Permeability of a 3-D porous medium


In an attempt to verify Palabos code for computing permeability of a 3-D porous medium I made an artificial volume of cylinders on a uniform cubic grid, for which analytical solution exists and can be computed as the following: permeability = (size of a cube)**2 * (porosity)**2 / (72 * Pi).
However, permeability computed with the code is very different than analytical solution. Could anybody help me to interpret the result from Palabos correctly?

Thank you so much!