VOF Modeling: two-phase flow in microchannel, getting strange flow spots

M_H215 · August 21, 2015, 02:12

Hi
I have modeled a two-phase flow(VOF) in microchannel. Red color is water and Blue color is air. I have done my problem's mesh by ICEM. In your opinion, what is the problem with my modeling?
As you see in the picture I have something strange in my flow(I circled around them).
Inlet BC is mass flow inlet and Outlet BC is pressure outlet.
Thank you

Parisa · August 21, 2015, 05:41

Hi,

Is this a transient simulation or steady state? I have had similar problem in another VOF simulation in steady state. I reinitialized the solution ran transient for some time to start with and then switched to a steady state.
So basically a better starting guess.
So if your model is not too large, start with a transient simulation.
Otherwise try to improve your initial conditions.

//Regards

M_H215 · August 21, 2015, 07:12

Hi parisa
Thank you for your reply
This is a steady state problem.
I will certainly use your advice.
It is highly appreciated if you could reply my another questions.
1-Why my residuals for continuity equation do not decrease more down 10^-2 or 10^-3?
2-How can I choose the best initial guess for my problem?

Parisa · August 24, 2015, 06:04

1- As long as I am concerned residuals are scaled based on your initial conditions (I am not 100% sure how they are scaled), they are not the best measure of convergence. You need to create monitor points that could be indicators of stabilized solution. For exmple you can track the mass balance in your system, velocity and etc..
Sometimes if you for example have back flow in your system it can be hard to converge continuity eq.

2- As I said you can generate good initial conditions with a transient solution. You can use large time steps if your simulation allows for that. Otherwise try starting the steady st. with low values of turbulence energy and high values of dissipation rate. You can maybe assign a small velocity in the direction of the flow. Start the solution with first order. These are some methods that are likely to help but it always depends on the system of course.
Hope it helps.

//Parisa

M_H215 · August 25, 2015, 15:20

Hi parisa,
Thank you very much for your reply again.
I have considered Re=400 in my channel, so I have assumed laminar flow.
As you said, I considered first order upwind for my problem but it didn't help,

also I checked transition flow but it didn't help again

and my problem happened again.
If something else come to your mind, please notify me.
Thanking you in advance

August 21, 2015, 05:41	Initial conditions	#2
Parisa New Member Join Date: Aug 2014 Posts: 13 Rep Power: 12	Hi, Is this a transient simulation or steady state? I have had similar problem in another VOF simulation in steady state. I reinitialized the solution ran transient for some time to start with and then switched to a steady state. So basically a better starting guess. So if your model is not too large, start with a transient simulation. Otherwise try to improve your initial conditions. //Regards

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August 21, 2015, 07:12		#3
M_H215 New Member Mohammad Join Date: Apr 2015 Posts: 17 Rep Power: 11	Hi parisa Thank you for your reply This is a steady state problem. I will certainly use your advice. It is highly appreciated if you could reply my another questions. 1-Why my residuals for continuity equation do not decrease more down 10^-2 or 10^-3? 2-How can I choose the best initial guess for my problem?

August 24, 2015, 06:04		#4
Parisa New Member Join Date: Aug 2014 Posts: 13 Rep Power: 12	1- As long as I am concerned residuals are scaled based on your initial conditions (I am not 100% sure how they are scaled), they are not the best measure of convergence. You need to create monitor points that could be indicators of stabilized solution. For exmple you can track the mass balance in your system, velocity and etc.. Sometimes if you for example have back flow in your system it can be hard to converge continuity eq. 2- As I said you can generate good initial conditions with a transient solution. You can use large time steps if your simulation allows for that. Otherwise try starting the steady st. with low values of turbulence energy and high values of dissipation rate. You can maybe assign a small velocity in the direction of the flow. Start the solution with first order. These are some methods that are likely to help but it always depends on the system of course. Hope it helps. //Parisa

August 25, 2015, 15:20		#5
M_H215 New Member Mohammad Join Date: Apr 2015 Posts: 17 Rep Power: 11	Hi parisa, Thank you very much for your reply again. I have considered Re=400 in my channel, so I have assumed laminar flow. As you said, I considered first order upwind for my problem but it didn't help, also I checked transition flow but it didn't help again and my problem happened again. If something else come to your mind, please notify me. Thanking you in advance