[Manuel Berger]

Dear Mathias,

thank you very much!

Best,
Manuel

[Manuel Berger]

Hi Max,

thanks for the explanation. Thus, my implementation of gravity is correct. See you at the Spring School!

Now I have to find a way to make this simulation numerically stable. Any recommendations based on personal experience for boundaries? So far I made the experience that increasing the resolution N helps sometimes, but with the disadvantage that the computational time increases dramatically.

Boundaries for numerical stability would be interested for: (in brackets are the used values)

gravitational force in lattice units (0.0187673)
latticeRelaxationTime (0.557646)
maximum velocity in lattice units (0.0505667)
maybe lattice viscosity
….

What are the steps you do, do make a LB simulation numerically stable.

Thanks a lot!

Best
Manuel

• This reply was modified 4 years, 6 months ago by Manuel Berger.

[Manuel Berger]

Dear Mathias,

I have already registered for the spring school in Berlin.

I have tried your conversion factor and found out that

converter.getConversionFactorMass()/converter.getConversionFactorForce();

is equal to

converter.getPhysDeltaT()*converter.getPhysDeltaT()/converter.getPhysDeltaX();

thus, my conversion is also fine.

The only question that still remains, which is independent of this code line is: How to convert gravitational acceleration (9.81 m/s^2) to body force? – or is it the same?

Thanks a lot!

Best
Manuel

[Manuel Berger]

Dear Mathias!

Thanks for the fast response and the hints:

1/ I have used the converter to convert quantities into lattice units (i.e. gravitation code line 174)

T gLB= gravity*converter.getPhysDeltaT()*converter.getPhysDeltaT()/converter.getPhysDeltaX();

I have also used the poiseuilleXd file as a starting point. However, it shows how to apply a body force, but not how the body force is connected to gravitation. I think in the LB framework OpenLB, there is so far no code that shows this connection. The LB framework palabos (palabos-v2.0r0\examples\showCases\vofMultiPhase\damBreak3d.cpp) shows that body force and gravitation (in lattice units) is the same. Code line 70,71. I assume my implementation of gravitation is fine in the bifurcation3d (see dropbox)?

2/ You are right Buoyancy forces are not considered.

3/ I get already a stable simulation result with changed boundary conditions (without gravitation). Adding gravity (1 thing changed) does not work.

I would be great if you could find a specific mistake in my code or recommendation i could change.

Thanks you very much for the help in advance

Manuel Berger

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