Multiphase VOF not converging
 

I am simulating a steady state stratified two phase 2D channel flow with periodic boundary conditions.

The residuals (all) oscillate for the default values of URFs. However, when I reduce the residuals of k, w & P to 0.1 the oscillations vanish and the solution converges (extremely slowly).

Is it advisable to do so?


P.S: I have changed grid sizes, ran unsteady state simulations. Nothing seems to prevent the oscillations when the URFs are at default values.

what Courant criterion do you use?

Is there a Courant criterion for steady state simulations?

The VOF method meanwhile was implicit and not explicit. So no Courant Number here either.

Am I missing it somewhere?

No, there is no Courant criterion for steady state simulations. Then again, it is questionable how much of VoF remains in a steady state implicit method. I've seen some discussions on this topic on the forum before and don't think they were ever fully conclusive. In the implicit steady state method, the sharp interface tracking (PLIC) method is - as far as I know - not available.

Anyway, what is the goal of your simulation? Because most applications I can imagine you would use VOF for, do not benefit really from a steady solution. What is it you want to study?

The topic is: Modelling of Turbulent Stratified Two Phase Flow in a Channel.

Presently, Fluent fails to accurately predict a simple turbulent two phase channel flow. The interface behaves as a rough wall and the turbulent kinetic energy here is not damped.

An in house MATLAB code successfully predicted the flow with an imposed value of omega at the interface using the Wilcox slightly rough boundary condition (Wilcox, 2006).

I have already used a UDF in fluent to incorporate the same Wilcox condition at the interface, it works and the outputs are in agreement with the MATLAB code. However, to be completely sure of the solution I need to get rid of the oscillations and obtain convergence.

I've read through an ANSYS document and it suggests that the VoF can be used in steady state if the solution is independent of the initial conditions.

Did you try turbulence damping in Fluent? You may not need UDF for that.

Yes, I have tried turbulence damping indeed it does work.
However, the damping coefficient is dependent on the grid size and this value needs to be obtained through trial and error.
The UDF meanwhile is grid independent.