Using Blanking for CFD-FASTRAN Chimera Cases
CFD-FASTRAN uses closed surfaces, such as walls or
blocked regions, to create Chimera holes (or blanked
regions) in a grid. By default, the number of buffer
layers is set to 1. This means that a closed surface
will cut a hole in any mesh that overlaps it, and the
initial "blanking" of grid cells defines the hole.
With a buffer layer of 2, grid cells adjacent to the
initial hole cut are identified and are also blanked.
Having multiple Buffer layers is a good thing. Zone
boundaries should not receive interpolated data too
close to a wall surface, because the solution
gradients are usually high there because of boundary
layer effects. The rule of thumb is to use as many
buffer layers as possible, but not so many that you
move the hole-boundary beyond (outside) of the donor
zone; that would correspond with the creation of
"orphan" grid cells (cells without any interpolant
from the chimera grid). Make sure that there is
sufficient overlap between chimera and background
grids. Generally an overlap of 8 to 10 layers of
cells is recommended.
It is recommended to first conduct a steady-state
analysis with chimera-overset grids without any
motion module, to ensure that the hole cutting is
good and there are no orphan cells.
Figure 1 shows the background grid overlapping the
pitching airfoil. Figures 2 and 3 show the chimera
hole-cutting boundaries for buffer layers set to 1
and 2 respectively. A larger blanked region can be
clearly seen in the Figure 3. The chimera gird around
the pitching airfoil exists, but is not shown in the
figures.
Figure 1. Background grid over a pitching
airfoil.
Figure 2. Blanked region in the background gird
for Buffer Layer setting of 1.
Figure 3. Blanked region in the background gird
for Buffer Layer setting of 2.
"Blanking" Option in CFD-VIEW
In order to view the chimera hole boundaries (and the
blanked region) in the model, as shown in Figure 1,
the user needs to use the "Blanking" option under
Data Type when importing Data files in CFD-VIEW, as
shown in Figure 4.
Figure 4. Import Data File panel in CFD-VIEW with
Blanking option selected.
The "Blanking" option uses blanking information to
prevent drawing blanked grid cells. If you would like
to display these blanked cells, you can ignore
blanking information by toggling the "Use Blanking"
command on the View menu, as shown in Figure 5.
Figure 5. Using Blanking from the View menu.
The blanking option is available when either the DTF
data source or the Plot3D data source is selected. If
checked, CFD-VIEW will allocate one extra column to
contain a blanking array, typically containing 0's
and 1's, which indicates if a certain grid node is
blanked.
Note:
While CFD-FASTRAN is used as an example in this
tip, the Blanking option also applies to CFD-ACE+
cases that use the Chimera grid option.
If you have any questions about this tip or would
like us to discuss some other topic in the future,
please let us know.
Regards,
Amit Saxena
Senior Applications Engineer
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