Here Dp represents the equivalent diameter of porous fibers, which
can be solved using the effective volume method. Using the method
discussed in our earlier work [7], fiber shape factors, ˇ and in
present work are set as 120 and 1.8, respectively.
When the porosity ε is greater than 0 and less than 1, the air
flows through the porous fiber zone; otherwise, the air flows in the
non-porous fiber zone, including the cavity and outer local zone
close to FADS. The corresponding velocity and pressure are both
degenerated into the macroscopic values, U and P. The relationship
between the microscopic values and the macroscopic values satis-
fies the Dupuit–Forchheimer relation [20], i.e. U = ε u, P = ε p.