Conservation of mass:
ε
∂u
i
∂xj
=
0, 0 < ε
≤
1 (1)
Conservation of momentum:
u
j
∂u
i
∂xj
=
−1
∂ p
∂xi
+ ∂
∂xi
eff
∂u
i
∂xj
+
∂u
j
∂xi
−
ε
˛
u
i + εF √
˛
|u
i|u
i
, 0 < ε
≤
1 (2)
Here ε is the fiber porosity, which can be calculated using the
method in literature [11]. u is the intrinsic velocity (physical
velocity). p represents the intrinsic pressure. and eff are the
kinetic viscosity and effective viscosity of the airflow, respectively.
is the air density. ˛ and F represent the fiber permeability and a
dimensionless form-drag constant [19], respectively, which can be
expressed as follows:
˛
=
D2
pε2
ˇ(1
−
ε)2 (3)
F
=
ˇε3/2