[Maggakahn]

Hello everyone,

I am currently trying to validate my numerical model by comparing the pressure gradient obtained by the CFD model to semi-empirical equations derived by others. However, I am having trouble establishing the real pressure gradient.

I am investigating a vertical multiphase pipe flow (bubble flow regime), which is open to the atmosphare at the top and have some mass flow inlet for gas and liquid. I am currently using the VOF model to acheive this, but as there is pockets of gas throughout the pipe, the plotted pressure varies quite a lot.

I am also aware that the correct pressure gradient should not include the entrance length.

Any ideas on how to find the proper pressure gradient for this case?

I am greatful for any reply, thank you.
Regards,
Magnus

[vinerm]

One field that is common across all phases, even with the inhomogeneous models, is pressure. Hence, it does not matter whether you use VOF or Eulerian, the pressure and its gradient should be continuous. If that is not the case, then there is some problem with the setup.

[Maggakahn]

Hello Vinerm, thank you very much for your quick response.

That might indeed be the problem in my case. Do you have any experience with this pressure fluctuation, and how it may be resolved? What can cause such an error? I have tried to change the setup to see if it resolved the issue, but nothing i have done seem to help. I have enclosed a screenshot of a graph of the static pressure along a centerline of the pipe.
p_plot.jpg

Thank you again,
Best Regards,
Magnus

[vinerm]

This, most likely, is due to issues with the setup. Either you do not have properly set up the operating conditions, including gravity and operating density or the boundary conditions are wrong. Gravity is must for multiphase flows and the operating density must be equal to density of the lightest fluid. Reference pressure location should also be the highest position of the domain.