Biomedical Engineering Reference
In-Depth Information
Fig. 6.5
Multiblock-structured mesh with matching and nonmatching cell faces at each block
interface
a
matching cell faces (conformal)
b
non-matching cell faces (non-conformal)
6.2.3
Multi-Block Mesh
Block-structured or
multiblock mesh
is another case of a structured mesh. This is
particularly effective for complicated shapes where it is difficult to apply a single
block. The multiblock mesh allows the computational domain to be subdivided into
topological blocks. Figure
6.5
shows mesh assembled from a number of structured
blocks connected to each other. The attachments of each face of adjacent blocks
may be regular (i.e. having matching cell faces, also referred to as
conformal mesh
)
or arbitrary (i.e. having nonmatching cell interfaces, also referred to as
non-confor-
mal mesh
). Generation of grids especially with nonmatching cell interfaces is much
simpler than creating a single-block fitted to the whole domain. This approach pro-
vides flexibility in selecting the best grid topology for each of the subdivided block
regions.
Special types of topologies for each block include structured
O
- and
C
-grids.
These are in conjunction with the usual structured orthogonal mesh described ear-
lier, as well as any unstructured mesh (discussed in Sect. 6.3) of triangle or tetrahe-
dral elements.
An O-grid is a series of sub-blocks that arrange the grid lines into an “
O
” shape
where the last point on a grid line joins up with the first point of that line, hence
creating a circular '
O
' shape. Similarly a
C
-grid, which is half an
O
-grid, has lines
that bend in a semicircle shaped like the letter '
C
'. Each series of sub-blocks can be
found within the O- or C-block itself and represented in Cartesian coordinates in the
computational domain (Fig.
6.6
).
If we use a body-fitted structured mesh, then highly skewed and deformed cells
are found at the perimeter of the geometry (Fig.
6.7
), since the interior of the do-
main must satisfy the geometrical constraints imposed by the circular boundary.
This mesh generally leads to numerical instabilities and deterioration of the compu-
tational results. Hence, it is better to mesh the geometry with an O-grid. In this op-
eration, the O-grid fills the external circular cross-sectional conduit, while a square
block fills the centre.
Finally, grids may be overlapped on top of one another to cover an irregular flow
domain referred to as Chimera or overset grids. This differs from a single flat block-
structured mesh which connects neighbouring blocks together. Chimera grids allow
rectangular, cylindrical, spherical, or non-orthogonal grids to be combined with the
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