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UDF for capillary driven flow in porous media

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Old   June 4, 2020, 13:38
Exclamation UDF for capillary driven flow in porous media
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Hello Everyone,

I am trying to simulate the capillary driven flow in porous media using VOF model. I have considered an upward wicking case where the capillary force acts in the opposite direction to the gravitational force. This capillary force is the only force that pulls liquid up. The inlet and outlet boundary conditions are atmospheric pressure conditions. I have developed the following code to add capillary force to the secondary phase-only but I not getting satisfactory results. any suggestions will be very helpful.
The code interprets with no errors. I don't see any changes in liquid volume fractions



#include "udf.h"

#define AIR_PHASE_ID 2 /*primary phase*/
#define LIQUID_PHASE_ID 3 /*secondary phase*/

#define POROUS_CELL_ZONE_ID 2 /*cell zone ID*/

#define C_CAPILLARY_MOM_SOURCE_X(C,T) C_UDMI(C,T,0)
#define C_CAPILLARY_MOM_SOURCE_Y(C,T) C_UDMI(C,T,1)
#define C_CAPILLARY_MOM_SOURCE_Z(C,T) C_UDMI(C,T,2)
#define CAPILLARY_PRESSURE 12.35 /* 2.gamma.cos(theta)/R */

real area_factor(real volume, real vof)
{
real factor;

factor = pow(volume, 2.0 / 3.0);
factor *= 1.0 - 4.0*(vof - 0.5)*(vof - 0.5); /* = 1.0 at VOF=0.5, 0.0 at VOF = 0.0 or 1.0 */

return factor;
}

void set_capillary_pressure_sources(Thread *lct, real *direction)
{
cell_t c;
real vol;
real capillary_force;

begin_c_loop(c, lct)
{
vol = C_VOLUME(c, lct);

capillary_force = CAPILLARY_PRESSURE * area_factor(vol, C_VOF(c, lct)) / vol; /*source term with unit N/m3*/

ND_VS(C_CAPILLARY_MOM_SOURCE_X(c, lct), C_CAPILLARY_MOM_SOURCE_Y(c, lct), C_CAPILLARY_MOM_SOURCE_Z(c, lct), =, direction, *, capillary_force);
}
end_c_loop(c, lct)
}
DEFINE_ADJUST(capillary_sources, mixture_domain)
{
Domain *liquid_domain;
Domain *air_domain;
Thread *lct;
Thread *act;
real capillary_direction[ND_ND];

liquid_domain = Get_Domain(LIQUID_PHASE_ID);
air_domain = Get_Domain(AIR_PHASE_ID);


printf("Setting sources in the domain");

lct = Lookup_Thread(liquid_domain, POROUS_CELL_ZONE_ID);
act = Lookup_Thread(air_domain, POROUS_CELL_ZONE_ID);

NV_D(capillary_direction, =, 1.0, 1.0, 0.0);
set_capillary_pressure_sources(lct, capillary_direction);

printf("Done.\n");
}

DEFINE_SOURCE(caplillary_mom_x_source, c, ct, ds, eqn)
{
return C_CAPILLARY_MOM_SOURCE_X(c, ct);
}

DEFINE_SOURCE(caplillary_mom_y_source, c, ct, ds, eqn)
{
return C_CAPILLARY_MOM_SOURCE_Y(c, ct);
}

DEFINE_SOURCE(caplillary_mom_z_source, c, ct, ds, eqn)
{
return C_CAPILLARY_MOM_SOURCE_Z(c, ct);
}
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Old   June 4, 2020, 16:06
Default Udm
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You are defining sources using UDMs but what is contained in those memories? No where in the code are any values being assigned to UDMs. So, the sources are 0.
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Old   June 5, 2020, 07:19
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Yes, I did not realize this before. Could you suggest some corrections in the code? I created this code by modifying the previous codes available.

