Case Studies in the Human Respiratory System - Computational Fluid and Particle Dynamics in the Human Respiratory System - page 235

pressure
inlet
0 Pa

pressure outlet
≈ 18 Pa

a b

Fig. 8.2 Applying pressure boundary conditions when the pressure outlet is unknown. a initial
simulation to establish pressure difference, b applying the known pressure difference from initial
simulation

pressure at the inlet is known (atmospheric at the nostrils) but the flow rate and/or
velocity is unknown. A pressure outlet is used to define the static pressure at the flow
outlets (and also other scalar variables, in case of backflow). However, the problem
with the pressure outlet is that it is unknown in relation to the nostril inlet to induce
the required flow rate. A modelling strategy to overcome this is to first simulate the
flow field with the known mass flow rate in mind using a mass flow rate condition
(Fig. 8.2 ). Upon completion of the simulation, the pressure difference between the
nostril inlet and nasopharynx will be known, and thus the pressure at the outlet can
be defined.

Heat Transfer Setup Two climatic conditions are used for the inspired ambient air,
the first is air at 25 ◦ C, 35 % relative humidity, which is similar to the ambient
temperatures that have been used in in-vivo studies (Garcia et al. 2007a; Holden et al.
1999; Keck et al. 2000a). This condition is referred to as Normal Air Condition. The
second condition is air at 12 ◦ C, 13 % relative humidity which is referred to as 'Cold
Dry Air Condition'. These air properties are applied at the inlet boundary condition.
The inner mucosal walls which are covered by a wet mucus layer are assumed to be
fully saturated with water vapour and to have an unlimited supply of heat and water.
Lienar et al. (2003) were able to measure local nasal mucosal wall temperatures after
exposure to air at different climatic conditions which ranged from 32.3-34.7 ◦ C for
normal air conditions and 30.6-33.7 ◦ C for cold dry air. Therefore a value of 33.5 ◦ C
and a value of 32.1 ◦ C were applied at the wall boundary condition for the normal
air and the cold dry air cases, respectively.
In addition to the continuity, momentum, and energy equations, the following
definitions are applied:

p
ρ +

u 2
2

E = h − (8.3)