About Expert Numerics Controls

The expert numerics controls allow you to disable secondary diffusion and second-order advection for all elements in your simulation or for the "bad" elements only. You can also specify the criteria that define bad elements in your model, such as chevron elements or elements with a high skewness angle or volume ratio.

This page discusses:

Effects of Disablement

Disabling secondary diffusion and second-order advection increases the robustness for the solution overall, while reducing the order of accuracy. You can define the effect of these second-order terms for elements in your simulation in the following ways:

  • Disable the terms in the entire domain: Increases solution robustness but decreases accuracy.
  • Disable the terms in bad elements only: Meshes often have a large number of high-quality elements and a small number of bad elements that cause convergence issues. You can disable second-order terms only for elements that do not meet your quality standards. This approach increases solution robustness with a smaller impact on accuracy than disabling second-order terms for the entire model.
  • Never disable the terms: The solver always uses second-order schemes for maximum accuracy. This approach can cause convergence issues if low-quality elements are present in the mesh.

Bad Elements Types

When you disable the secondary diffusion or second-order advection in bad elements only, you can specify any of the following as a bad element:

  • Chevron elements
  • Elements whose skewness angle exceeds a value that you specify
  • Elements whose volume ratio exceeds a value that you specify

Chevron elements are elements with a line that joins the cell centers, but the line does not pass through their common face. Chevron elements are common at sharp trailing edges in aerospace simulations.

Viewing Bad Elements

You can view any elements you flag as bad elements when you review your simulation results. All elements in the simulation are now associated with the Boolean output variable BADCELL, which displays good elements in green and bad elements in red. When your analysis is complete, you can locate the bad elements by creating a new display group with the following parameters:

  • Variable: BADCELL
  • Values at: Elements
  • Range: Above a minimum value of 0.5

You can also apply transparency or use display groups to view bad elements in the interior of the model.

The default values used to identify bad elements detect only the elements that are nearly degenerate. If you still have convergence issues after disabling second-order options using the default parameters, you should identify the bad elements in your mesh and improve the quality of the mesh in those areas. If the issue persists, relax the bad element definition progressively to achieve a converged solution while minimizing the impact on accuracy.