Thanks
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Old   June 5, 2020, 08:38
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To help you, I need to know what is it that you want to do, what is the objective of the code. As of now, there are redundancies in the code. UDMs may not even be required. Two functions have been written outside DEFINE_ macros, however, only called once. That makes it difficult instead of being easier to read.
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Old   June 5, 2020, 09:09
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I am trying to simulate the upward wicking flow in porous media. To do so, I am using the VOF method. The inlet and outlet both have atmospheric pressure conditions. The action of gravity is in negative y-direction. So the main mechanism for flow is the capillary action. The main objective of this code is to add a positive momentum source to the secondary phase which is based on the capillary force.

Hope this helps
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Old   June 5, 2020, 09:19
Default UDMs
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Replace all components of C_CAPILLARY_MOM_SOURCE_X(c, lct) in ND_VS function with three C_UDMIs.
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Old   June 5, 2020, 09:29
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line 35 - ND_VS(C_UDMI(C,T,0), C_UDMI(C,T,1), C_UDMI(C,T,2), =, direction, *, capillary_force);

I replaced it but fluent shows following error,

line 35: T: undeclared variable.

So I changed, t with lct everywhere. The code does not show any errors related to syntax.
When I hook UDF as follows -
Define adjust- capillary_sources
user defined memory locations - 5
y momentum source in cell zone conditions - caplillary_mom_y_source.

I dont see any variation in liquid volume fraction.
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Old   June 5, 2020, 10:54
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If you are hooking these as momentum sources, then there won't be any effect on the volume fraction. The sources will only affect the flow.
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Old   June 5, 2020, 11:05
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Okay. Can you suggest any method so that I get it working?
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Old   June 5, 2020, 11:07
Default Flow and Volume Fraction
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As per your objective, you need to add momentum so that the liquid moves. That part is being done by your code. However, the issue you are reporting is related to volume fraction. The UDF you have will not affect the volume fraction.
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Old   June 5, 2020, 11:16
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Okay. Yeah, that right. Do you have any suggestions on which logic should I be using that causes the secondary liquid to pump up according to capillary pressure?

It will be helpful to have your comments on this?

Thanks
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Old   June 5, 2020, 15:56
Default Capillary Force
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The code that you have should work after the modifications suggested, provided you hook it at right place. All the source UDFs must be hooked in the Cell Zones to the corresponding fields, i.e., momentum fields. If the source values are non-zero, then you should see its effects. To check, you can plot UDMs and see if the values are non-zero. For z-momentum, the values would be 0.
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Old   June 8, 2020, 10:12
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Ok thanks, I will try to run simulation to see why it is happening.
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Old   June 8, 2020, 13:06
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Hello, I did a simulation by specifying the smaller amount for the initial liquid volume fraction with the modified UDF. Now I can see the change in liquid volume fraction. Thanks for the suggestion that you provided.

I even tried simulating the case using original UDF that also worked fine but the variation in liquid volume fraction w.r.t time is very fast than modified UDF (modified UDF results are logically correct). I am curious about, how should I make sure that the momentum source is being attached to the secondary phase (liquid) only.? When I plot UDM in XY plot, I only see 0 value.

Thanks.
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Old   June 15, 2020, 04:26
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Since you are using VOF, there is only one momentum equation; not two separate equations for two phases. To ensure that the momentum is added only within the liquid phase, just multiply the source with the volume fraction of liquid phase.
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Old   June 16, 2020, 12:48
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Thanks for the reply. Yes, only a single set of momentum equations is solved for the VOF model. However, I did not get why I need to multiply source term with volume fraction. could explain it in few words??

Best,
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Old   June 16, 2020, 13:07
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Hi
This so good and perfect, I used it.
Thank you very much for your helps and guides.

chiropractic
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Old   June 16, 2020, 13:51
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can you explain the problem that you are trying to simulate?
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Old   June 16, 2020, 14:32
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Volume fraction is being used as weight. Source will have a non-zero value only if volume fraction is greater than 0, otherwise, it will be 0.
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Old   October 4, 2020, 03:27
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I like this post very much, I hope the host can share more, thank you

Last edited by Wyj; October 5, 2020 at 21:57.
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