Mesh Adaption for Reacting Flows Reference

The Adaptive Mesh model refines and coarsens cells in flame zones based on the secondary gradient of selected combustion variables, such as species mass fraction, or mixture fraction and progress variable.

Mesh Adaption for Reacting Flows Properties

Max Refinement Level

Sets the maximum limit on the number of times a cell can be refined in a simulation.

One refinement level for a cell means one step of midpoint subdivision. The resulting number of child cells are equal to the vertices of the parent cell.

For trimmed cells, one refinement level means 8 subdivided child cells from one parent cell as shown in the image below:

For poly cells, the midpoint subdivision results in as many child cells as there are vertices in the parent cell, that is, typically 12-15 child cells. One example is shown in the image below:

The refinement level increases or decreases only by one level in each adaption—mesh adaption is a successive procedure.

Variables

Variables

The following table shows which variables are available with which combustion models.

Variables	Complex Chemistry / EBU models	FGM model	SLF / Chemical Equilibrium	Any other combustion model
Mixture Fraction 0		*	*
Progress Variable		*
Temperature				*
[species] H2O and OH (if present) are selected by default. Other species are also available.	*

* default variables

Error Indicator Limits

The error indicator limits are available to set for each variable as a scalar range [low, high]. The Adaptive Mesh model calculates the error indicator (EI) as the output of the secondary gradient of the selected Variables and uses the error indicator limits to determine if cells are marked for refinement as follows:

• If EI > high, cells are marked for refinement.
• If EI < low, cells are marked for coarsening (cannot be coarser than the starting mesh).
• If low <= EI <= high , cells are kept unchanged.

Default [low, high] values are suitable for most cases. Otherwise, follow these guidelines:

Requirement	Action
Adapt fewer cells.	Increase the high limit, and optionally also increase the low limit.
Adapt more cells.	Lower the high limit, and optionally also lower the low limit.

If several adaptive mesh criteria are active at the same time, their relative influence on the adaptive mesh refinement is prioritized as follows:

• Refine and Coarsen → Refine
• Refine and Keep → Refine
• Keep and Coarsen → Keep

By this approach a finer mesh takes precedence over a coarse mesh.

Field Functions

Error Indicator of [variable]

$EI$ , a normalized second gradient, is used to determine if the adaptive mesh model refines or coarsens cells based on [low, high] values that are specified for each selected variable.