**4.2.2 Effect of artefacts on specific surface area and pore size distribution**

Specific surface area of the wool fibre web sample was analysed as a function of threshold value and noise level, see the results in Fig. 15. The specific surface area of the geometry thresholded by the forest fire method had a value of 0.175 m-1. The value of specific surface area of the sample geometry decreased as a function of increasing threshold value and increased as a function of increasing noise level. According to Eq. (3), when a sample geometry has a high specific surface area, it has a small permeability value. This could also be seen from the results in Fig. 15 and in Figs 13a and 14a.

Pore size distribution of the wool fibre web sample geometry was analysed as a function of threshold value, see the results in Fig. 16a. When compared to the experimental permeability results, the optimal threshold value was found to be between 20 and 30, see Fig. 13a. The results in Fig. 16a clearly show that with the threshold values of 20 or 30 the pore size distribution differs highly from the pore size distribution of the geometry that was segmented using the forest fire method. These results support the interpretation related to the permeability values obtained using the geometries segmented by conventional grey value -based thresholding. Combination of too high specific surface area and too thin fibres (high porosity) resulted to the permeability values very close to the experimental results. These interpretations were found to be similar for the packaging board and for the sand stone samples also.

Pore size distribution as a function of noise level for the wool fibre web sample is presented in Fig. 16b. The effect of added noise was found to be not as high as for the hexagonal array of cylinders, see Fig. 7. For noise level of 2 %, the mode value of the distribution was approximately a half of the mode value of the noise free geometry.

The Effect of Tomography Imaging Artefacts on

Structural Analysis and Numerical Permeability Simulations 487

Fig. 16. (a)

(b) Fig. 16. Normalized pore size distribution for the wool fibre web sample as a function of

Approximate factors of how certain levels of noise and edge roughness effect on permeability value, mode value of pore size distribution and specific surface area of the hexagonal array of cylinders are summarised in Table 1. According to the results, even a small amount (0.01 % of sample volume) of noise in the void space has a drastic influence on the permeability values. The noise level of few percent caused magnitudes decrease in the permeability values. Increase in the amount of noise increases the specific surface area of the simulation geometry and thus decreases the permeability value. Also the pore size

pore size with different threshold values (a) and with different noise levels (b).

**5. Conclusions** 

Fig. 15. Specific surface area of the wool fibre web sample as a function of threshold value (a) and noise level (b).

Fig. 15. (a)

(b) Fig. 15. Specific surface area of the wool fibre web sample as a function of threshold value

(a) and noise level (b).

Fig. 16. Normalized pore size distribution for the wool fibre web sample as a function of pore size with different threshold values (a) and with different noise levels (b).
