Permeability.cpp, lattice to physical

My interest is in finding permeability in Porous media; in my case petroleum rocks.
Typically the input is a 3-D image of a rock in .raw format, from where I need to evaluate the permeability.

Coming to the issue with which I need help.

I am trying to find out the permeability of cylindrical pipes, so that I can validate the simulation results with the values obtained from analytical expressions.
I have created 3 different pipe geometries.

  1. Pipe diameter 10 voxels, length 2000 voxels.
  2. Pipe diameter 20 voxels, length 2000 voxels.
  3. Pipe diameter 30 voxels, length 2000 voxels.

After running the permeability.cpp script of Palabos on these geometries, I am getting the results as

  1. 0.06124
  2. 0.06242
  3. 0.06417

Where even the ratios are not analytically concordant, plus I am unsure about how to convert these units to physical units. I request you to tell me what is wrong with my approach, or suggest me a new one.

Thank you.

With regards,