TimBingert
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If you base your case on the Euler-Euler model, the drag force is necessary. The buoyancy force can be added through the descriptors::EXTERNAL_FORCE field so that you have both forces active.
Hi Callum,
the idea of the free surface models is that you are only solving the momentum equations for one phase, i.e. the liquid. If you want to do a crossflow of water and air you need to solve momentum equations for both phases. So a phase field model would be more appropriate for you. There is a so-called “free energy” model in OLB but it only has equal densities for the two phases yet. In the next release, there will be high density ratio phase field models that can be used for your problem.
Best
TimHi danial,
the division by the density is always needed, as it is once again multiplied by in the forcing scheme.
Please note, that the pseudopotential models as you mentioned are from different works and hence quite different. The old ShanChen model is using a ShanChen forcing scheme so it shows the relaxation frequencies omega at certain points. This makes the equilibrium depend on the relaxation time and therefore on viscosity which can be problematic as well as the old model only provides low density ratios.
The model used in MCMPForcedPostProcessor is using a Guo forcing scheme and has conservation laws to determine total densities and shared velocities.Kind regards
TimHi Callum,
I want to emphasize again, that this model for thermodynamic multicomponent multiphase mixtures has been introduced to OpenLB without boundary conditions as you can see from the example cases. Even for the inlet, it might need extra work to have a functioning velocity boundary condition. E.g. how does the pseudopotential force work on your bc as there will be some neighbouring nodes that are missing? Or as this model has density ratios, how do you determine the density at the inlet, it needs to be set as well as the velocity as in these multiphase models the density is not uniform everywhere.
If you would just like to have a simulation with water and air without the exact thermodynamics, you could also have a look into the free surface models. They are easier, but have the drawback that you only solve the momentum equation for the liquid. In the future, there will be high density ratio phase field models in OpenLB, too, with which it will be possible to simulate water-air systems too (without the mixture thermodynamics though).
Kind regards
TimHi Azmir,
please be aware that thermodynamically speaking, in a multicomponent mixture, there are no “partial pressures”. The partial pressure is just a simplification of a component’s fugacity in the mixture which is related to the component’s chemical potential. I would recommend to derive a free energy (Helmholtz energy) from the bulk pressure term you showed above and then derive chemical potentials for components 1 and 2. As you are using the pseudopotential model, you need to take into account how the psi_i are a function of the rho_i.
All in all, this looks like another mixing rule for a multicomponent equation of state.Kind regards
TimHi Aaron,
in our code, the cs^2 is defined depending on the DESCRIPTOR, so in most cases this is 1/3.
Hence, the pressure should be p = \rho * cs^2.
When you take a closer look at the heat conductivity lambda, it is connected to the thermal diffusivity alpha via alpha = lambda/(\rho*C_p). In your first equation, there is no \rho to be seen since this is all in lattice units where we choose \rho = 1 for simplicity.Hope this helps.
Kind regards
TimHi Callum,
this outflow boundary condition topic is also a part of my current or future research, so I can’t give a final answer. But I read about a convective type boundary condition which might help in that case, but this is not more than a hint. Glad you managed to find your bug.
Kind regards
TimHi Callum,
are you sure you haven’t changed something about the case setup? When I download, unpack and compile OLB1.7 the waterAirflatInterface2d case works for me. This model uses an empirical Peng-Robinson equation of state with Huron-Vidal mixing rules, which will explain the parameters that you are wondering about.
About adding inlet and outlet boundary conditions: This model is designed to show how well this multi-component pseudo-potential can recreate thermodynamic equilibrium for the multi-component mixture. This model will need more specific implementation at least when it comes to the outflow/pressure boundary condition, as there won’t be a constant pressure at the outflow for such a multi-phase fluid because of surface tension effects.I hope this can clarify your questions.
Tim
Hi Callum,
the microFluidics2d example uses a phase-field model that might be incapable of simulating a high density ratio such as water/air. For this, you can use the model from the example “phaseSeparation3d”. It is a pseudo-potential approach that uses Carnahan-Starling EoS. With the right reduced temperature, the EoS will give you a density ratio of the water/air system. The implementation with this model is easier, too, as you will only need one lattice.
Best regards,
Timtypedef D2Q9<CHEM_POTENTIAL,FORCE,EXTERNAL_FORCE> DESCRIPTOR;
sorry typoHi Callum!
In the line where the Descriptor is set, you have to include the Field “EXTERNAL_FORCE” like this:
typedef D2Q9<CHEM_POTENTIAL,FORCE,EXTERNAL_FORC> DESCRIPTOR;
This field will be added to the multi-phase force in the free energy post-processor, just fyi. In the “prepareLattice” method, you can now initialize the external force field with the desired gravitational acceleration.Hope this helps, kind regards
TimExcuse me, I have to make another comment on your case: There is a reason why the Rayleigh Taylor cases used a Guo forcing which is the existence of another body force as gravity. But as the Shan Chen forcing scheme is also a general forcing scheme, see page 241 in the Krüger book, the gravity will be considered in the velocity shifted equilibrium as the external force is being added to the Shan Chen force as a final step of the post processor.
You are correct, the Guo forcing was chosen which does not work in this case as the coupling strictly requires Shan Chen forcing as there is no force to add after the averaging of velocities.
In the next OpenLB release, there will probably be a Shan-Chen-like model that uses Guo forcing instead of Shan Chen forcing. For now, it would be easiest to change the dynamics from “ForcedBGKdynamics” to “ForcedShanChenBGKdynamics” in the Rayleigh Taylor cases.Tim
Hi Stefano,
regarding the question what multi-component model you should use, I would recommend to look into phase field models with high density ratios. There is a similar model already implemented to OpenLB which is called “Free Energy”, for example used in the Young Laplace or Contact Angle examples, but it does not allow different densities for the two phases which you can find a lot of literature about.
For the loading of the .dat file and the boundary definition you might get another reply by someone else.
Hope this helps.
Best regards,
TimHi Jendrik,
there is an actual issue with the rayleighTaylor cases as they use the Shan-Chen post processor, but they use dynamics with the Guo forcing scheme. The model that is meant to be iplemented is the multi-component Shan-Chen model from the Krüger book, where you have the Shan-Chen forcing scheme, see equations 9.125 and 9.126. So one should change the dynamics from “ForcedBGKdynamics” to “ForcedShanChenBGKdynamics” that use the velocity shifted equilibrium, similar to the phase separation cases. This will match the post processor, as there is no addition of F/2 to the velocity in this model which is what I suppose you tried to do with these IdentityForcedBGKdynamics?!
Hope this helps
Best
Tim