Generation of Computational Mesh for CFPD Simulations - Computational Fluid and Particle Dynamics in the Human Respiratory System - page 84

Fig. 4.13 Mesh construction based on advancing front method on a 90 ◦ bend pipe. a Initial front
is established on the edge mesh setting. b New triangular mesh element is added by inserting one
new point to form a triangle and this continues to create a front. c Final completed mesh

triangular mesh on the surface boundaries of the domain. This mesh forms the initial
front, which is then advanced into the physical space by placing new points ahead of
the front and forming tetrahedral elements. The required intersection checking now
involves triangular front faces rather than edges as in the two-dimensional case. For
detailed descriptions of the advance front method the reader is strongly encouraged
to refer to the work by Lo (1985), and Marcum and Weatherill (1995) (Fig. 4.13 ).

4.3.5 Comparisons Between Structured and Unstructured Mesh

The main advantage of using an unstructured mesh is its ability to conform onto
complex geometries where it may be impossible to apply a structured mesh. With
advances in mesh generation algorithms, combined with a user friendly graphical
interface within the software, an unstructured mesh can usually be created automat-
ically or with minimal user interaction. In comparison, a structured mesh requires
laying out a single mesh block that is then split, merged, or has O-, and C- grids
applied, even within commercial mesh packages. This process is much more difficult
to perform and requires experience in the blocking strategy.
Some drawbacks of unstructured mesh include the treatment of the nodal points
of any cell which cannot be simply treated or addressed by a double of indices ( i , j )in
two dimensions or a triple of indices ( i , j , k ) in three dimensions. Instead additional
data calculations are needed to connect an arbitrary number of neighbouring nodes,
which requires additional computational memory and further complicates the solu-
tion algorithms that are used to solve the flow-field variables. This typically results
in increased computational times to obtain a solution and erodes the gains in com-
putational efficiency compared with a structured mesh. Furthermore an unstructured
mesh is ineffective in resolving wall boundary layers since triangles and tetrahedrals
do not deform (stretch or bend) during local refinement to make the cells very small.