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UDF: how to identify interface in VOF model

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Old   February 4, 2012, 06:52
Default UDF: how to identify interface in VOF model
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Chemflame
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Hello,

I am doing unsteady state analysis of a 2-D droplet evaporation in an square box. I have considered 1 mm radius semi-spherical water droplet placed in a square box of 5x5 mm. The surrounding of the droplet is air. I am trying to give a constant evaporation rate as my source term at the interface of the water droplet and air (i.e. only the water on the interface will evaporate into the surrounding). I have tried to write a UDF but I am unable to understand how to find the interface at which I should apply Source term .


Since this is VOF model so to find the interface I can find VOF in each cell and if it is 1, then I can say that it is water and if it comes less than 1 than I can say that it is mixture of water and air. As:

#include "udf.h"
DEFINE_SOURCE(mass_rate_source,c,t,dS,eqn)
{
Thread *t;
cell_t c;
real source;

C_CENTROID (coord,c,t);
if (C_VOF(c,c_t) ==1)&&(MISSING CONDITION)
dS[eqn]=0.0;

if (IT IS INTERFACE)
source = 0.0001;
else
source=0;

return source;
}

In the missing condiiton i need to find the VOF value of a vertically above cell and if it is less then 1 then it can be said that this is the interface.
I am unable to understand how to write this portion .
Any help would be much appreciated,

Thanks!!

Abhishek
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Old   February 11, 2012, 09:59
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Here is how you can identify the cells at the interface:

http://cape-forum.com/index.php/topic,529.0.html
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Old   April 19, 2012, 02:11
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Jacky
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Quote:
Originally Posted by process.abhi View Post
Hello,

I am doing unsteady state analysis of a 2-D droplet evaporation in an square box. I have considered 1 mm radius semi-spherical water droplet placed in a square box of 5x5 mm. The surrounding of the droplet is air. I am trying to give a constant evaporation rate as my source term at the interface of the water droplet and air (i.e. only the water on the interface will evaporate into the surrounding). I have tried to write a UDF but I am unable to understand how to find the interface at which I should apply Source term .


Since this is VOF model so to find the interface I can find VOF in each cell and if it is 1, then I can say that it is water and if it comes less than 1 than I can say that it is mixture of water and air. As:

#include "udf.h"
DEFINE_SOURCE(mass_rate_source,c,t,dS,eqn)
{
Thread *t;
cell_t c;
real source;

C_CENTROID (coord,c,t);
if (C_VOF(c,c_t) ==1)&&(MISSING CONDITION)
dS[eqn]=0.0;

if (IT IS INTERFACE)
source = 0.0001;
else
source=0;

return source;
}

In the missing condiiton i need to find the VOF value of a vertically above cell and if it is less then 1 then it can be said that this is the interface.
I am unable to understand how to write this portion .
Any help would be much appreciated,

Thanks!!

Abhishek
Well as u need to find cell top of ur current cell u can do following :

begin_c_loop(c,t)
{
V[0] = C_U(c,t);
V[1] = C_V(c,t);
mag = NV_MAG(V);
NV_D(xunit, = ,1,0,0);
NV_D(yunit, = ,0,1,0);

c_face_loop(c, t, n)
{
F_AREA(A, f,t);
C_CENTROID(dr0, F_C0(f,t),t->t0);
F_CENTROID(xf, f,t);
NV_VV(dr0, =, xf,-,dr0);
ds = NV_MAG(dr0);
NV_VS(es,=,dr0,/,ds);

XDOT = NV_DOT(es,xunit);
YDOT = NV_DOT(es,yunit);

if( YDOT <= 1.0 && YDOT > cos(15.0))
{
top = n;
}
} /* end of c_face_loop */

ftop = C_FACE(c0,t,top);
c1 = F_C1(ftop,t);

/* This is ur condition */
if (C_VOF(c1,t_liquid) < 1.0)
{
message(" IT IS INTERFACE \n");
}

}
end_c_loop(c,t)




HOPE this helps and is self-explanatory.....
Regards,
Bharat
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