[big worm]

Hello guys, I am attempting to do a simulation about gas diffusion using UDS and have a problem with the boundary conditions.
The simulation is simple—CH4 flows through a pipe. The problem is that CH4 can penetrate the wall, and its concentration on the wall is Linear with the concentration gradient normal to the wall.
Could someone help me. Any help is much appreciated!

#include "udf.h"

DEFINE_PROFILE(myprofile,thread,i)
{
real factor;
real NV_VEC(A);
cell_t c0;
Thread *t0;
face_t f;
factor=1.26e-4;
begin_f_loop(f,thread)
{
c0=F_C0(f,thread);
t0=THREAD_T0(thread);
F_AREA(A,f,thread);
F_PROFILE(f,thread,i)=factor*NV_DOT(C_UDSI_G(c0,t0 ,0),A)/NV_MAG(A);
}
end_f_loop(f,thread)
}