine command is proactive. Instead of looking for problems after the mesh generation, Rules can be evaluated at any time during the process. It avoids the accumulation of mesh issues for later stages of the solution.

Figure 1. Structured grid for a flange.

Considering the grid shown in Figure 1, suppose an organization's best practice for structured grids includes the criterion that the spacing between grid points on connectors should not vary by more than 30 percent from one interval to the next. The user can create a Rule for this in the Rules command, as listed below:

1. Under the Examine menu, open the Rules panel.
2. Click the Connector icon. This means the rule is to be applied only to connectors and limits the metrics to those that apply to connectors.
3. Choose the function Connector Length Ratio I from the drop-down menu.
4. For the Limit, choose < (Less Than) from the drop-down menu and set the value to 1.3. (In other words, the rule says that all grid length ratios should be less than 1.3.)
5. Press the Save Rule button.

Figure 2. Locations where the connector rule for length ratio is broken are displayed in red.

Two observations are made after the rule's computations have been refreshed and updated. First, the Display window has been updated, and the locations where the rule is violated are displayed in red (see Figure 2). Second, the status icon has changed to a ruler with an “x” meaning that a rule is broken (in other words, there are grids that violate the criterion you set for the rule).

Advanced Use of Rules Command

In the above example, only one rule is created. But the organization's best practices might include other mesh quality guidelines, and it is possible to accommodate these guidelines by creating additional rules on Fidelity Pointwise. Figure 3 illustrates the addition of a second rule that says the minimum included angle on a surface grid can be no less than 60.

Figure 3. Color and naming options help you distinguish one rule from another.

Figure 4. Cells that break rules are drawn on the grid using their assigned color. In this case, the connector rule is red, and the domain rule is yellow.

The user can track the issue by looking at the table of Rules on the top panel to find out more about a broken rule. Each rule is listed by name, and by opening the rule, a list of every entity to which the rule is applied and the number and percentage of violating cells can be viewed.

If the concerns are about the window filling up with a display of broken rules, the user can control the setting in the Edit, Preferences command. The default is to display 1,000 violating cells per rule. When the limit is exceeded, the worst violators are shown first.

Summary

Mesh generation benefits from the imposition of well-defined boundaries. Rules help the user accomplish this. By creating rules for mesh quality that reflect an organization's best practices, you significantly improve your ability to generate a mesh that can be used for accurate and efficient CFD. When Rules is used in conjunction with the other mesh quality diagnostic tools in Fidelity Pointwise, much of the “magic” from mesh generation can be removed and turned into a well-defined engineering process.

Watch this video on how to use Examine rules in Fidelity Pointwise for proactive alerts on grid quality.

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If you'd like to evaluate your mesh's quality using Fidelity Pointwise or set some rules for generating a new mesh, try a free demo of our software:

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