2 Phase flow simulation in a T-junction microchannel

prohackrav · November 13, 2013, 04:11

Hi. I am doing a transient laminar flow simulation to get the slug flow of 2 phase immiscible liquids in a t junction micro channel using VOF model. Its a simple 2d geometry and surface tension modelling is enabled. Whenever my timestep advances I get an error stating that the global COurant number is greater than 250. I reduced the time step to values as low as 1e-5. Still the problem persists. I also reduced the UR factors and increased the iterations per time step. (Pressure-0.2, Density-0.5, Body forces-0.5 Momentum- 0.3 As seen in some forum). The velocity seems to be fine at time 0 but drops to zero as soon as the timestep advances. I have included the snapshot. Can somebody help me?

flotus1 · November 13, 2013, 04:36

Did you estimate the time step size necessary to achieve a Courant number below 250?
When you say "micro channel" and the velocity magnitude is around 2.5 m/s, I estimate a time step size in the order of $10^{-6} - 10^{-7}$ to meet this requirement.

prohackrav · November 14, 2013, 01:01

Quote:

Originally Posted by flotus1

Did you estimate the time step size necessary to achieve a Courant number below 250?
When you say "micro channel" and the velocity magnitude is around 2.5 m/s, I estimate a time step size in the order of $10^{-6} - 10^{-7}$ to meet this requirement.

Thanks for the reply. Its 0.8mm microchannel t-junction. I have two opposite inlets one of kerosene and other water. Both with velocity ranging from 0.02-0.1 m/sec. So what would be the estimated time step size?

flotus1 · November 14, 2013, 04:20

The Courant number is defined as

$C = \frac{u \, \Delta t}{\Delta x}$ thus $\Delta t = \frac{C \, \Delta x}{u}$

where $\Delta x$ is the size of a cell. Just to make sure, decrease the time step size further and see if the error persists.
Maybe the problem arises from the initial conditions. Did you try to initialize each half of the domain with the substance at the inlet?

prohackrav · November 21, 2013, 01:55

Quote:

Originally Posted by flotus1

The Courant number is defined as

$C = \frac{u \, \Delta t}{\Delta x}$ thus $\Delta t = \frac{C \, \Delta x}{u}$

where $\Delta x$ is the size of a cell. Just to make sure, decrease the time step size further and see if the error persists.
Maybe the problem arises from the initial conditions. Did you try to initialize each half of the domain with the substance at the inlet?

Do you mean to patch both the inlets till the junction with their corresponding inlet fluids? No. I havent tried that. Will coarsening the mesh help for increasing the time step? I did that instead of reducing the time step further from 1e-5.

flotus1 · November 21, 2013, 09:47

Would it be such a mayor inconvenience to reduce the time step size to lets say 10e-8s just to see if the error is really related to the time step size?

Zaktatir · November 21, 2013, 17:00

Could you please give more input about your Simulation?

-Reference density
-I think you are using a tracking method (explicit that's why CFL-dependency) Geo-REconstruct
-Are both phases well separated like a normal slug flow simulation or do they interpenetrate each other? Are small bubbles and liquid entrainment expected to occur or not?
-Which interpolation of Pressure are you using?
-Reference Location of the operating Pressure
-Operating Pressure..

amasoudi · November 21, 2013, 17:53

Hi,
Does anybody know how can I monitor the rising velocity of single bubble in fluent?

maryam74121 · August 27, 2019, 13:22

hi
i am simulating a model with two vents and one outlet.the water patched into the main chamber and air can goes in and out from two vents. i want to simulating water flow in channel.
i dont have any parameters in vents what is the best boundary condition for them? the vents are in the atmosphere pressure but the whole model is rotating so i dont know the gauge pressure=0 in correct for these vents or not?
i use the out flow for outlet which it is incompatible with pressure boundaries.
i cant see the right flow in channel is there any one could help me?
the residuals cant decrease enough and continuity residuals are in order of 10
i appreciate your recommendations

November 21, 2013, 17:53	"Monitoring bubble rising velocity in Fluent"	#8
amasoudi New Member Ali Masoudi Join Date: Nov 2013 Posts: 4 Rep Power: 13	Hi, Does anybody know how can I monitor the rising velocity of single bubble in fluent?

August 27, 2019, 13:22	vof modeling in micro channel in fluent	#9
maryam74121 New Member maryam Join Date: Jul 2017 Posts: 10 Rep Power: 9	hi i am simulating a model with two vents and one outlet.the water patched into the main chamber and air can goes in and out from two vents. i want to simulating water flow in channel. i dont have any parameters in vents what is the best boundary condition for them? the vents are in the atmosphere pressure but the whole model is rotating so i dont know the gauge pressure=0 in correct for these vents or not? i use the out flow for outlet which it is incompatible with pressure boundaries. i cant see the right flow in channel is there any one could help me? the residuals cant decrease enough and continuity residuals are in order of 10 i appreciate your recommendations

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November 13, 2013, 04:36		#2
flotus1 Super Moderator Alex Join Date: Jun 2012 Location: Germany Posts: 3,427 Rep Power: 49	Did you estimate the time step size necessary to achieve a Courant number below 250? When you say "micro channel" and the velocity magnitude is around 2.5 m/s, I estimate a time step size in the order of $10^{-6} - 10^{-7}$ to meet this requirement.

November 14, 2013, 04:20		#4
flotus1 Super Moderator Alex Join Date: Jun 2012 Location: Germany Posts: 3,427 Rep Power: 49	The Courant number is defined as $C = \frac{u \, \Delta t}{\Delta x}$ thus $\Delta t = \frac{C \, \Delta x}{u}$ where $\Delta x$ is the size of a cell. Just to make sure, decrease the time step size further and see if the error persists. Maybe the problem arises from the initial conditions. Did you try to initialize each half of the domain with the substance at the inlet?

November 21, 2013, 09:47		#6
flotus1 Super Moderator Alex Join Date: Jun 2012 Location: Germany Posts: 3,427 Rep Power: 49	Would it be such a mayor inconvenience to reduce the time step size to lets say 10e-8s just to see if the error is really related to the time step size?

November 21, 2013, 17:00		#7
Zaktatir Senior Member Join Date: Jan 2010 Location: Germany Posts: 268 Rep Power: 17	Could you please give more input about your Simulation? -Reference density -I think you are using a tracking method (explicit that's why CFL-dependency) Geo-REconstruct -Are both phases well separated like a normal slug flow simulation or do they interpenetrate each other? Are small bubbles and liquid entrainment expected to occur or not? -Which interpolation of Pressure are you using? -Reference Location of the operating Pressure -Operating Pressure..