OpenLB – Open Source Lattice Boltzmann Code Forums on OpenLB General Topics Two component lattice density setting

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• #5718
Gloriousface
Participant

Hello，everyone

I’m setting the lattice density of the two components. Does the sum of the lattice densities of the two components need to be equal to 1? Or the lattice density of each component needs to be around 1. If you can help me solve this problem, I would be very grateful.

#5720
stephan
Moderator

The lattice density for a Navier-Stokes lattice (population which approximates NSE) can in general be initialized with unity.

Please note that dependent on what you are simulating, and which model you are using the physical densities of each lattice have separated meanings.
This means that the lattice densities are kinetic densities in the first place and are only mapped one-to-one on macroscopic conservables if constructed to do so.

For e.g. Free Energy Model, the lattice densities of the second (and maybe third for ternary systems) are macroscopic variables (order parameters) in the additional Cahn-Hilliard equations.
These variables are hence not directly the component densities, but instead recover them by a simple additive relation.
Dependent on exactly these relations the additional Cahn-Hilliard lattice densities should be initialized (see e.g. the microFluidics2d Example in OpenLB).

For more information please have a look at the user guide and the standard literature for Multiphase/Multicomponent LBM (e.g. Krüger et al. 2017 “The Lattice Boltzmann Method” Chapter 9).

BR
Stephan

#5724
Gloriousface
Participant

Thank you very much for your reply. When I used SC model to simulate oil-water two-component flow, ZouHe velocity boundary was set at the inlet and ZouHe pressure boundary was set at the outlet. During the simulation, the average density of component 1 and component 2 kept increasing and diverged after 10000 iterations. What is the reason for this? I would appreciate it if you could help me with this problem.

Best
Gloriousface

#5726
mathias
Keymaster

Dear Gloriousface,

my team helped you to get a basic version running. Finxing stability issues by means of our forum, however, is more that we can do.

Best
Mathias

#5727
Gloriousface
Participant

Thank you very much for your help.I don’t quite understand “getAverageRho ()”. What density does this function output and why is it different from the initial density I set？

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