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By Antoine Dozolme and Thierry Marchal, Fluent Benelux
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The surface mesh, and contours of
deformation on the inner trachea
wall during inspiration
Below, deformation of the outer
trachea wall with velocity vectors
at the bronchi exits, with the
shadow of the inner trachea
wall shown
The airflow through the trachea and
bronchi can be impacted by disease
or injury. It can also be used to transport
drugs from nasal sprays or inhalers
to the blood stream through the airway
walls. Recently, the fluid-structure interaction
(FSI) model in FIDAP was used to
study the airflow and induced wall deformation
during a normal breathing cycle.
When FSI is used, the stresses on, and
subsequent deformation of the airways
are calculated as a consequence of the
airflow. Two approaches were investigated.
For the simpler case of a 3D branched
duct, the shell element option was used
to model the trachea wall. This option
treats the wall as a membrane of negligible
thickness with an associated bending
resistance, so that small deformations
are possible. A more rigorous – but more
computationally expensive – approach
was applied to a patient-specific geometry.
For this case, the trachea wall was
modeled as a deformable entity of finite
thickness. While the simplified approach
yields quick qualitative results that are
satisfactory for a preliminary study, the
second approach is more appropriate
for patient-specific geometries, when
stresses and deformation of the trachea
wall are the major focus.
PreSTO, the new template capability
in FIDAP, was used to setup the simulations,
using a realistic geometry
obtained from computerized tomography
(CT) scans of a patient. To prescribe
the inlet velocity boundary condition,
breathing was modeled as a sinusoidal
function of time. A breathing cycle of 5
seconds, appropriate for normal adult
activities, was considered. Modifications
to the breathing rate and/or amplitude
were studied to investigate the differences
between periods of exertion or sleeping.
Only radial deformations of the airway
walls, resulting from normal forces,
were allowed to occur. Following such
deformations, the elastic remeshing
technique was applied. The mixing
length model was used for turbulence.
The results showed that deformations
of the trachea were small, occurring at
the junction of the trachea and bronchial
pipes, as expected. The computed
deformations are directly related to the
pressure applied by the moving air. The
airflow was found to separate into
equal proportions between the right
and left lung during inspiration, with no
direct air exchange between the lungs
during expiration.
The authors wish to thank Dr. Thomas
Frauenfelder, University Hospital of
Zurich, for providing the CT data.
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