Biomedical Engineering Reference
In-Depth Information
flow does not necessarily imply greater accuracy (see Sect. 8.2.2). Furthermore
in local regions of low flow rates, where laminar effects are dominant, turbu-
lence models provide greater diffusion due to the turbulence production inherent
in turbulence models. Sophisticated models such as LES and Direct Numerical
Simulations, however, do resolve the transitional behaviour but are quite compu-
tationally expensive. The absence of the laryngeal jet is expected to produce an
under-prediction of the deposition efficiency in the first bifurcation at the carina.
However, deeper into the lung airways the effects of the laryngeal jet diminishes and
eventually become negligible. A steady flow was applied based on a variety of criteria
(Isabey and Chang 1981; Slutsky et al. 1981; Sullivan and Chang 1991) such as the
Womersley parameter,
α
D/
2(
ω/ν
g
)
0
.
5
; a variant of the Womersley parameter,
=
α
∗
1
/
2(
ωD/
0
.
0075
u
ave
)
0
.
5
=
<
1 (Pedley et al. 1977); and the Strouhal num-
ber,
S
ωD/u
ave
where
u
ave
is the mean velocity,
ω
is the angular frequency of
oscillation (
=
2
πf
), D is the diameter of the tube; and
ν
g
is the viscosity.
The outlets were artificially extended downstream given by,
L
extension
=
0
.
05 Re
D
to obtain fully developed profiles and hence avoid any reverse flow that
may be caused due to an abrupt end to the flow field. Each model underwent mesh
refinement by cell adaption techniques that included refining large volume cells, cells
that displayed high velocity gradients, and near wall refinements. Velocity profiles
were compared between each subsequent model until the profiles remained the same
and hence became independent of the grid size. The final models had a mesh size
between 1.1 to 1.5 million cells (Fig.
8.58
).
=
Fig. 8.58
Computational
model of the upper
tracheobronchial airway tree
with branch identification.
The first two characters define
the branch generation. The
third character represents the
right
or
left
airway. The
fourth character may be U, M
or L representing
upper
,
middle
and
lower
,
respectively
G1
G4RU
Surface mesh at
trachea section
G2R
G2L
G3RU
G3L
G3RL
G3LL
G4RL
Airway tree of Patient-A after
recovery from asthma episode
Cross-sectional mesh at
trachea section shown above
8.5.2
Airway Geometry Recovery
The reconstructed model of the bronchial tree exhibits an asymmetric dichotomous
branching pattern. The beginning of the bronchial tree begins with the trachea, which
is a hollow cylinder in the shape of a horseshoe due to the C-shaped supporting
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