Hello Palabos developers and users,
I am working in the area of snow microstructure characterization using Microcomputed tomography and structure-property modeling for developing snow cover simulation models. Recently I was introduced to Palabos for fluid flow simulation in porous media using LB technique (Degruyter et al 2010). I must congratulate and thanks the developers for providing such a useful and powerful tool to the scientific community.
I started with Geophysics tutorial for permeability estimation of porous media and applied on the 3D binary mask of snow microstructure obtained from tomography data. In this regard, I have following questions:
- Convergence criteria: As per the tutorial, steady state is reached when the standard deviation of the average energy, over 1000 time steps, fall below the threshold of 1e-4. Also a max number of iteration, 30000, is specified. For my simulations (system size of 150x150x150, 200x200x200, 150x150x300, resolution ~ 25.7 microns), the average energy never falls below 1e-4, even after max 30000 iterations. The permeability results were also not much different when I go for max iterations i.e. 30000 or only 2000.
Max Iterations Permeability stdDev/average
2000 0.521308 0.0342814 (after 1500 iterations)
30000 0.520902 0.0344371 (after 29500 iterations)
My question is whether the solution has converged in the first say 5000 iterations itself or my system is ill-defined. Although I must say that the estimated permeability values are of the same order to what I expect. Since the average energy is not changing much, can I safely restrict the max iterations up to say 5000, without bothering about the criteria of < 1e-4.
Choice of omega: What I understand that a value of omega=1 is the safest option for a stable solution in single relaxation BGK model, however there is a finite size effect and omega (kinetic viscosity) dependency in the LBM. Is there any criteria for deciding upon the appropriate values for omega and discretization level.
Unit conversion: How the time step / resolution delta t is defined? Specifically how can we convert the various parameters like, average velocity, Grad P and viscosity from LB units to physical units.
I would appreciate if some one can clarify about any silly mistakes I am making. Cheers