A grid created in CFD-VisCART will have a high percentage of Cartesian cells (100% for Stair-Step Mesh; as high as 90-95% for Projected Mesh). Since these form the bulk of the grid, it helps to make sure that these cells are built in an efficient manner.

The Cartesian cells split repeatedly in subsequent layers as they approach a source (line, plane, box, etc.) or a surface where a smaller cell size has been specified.

Figure 1.  Cartesian cell splitting

There are a couple of cell-splitting rules that the user should know about:

1. The cells will split and reduce in size until they are equal to or smaller than the specified cell size on a surface or source - the size is not matched.
2. When a cartesian cell splits in the subsequent layer, the cell size in each direction of split is reduced to half. This is shown in Figure 2.

Figure 2.  Splitting rule

The above cell-splitting rules should be kept in mind while assigning cell sizes in CFD-VisCART. This will prevent drastic increases in cell count (grid size) due to minor changes in user-specified cell size values. The following example is a simple demonstration of how the cell-splitting rules apply.

This model (Figure 3) contains a cube of side 100 with two nested (one inside the other) box sources inside it.

Figure 3.  Simple Model (3 surfaces of the cube blanked out)

Case 1:

Largest cell size in simulation: x = y = z = 10
Normal cell size = Tangential cell size = 10
Source Box_1: x = y = z = 5
Source Box_2: x = y = z = 2.75

Cells split:

• from size 10 to 5 inside Box_1 (Box_1 specification of 5 satisfied)
• then from size 5 to 2.5 inside Box_2 (Box_2 specification of 2.75 satisfied)

Grid Created: 11584 cells

Figure 4.  Case 1 - Grid on cutting plane passing through the center of the cube

Case 2:

Same as Case1 except - Source Box_2: x = y = z = 2.25

Cells split:

• from size 10 to 5 inside Box_1 (Box_1 specification of 5 satisfied)
• then from size 5 to 2.5 inside Box_2 (Box_2 specification of 2.25 not satisfied)
• then from size 2.5 to 1.25 inside Box_2 (Box_2 specification of 2.25 satisfied)

Grid Created: 52408 cells

Figure 5.  Case 2 - Grid on cutting plane passing through the center of the cube

As seen in Case 2, a small change in the specified cell size for box source Box_2 resulted in a 5-fold increase in the cell count.

Real world models are not as simple as the one above and will have a number of sources of various kinds and sizes making it difficult to come up with the best set of cell size values for the various sources. It is still a good practice in general to keep this behavior in mind and tweak the global, surface and source cell size values before generating a grid or if a steep increase in the cell count is observed.

If you have any questions about this tip or would like us to discuss others topic in the future, please let us know.

Regards,
Santosh Kini
Applications Engineer