What are the two phases, schiller naumann model is for drag around particles, so i assume you have a fluid and then disperse particles. Is that the case? If so you need to consider the diameter of the particles.

For the porous media you need to specify the resistance coefficients of the porous media which will depend on the pore size or diameter of the holes in the porous media. A good way to do this is to use the Ergun equation to determine linear and quadratic resistance coefficients for the porous media

Tristan Davenne

Thanks for your answer.

The two phases are oil and gas (air). I explore the process of filtration of aerosols. I solved my problem with the diameter by setting a source term of oil for the Cell Zone (Porous Zone) and the Inlet now is exclusively gas. With these settings, the diameter has no more an effect by disabling the drag force and all other phase interactions.

The resistance, pore size and other filter parameters I get from other simulations and experiments. Here the diameter should not matter. This point of view also has the support.

It is quite simple, if you disable the interphase drag between phases, than in the Eulerian multiphase flow equations there is no more mechanism present, which is binding the velocity fields of the two phases to each other. So the velocity field of a buoyant phase e.g. could be infinitely get accelerated without violating the conditions which you have defined. With existing drag force, the drag force would increase the larger the slip velocity between phases is developing, and this leads in the consequence to what is called "terminal rise velocity" of e.g. buoyant bubbles in a stagnant liquid. So you see, that drag force is essential and cannot be just switched off artificially.

Schiller-Naumann drag is only for submerged solid particles in a fluid. It is using a correlation for drag force coefficient, which is only valid for rigid particles (i.e. not for flow morphologies with bubbles or droplets; for such multiphase flows other drag force coefficient correlations apply).

Furthermore the Eulerian MPF equations heavily depend on flow morphology. Formulations can be found for other flow regimes than just dispersed particulate MPF mixtures. But without having more details about the morphology of your multiphase flow (with the addition that you like to study flow behavior of a multiphas flow through a porous media) it is nearly impossible to give more detailed advices.

Regards, Thomas Frank.

Thank you very much for your answer and the explanation, Thomas Frank

Your answer fits totally to my results. The velocity fields are extremely high when the residuals gets instable.

Is there an alternative way to disable the influence of diameter, as to disable all interphase reactions?

If you select homogeneous flow, then the dispered and continuous phases will be combined together to make one new phase modelled with a single continuity and momentum equation. In this case the velocity of the two phases are the same at any cell in your model. This approach is only reasonable if you think you have low slip velocity between the two phases.

Tristan Davenne is entirely right. Essentially what you mentioned about your current solution points in the same direction. It seems to me, like ANSYS support has recommended to you to switch from a disperse multiphase flow formuölation to a VOF-like (VOF = volume-of-fluid) formulation. VOF exists in the ANSYS software in two flavours - homogeneous and inhomogeneous. The homogeneous model with just 1 set of momentum equations follows the assumption, that both phases move with the same velocity field. In that case no assumptions or knowledge about interfacial momentum transfer is needed. ANSYS has implemented an inhomogeneous VOF model as well. In that case from the local ratio in volume fractions of both phases an inherent drag term is deduced, but in that case you don not have to specify it yourself. It assumes, that separatign interfaces between the two phases are rather macroscopic and resolved by the simulation like typically done in free surface flows.

Regards,

Thomas Frank.