When beta features are enabled (as described in Introduction), the following geometric-based metrics are available, which are designed for identifying poor quality cells that could negatively affect local or global solver stability:
Tangent Skewness Quality
This is a face-based metric that uses information defined within the cell for calculating the quality. The maximum value calculated for the faces of the cell is reported.
Cell-Based Tangent Skewness Quality
This is a cell-based metric that incorporates neighboring cell information for calculating the quality.
These field variables calculate a skewness in a cell that can better predict the stability behavior of the solver. The larger the value for these field variables, the worse the quality of the cell. They can be used during mesh generation and prior to a numerical simulation to make sure that maximum value of the tangent skewness in the domain is in a reasonable range.
These field variables are available in the Mesh... category, and you can use the Contours Dialog Box, Field Variable Register dialog box (see Field Variable), and/or Histogram Dialog Box to ascertain the values and/or identify the distribution. You can also use the button in the General task page, and the maximum tangent skewness quality value and location will be reported in the console.
If it is not possible or desirable to revise the mesh so that the cells have better skewness values, you can use the following beta feature text command to try and improve robustness by applying poor mesh numerics (that is, a numerical correction of the transport equation discretization) to the cells with a Tangent Skewness Quality value above a defined threshold. For further details on poor mesh numerics, see Robustness with Meshes of Poor Quality.
solve → set →
poor-mesh-numerics →
tangent-skewness-quality-based?
The threshold value for the previous text command is set to 5 by default, as this value generally works well with the default Fluent settings. However, you may need to adjust the threshold value depending on your case. For example, you may want to increase the threshold for the following:
smooth solutions on fine meshes
solutions with decent gradient quality
solutions with small variations between iterations / time steps
On the other hand, you may want to decrease the threshold for the following:
non-smooth solutions on coarse meshes
solutions with poor quality gradients
solutions with abrupt changes due to large under-relaxation factors / time step sizes
solutions with geometrical or physical discontinuities (for example, shocks, porous media, or multiphase flows)
When you have used the
solve/set/poor-mesh-numerics/tangent-skewness-quality-based? text
command, note the following:
An additional field variable is available in the Mesh... category: Tangent Skewness Index. This reports the number of faces in a cell that have a Tangent Skewness Quality value that is greater than your defined threshold. Tangent Skewness Index is an additional indicator to identify the worst cells among bad cells based on the tangent skewness metrics. The larger the value, the more severe the effect on solver discretization.
If you use the Mark Poor Elements field variable (in the Mesh... category) to visualize or display poor mesh elements, cells that have a Tangent Skewness Quality value that is greater than your defined threshold will be identified with a value of 7.