Fig. 3. Subdivision of the cylindrical model of radius r₀ and height h₀ for numerical analysis. This subdivision yields coaxial cylindrical and hollow-cylindrical volume elements of height (h) with radial extensions being multiples of 0.5r. The numerical analysis aims to determine the oxygen partial pressures for the discrete set of grid points (filled circles) representing the volume elements. The axial and radial coordinates of the points are (j+0.5)h and ir, where the indices j and i are integers with j=0,..., N_ax-1 and i=0,..., N_rad. The oxygen partial pressure (P_j,i) of a representative volume element (white rectangle) is affected by diffusive (broken arrows) and convective (solid arrows) exchange processes with the adjacent volume elements. The hatched areas represent those fractions of the respective volume elements that are shifted to the left by convection during the time interval t. The white line on the left exemplifies the radial position of a compartment interface that has to be rounded to a multiple of r.