Tortuosities with respect to porous volume fraction, morphological heterogeneity and structural anisotropy. Average tortuosities with their corresponding confidence intervals l_σ, computed on 40 realizations of each model. (a) Porous volume fraction V_p decreasing with models (1), (2) and (3). (b) Morphological heterogeneity increasing with models (4) and (5). (c)–(d) Structural anisotropy with in (c) decreasing V_p with models (1’), (2’) and (3’), and in (d) average arithmetic M-tortuosity values of model (3’) with a screening of parameters α₁ = {−10, −5, −2, −1, 0, 1, 2, 5, 10} and α₂ = {−10, 0, 10}.