Multiphase VOF not converging

hazari.aaquib · March 18, 2016, 07:58

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.

CeesH · March 18, 2016, 08:05

what Courant criterion do you use?

hazari.aaquib · March 18, 2016, 08:56

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?

CeesH · March 18, 2016, 10:52

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?

hazari.aaquib · March 18, 2016, 11:24

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.

vinerm · March 22, 2016, 02:12

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

hazari.aaquib · March 22, 2016, 04:34

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.

March 18, 2016, 07:58	Multiphase VOF not converging	#1
hazari.aaquib New Member Syed Aaquib Hazari Join Date: Apr 2015 Location: Delft Posts: 16 Rep Power: 11	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.

March 22, 2016, 02:12		#6
vinerm Senior Member Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,946 Blog Entries: 1 Rep Power: 36	Did you try turbulence damping in Fluent? You may not need UDF for that. __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.

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March 18, 2016, 08:05		#2
CeesH Senior Member Cees Haringa Join Date: May 2013 Location: Delft Posts: 607 Rep Power: 0	what Courant criterion do you use?

March 18, 2016, 08:56		#3
hazari.aaquib New Member Syed Aaquib Hazari Join Date: Apr 2015 Location: Delft Posts: 16 Rep Power: 11	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?

March 18, 2016, 10:52		#4
CeesH Senior Member Cees Haringa Join Date: May 2013 Location: Delft Posts: 607 Rep Power: 0	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?

March 18, 2016, 11:24		#5
hazari.aaquib New Member Syed Aaquib Hazari Join Date: Apr 2015 Location: Delft Posts: 16 Rep Power: 11	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.

March 22, 2016, 04:34		#7
hazari.aaquib New Member Syed Aaquib Hazari Join Date: Apr 2015 Location: Delft Posts: 16 Rep Power: 11	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.