Biomedical Engineering Reference
In-Depth Information
by the diaphragm flattening out to increase the volume of the pleural cavity (see
Sect. 2.3.2). Flow rates for adults can range between 5-12 L/min for light breathing
and 12-40 L/min for non-normal conditions, such as during exertion and physical
exercise. Usually breathing switches from pure nasal flow to oral-nasal flow at this
higher range. Additionally, flow rates for extreme forced inhalation conditions have
been found to reach 150 L/min (Robert 2001). Issues pertaining to the geometry and
physiologic features discussed in Chap. 2 also need to be kept in mind when setting
up the numerical model and interpreting the results.
8.2.2
Boundary Conditions
The reconstructed computational model of the human nasal cavity from Chaps. 3 and
4 is used in this case study. Modelling the inhalation process raises some important
conditions that need to be determined:
• is the flow
laminar or turbulent
,
•
steady or unsteady
,
• the type of
inlet/outlet conditions
for the nasopharynx and nostril inlets
• additional equations that are needed to account for
heat transfer
.
Laminar or Turbulent
Laminar flows are normally characterised by a smooth mo-
tion where the fluid's viscosity dominates allowing high molecular diffusion and
dampening out any fluctuations in the flow. This leads to adjacent layers of fluid
sliding past each other in an orderly fashion (like layers of lamina). However for a
nasal cavity which has a complex geometry that is highly convoluted, flow separa-
tion and recirculation can exist especially in the region from the nasal valve to the
main nasal passage where a rapid increase in the cross-sectional area is observed
(Proctor 1982), enhancing flow instabilities. It must be mentioned that although flow
separation and recirculation are typical characteristics of turbulent flow, the presence
of these characteristics does not necessarily indicate that the flow is turbulent, since
they may be found in laminar flows as well.
The dimensionless parameter, the Reynolds number (
Re
), is often used to de-
termine the flow regime. For higher velocities the effects of turbulent disturbances
become significant with the presence of velocity fluctuations in the flow field while
for lower velocities the viscosity dominates to dampen out any fluctuations. The
critical flow rate and therefore the
Re
number at which the flow changes from a
laminar to a turbulent flow regime are difficult to succinctly define due to the com-
plexity of the airway, which has led to some debate concerning the type of airflow
regime to implement for numerical simulations. Experimental studies by Kelly et al.
(2000) have suggested that a laminar flow regime dominates for low flow rates around
10 L/min. While the results of Hahn et al. (1993) also agree, the authors mention that
the flow is a disturbed laminar regime. Churchill et al. (2004) studied ten nasal cast
replicates and found that at the lowest flow rate of 1.5 L/min, small local turbulent
characteristics were present. However they pointed out that some of the turbulence
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