Tortuosity factor τ versus pore volume fraction ε of the random models in Section 2.4. The symbols (circle, square, diamond and triangle) are numerically estimated with the FFT method. The estimate by Equation (6) with Weissberg’s bound p = 0.5 is plotted in green dotted curve. The estimate by the self-consistent method by Equation (25) is plotted in magenta dotted curve. The curve fittings with Equation (6) are performed with the data τ (ε ≥ 0.8), and are plotted in dotted curves in the same color as the symbols (subplot). The corresponding values of p are shown in Table 5.