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-   -   UDF for writing porosity as a function of distance from the wall (http://www.cfd-online.com/Forums/fluent-udf/105511-udf-writing-porosity-function-distance-wall.html)

rohinibc August 1, 2012 12:10

UDF for writing porosity as a function of distance from the wall
 
Hi,

I am unable to get my UDF for setting porosity variation to work out. Here is what my fn.c code looks like.

/*****************************
UDF to specifiy porosity as a function of distance from the wall of the packed bed reactor
************************************/

#include "udf.h"
#define H 0.1
#define dp 0.008


DEFINE_PROFILE(porosity_function,t,i)
{
cell_t c;
real x[ND_ND]; /*This will hold the position vectors*/
real y;
real a1;
real a2=6; /*to specifiy the porosity variation function*/
real eps_inf=0.37;
real eps;

a1 = (1./eps_inf)-1;

begin_c_loop(c,t)
{
C_CENTROID(x,c,t);
y = (H - x[1])/ dp ;
eps = eps_inf*(1. + a1*exp(-1*a2*y));
F_PROFILE(c,t,i) = eps;
}
end_c_loop(c,t)
}

I tried using both F_PROFILE and C_PROFILE inside the code. The code builds and loads alright during compilation. But when I try to use in in the porous zone as function for porosity, it doesn't work. It gives me an error which is :
Error = invalid argument; not a number[1]

Could anybody help me with this?

gearboy August 6, 2012 02:05

use the following code to specify cell profile. It seems your error is led by other problems.


begin_c_loop(c,t)
{
F_PROFILE(c,t,i)=......;
}
end_c_loop(c,t)


Quote:

Originally Posted by rohinibc (Post 374873)
Hi,

I am unable to get my UDF for setting porosity variation to work out. Here is what my fn.c code looks like.

/*****************************
UDF to specifiy porosity as a function of distance from the wall of the packed bed reactor
************************************/

#include "udf.h"
#define H 0.1
#define dp 0.008


DEFINE_PROFILE(porosity_function,t,i)
{
cell_t c;
real x[ND_ND]; /*This will hold the position vectors*/
real y;
real a1;
real a2=6; /*to specifiy the porosity variation function*/
real eps_inf=0.37;
real eps;

a1 = (1./eps_inf)-1;

begin_c_loop(c,t)
{
C_CENTROID(x,c,t);
y = (H - x[1])/ dp ;
eps = eps_inf*(1. + a1*exp(-1*a2*y));
F_PROFILE(c,t,i) = eps;
}
end_c_loop(c,t)
}

I tried using both F_PROFILE and C_PROFILE inside the code. The code builds and loads alright during compilation. But when I try to use in in the porous zone as function for porosity, it doesn't work. It gives me an error which is :
Error = invalid argument; not a number[1]

Could anybody help me with this?


rohinibc August 7, 2012 17:06

@gearboy
 
hey,

Thanks for responding. But then, I am still unable to figure out what you are actually saying. I did put in F_PROFILE(c,t,i) in my code. and, its still giving me the same error. Am I not catching something you are saying correctly?

thanks,

gearboy August 7, 2012 22:05

Quote:

Originally Posted by rohinibc (Post 375905)
hey,

Thanks for responding. But then, I am still unable to figure out what you are actually saying. I did put in F_PROFILE(c,t,i) in my code. and, its still giving me the same error. Am I not catching something you are saying correctly?

thanks,

I think there might be a math error in your computation. Try printing the "eps" for each cell.

#include "udf.h"
#define H 0.1
#define dp 0.008

DEFINE_PROFILE(porosity_function,t,i)
{
cell_t c;
real x[ND_ND]; /*This will hold the position vectors*/
real y;
real a1;
real a2=6; /*to specifiy the porosity variation function*/
real eps_inf=0.37;
real eps;
a1 = (1./eps_inf)-1;
begin_c_loop(c,t)
{
C_CENTROID(x,c,t);
y = (H - x[1])/ dp ;
eps = eps_inf*(1. + a1*exp(-1*a2*y));
Message("%g\n",eps); // monitor each cell's eps
F_PROFILE(c,t,i) = eps;
}
end_c_loop(c,t)
}

zainab August 8, 2012 05:04

Hi gearboy
i working on modeling 3D gas-liquid slug flow in horizontal pipe with fluent, but I can not get the true phase distribution for the fluids, I think that I need to use a volume fraction UDF can you help me the following UDF for profile of velocity, how can I change it to get one for VF where I wont to set lower half of the cylindrical pipe to 1 means filled with secondary phase while the upper half being take its values from the normal iterations from the mixture velocity and other initial and boundary conditions. I need UDF constant with time not like the below which is time dependent.
Thank you.


DEFINE_PROFILE(VOLUMEFRACTION_profile, t, nv )
{
face_t f;
real x[ND_ND];
real f_time = RP_Get_Real("flow-time")
begin_f_loop (f,t)
{
F_CENTROID(x,f,t);
if (f_time<=10e-1)
{F_PROFILE(f,t,nv) = 1*cos(PI*f_time/3);
}
else
F_PROFILE(f,t,nv) = 0;
}
end_f_loop (f,t)
}

rohinibc August 8, 2012 08:16

checked printing cells
 
Hi gearboy,

I checked printing out my porosity values outside of this udf function as a regular c function and it seems to be fine. There is no case where NaN comes up when I evaluate this function.


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