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VOF Model/Pipe Flow/Capillary Flow

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Old   August 26, 2015, 15:36
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Ashley Byers
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Hey there

I am doing a 2-d axi-symmetric case with water in a capillary. Water is going much, much farther in the capillary than the analytical solution. It is not even close.

-My mesh is on the +x-axis, gravity in the -x-axis.
-Using VOF model, explicit scheme
-Piso coupling
-Surface Tension 0.07 N/m
-Contact angle 70 degrees
-Pipe radius of 0.3 mm
-Pressure Inlet and pressure outlet are gauge 0

This should go about 16 mm, but it does not. Does anyone have any idea why?

Also, what is an acceptable residual for this problem?

Thanks in advance for any help.

Last edited by AshleyB; August 27, 2015 at 12:29.
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Old   January 15, 2017, 22:47
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Peter Parker
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I simulate two immiscible fluid, a channel fow with primary phase is water, there is air cavity with air bubble as secondary phase
VOF model, CSF, Geo-reconstruct solver; velocity inlet, pressure outlet BCs; surface tension force 0.072 N/m
But I get weird velocity vector at the interface (see attached picture).
Is there anyone know how to fix this?

Thanks in advanced

Attachment 53209

Attachment 53210

Last edited by letsroll; February 6, 2017 at 20:43.
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Old   February 2, 2018, 13:53
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Hardy
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Hello
I am modeling horizontal placed rectangular open microchannel with height, width, and length of 2 , 15, and 100 micrometers. The mesh is hexahedral mesh with all quadrilateral. In General section choose pressure-based and transient time. By marking absolute velocity. In setup use VOF model, which includes open channel flow, coupled level set method and Implicit body forces. Taking air as a primary phase and water as secondary phase. In which interaction include surface tension between water and air of 0.0735 mN/mM.

I am putting drop of water in contact with open microchannel.
In boundary condition, inlet(water) is pressure inlet with volume fraction 1. outlet (mixture of water and air) is pressure outlet with 0 volume fraction. Atmospheric(air) as pressure outlet. In all this boundary condition I select open channel flow in the multi-phase option with free surface level 2 micrometer. Also, choosing operating density by default. By considering mixture for Wall Zone with stationary wall, no slip condition with wall adhesion between water and air of 90 degree.

Solution method, scheme - PISO, gradient - least square cell based, pressure - presto, Momentum - quick, Volume frcation - Implicit scheme.

Solution Initialization, Compute from - Inlet and initialize it. In patch select water and variable volume fraction with value 1 and zone to patch is liquid.

Run calculation with time step 1.0e+6 microsecond with 1000 iteration.

Please, help me if anything wrong in the above explanation of model.
In this, I want to find out how much time fluid takes to travel through the end of channel with the effect of capillary force. But, I didn't got the correct results.

So. I hope somebody help me on this. Thank you in advance.
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Old   February 2, 2018, 13:54
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Hello
I am modeling horizontal placed rectangular open microchannel with height, width, and length of 2 , 15, and 100 micrometers. The mesh is hexahedral mesh with all quadrilateral. In General section choose pressure-based and transient time. By marking absolute velocity. In setup use VOF model, which includes open channel flow, coupled level set method and Implicit body forces. Taking air as a primary phase and water as secondary phase. In which interaction include surface tension between water and air of 0.0735 mN/mM.

I am putting drop of water in contact with open microchannel.
In boundary condition, inlet(water) is pressure inlet with volume fraction 1. outlet (mixture of water and air) is pressure outlet with 0 volume fraction. Atmospheric(air) as pressure outlet. In all this boundary condition I select open channel flow in the multi-phase option with free surface level 2 micrometer. Also, choosing operating density by default. By considering mixture for Wall Zone with stationary wall, no slip condition with wall adhesion between water and air of 90 degree.

Solution method, scheme - PISO, gradient - least square cell based, pressure - presto, Momentum - quick, Volume frcation - Implicit scheme.

Solution Initialization, Compute from - Inlet and initialize it. In patch select water and variable volume fraction with value 1 and zone to patch is liquid.

Run calculation with time step 1.0e+6 microsecond with 1000 iteration.

Please, help me if anything wrong in the above explanation of model.
In this, I want to find out how much time fluid takes to travel through the end of channel with the effect of capillary force. But, I didn't got the correct results.

So. I hope somebody help me on this. Thank you in advance.
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Old   March 12, 2018, 06:55
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Ani
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Sorry to revive an old thread and thanks @Stiboo for the explanation.

A general question, after the VOF run, the velocity that is obtained, is that a velocity of the mixture of phase-1 (air) and phase-2 (water)?

I want to compare the secondary phase (in my case, water) velocity with the results from experimental measurements, but all I get is the "mixture" velocity. Is there a way to retrieve a "phase velocity"?
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Old   June 19, 2018, 05:33
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Akashdeep Gangrade
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Quote:
Originally Posted by stiboo View Post
hi, i also simulate such problem in a vertical flow, so i try to give you some hints.

You have to define the contact angle between the water and the wall, typically less than 90 degree.

Absolute pressure = operating pressure + gauge pressure
you should define atmospheric pressure in the operating conditions, and 0 for gauge pressure at the boundaries.

Why do you define an axial velocity? I think you should not define an initial velocity, because the surface tension will trigger the flow to move, and 0.001 m/s is very high anyway, since you are in microscale.

Turn on double precision when you start fluent.

Instead of the simple, you should use the piso pressure-velocity coupling.

Instead of the velocity-inlet, you should have a pressure-inlet with volume fraction of 1 for water, and a pressure-outlet with 0 backflow volume fraction for water. Set the gauge pressure to 0 for both.
Define a domain initially filled with water (patch), and let the transient simulation begin.

Good luck!
i have done the same thing mine problem is that the water continue to rise
first i performed 3000 iteration than 3000 more two times during second time it rises more than previous one and at last it rises upto the top level
any help is deeply required if someone doing same i can share my wbj file
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Old   June 19, 2018, 07:46
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Akashdeep Gangrade
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Quote:
Originally Posted by stiboo View Post
Hi, I also simulate such problem in a vertical flow, so I try to give you some hints.

You have to define the contact angle between the water and the wall, typically less than 90 degree.

Absolute Pressure = Operating pressure + Gauge pressure
You should define atmospheric pressure in the operating conditions, and 0 for gauge pressure at the boundaries.

Why do you define an axial velocity? I think you should not define an initial velocity, because the surface tension will trigger the flow to move, and 0.001 m/s is very high anyway, since you are in microscale.

Turn on double precision when you start fluent.

Instead of the SIMPLE, you should use the PISO pressure-velocity coupling.

Instead of the velocity-inlet, you should have a pressure-inlet with volume fraction of 1 for water, and a pressure-outlet with 0 backflow volume fraction for water. Set the gauge pressure to 0 for both.
Define a domain initially filled with water (patch), and let the transient simulation begin.

Good luck!
i am doing the same but the capillary rise is continuous it is not stopping after 3000 iterations it filled half after another 3000 iterations it filled more and continue to do so it is more than what is predicted by Lucas Washburn equation
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Old   June 23, 2020, 13:47
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sorry to revive this old thread, but has anyone figured out how to fix this? The capillary height in the simulation has far passed the analytical solution and is still rising.
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