[xiaomimi123]

Hello everyone, I have used an UDF to imply slip boundary in the FLUENT,However, as soon as Fluent is run, it will diverge.This UDF calculates the wall's slip velocity by multiplying the velocity gradient in the normal direction of the wall by the slip length, and assigns the wall's motion velocity. Is there anything wrong with my code here?

Thank you very much for your answers.

/************************************************** **************************
Slip_Thermal_BC.c
UDF – User Defined Function
For Slip velocity(Maxwell) + temperature jump (SmoluchowsKi)
************************************************** ***************************/
#include "udf.h" /*must be at the beginning of every UDF */
#include "sg.h" /*must be at the beginning of this UDF */
#include "math.h" /*must be at the beginning of this UDF */
#define not SCHEMEMFP /* shall MeanFreePath be scheme var.? */
#define SCHEMEUDRLXCOEFF /* shall the under–relaxation Coefficient... */
#define SCHEMESpHR /* shall SpHR ... */
#define SCHEMESIGMASQUARE /* shall sigma-square value... */
#define SCHEMEAMBPRESS /* shall ambient pressure value... */
#define UNDERRLX /* shall slip veloc be under-relaxated in Severe cases? */
#define WALLMOTION /* shall wall-motion be regarded? */
#define sigma_square 1.35305239e-19 /* squared value of Sigma (molecule diameter ) */
#define ambpress 101325 /* ambient pressure */
#define TMAC 1.0 /* tangential momentum accomodation Coefficient */
#define ThAC 1.0 /* thermal accomodation Coefficient */
#define SpHR 1.4 /* specific heat ratio; Air, Oxygen, Nitrogen */
#define UDRLXCOEFF 0.02 /* under-relaxation coefficient */
#define Boltzmann 1.3806505e-23 /* Boltzmann constant */
#define PI 3.14159265358979323846 /* number pi */
#define SQRT_2 1.41421356237309504880 /* sqrt(2) */
#define knodson 0.02
#define hydraulic_diameter 5.0e-5
#define Twall 320
/*
==========================================
Velocity slip at wall boundaries,
a separate routine for every velocity coordinate
this is x-coordinate routine
with thermal creep term
==========================================
*/
DEFINE_PROFILE(maxwell_slip_velocity_x,f_thread,in dex)
{
face_t face;
cell_t cell;
Thread *c_thread;
real slip, thcreep, dveloc;
real normal_slip, tangential_slip, tangential_thcreep;
real Coeff1[ND_ND], Coeff2[ND_ND],a;
real u[ND_ND];
real MeanFreePath; //=6.8e-8;


real y[ND_ND];
real A[ND_ND];
real dr0[ND_ND]; //vector that connects the centroid of cell c0 to the face centroid
real A_by_es;
real ds; //distance between the cell centroid and the face centroid
real es[ND_ND];

begin_f_loop(face,f_thread)
{
F_CENTROID(y,face,f_thread);
cell=F_C0(face,f_thread);
c_thread=THREAD_T0(f_thread);
BOUNDARY_FACE_GEOMETRY( face,f_thread,A,ds,es,A_by_es,dr0) ;
ND_SET(u[0],u[1],u[2], C_U(cell,c_thread), C_V(cell,c_thread), C_W(cell,c_thread));
a=NV_MAG(u);
ND_SET(F_UDMI(face,f_thread,0), F_UDMI(face,f_thread,1), F_UDMI(face,f_thread,2), NVD_DOT(u,1,0,0)/a , NVD_DOT(u,0,1,0)/a, NVD_DOT(u,0,0,1)/a );
ND_SET(F_UDMI(face,f_thread,3), F_UDMI(face,f_thread,4), F_UDMI(face,f_thread,5), NVD_DOT(A,1,0,0)/A_by_es, NVD_DOT(A,0,1,0)/A_by_es, NVD_DOT(A,0,0,1)/A_by_es) ;
}
end_f_loop(face,f_thread)

// if (!Data_Valid_P())
// {
begin_f_loop(face,f_thread)
{

/* get cell and cell thread pointer */
cell=F_C0(face,f_thread);
c_thread=THREAD_T0(f_thread);


/* save the velocity coordinates into u[ND_ND] */
ND_SET(u[0],u[1],u[2], F_U(face,f_thread), F_V(face,f_thread), F_W(face,f_thread)) ;
/* save the transformation coefficients c_Mm: c-11, c_21, c_31 into Coeff1[] */
ND_SET(Coeff1[0],Coeff1[1],Coeff1[2], F_UDMI(face,f_thread,0), F_UDMI(face,f_thread,1), F_UDMI(face,f_thread,2)) ;
/* save the transformation coefficients c_Mm: c_12, c_22, c_32 into Coeff2[] */
ND_SET(Coeff2[0],Coeff2[1],Coeff2[2], F_UDMI(face,f_thread,3), F_UDMI(face,f_thread,4), F_UDMI(face,f_thread,5)) ;

/* evaluate the du/dl (tangential to surface) term in the local coord. System */
tangential_slip= NVD_DOT(Coeff1, NV_DOT(Coeff1,C_U_G(cell,c_thread)), NV_DOT(Coeff1,C_V_G(cell,c_thread)), NV_DOT(Coeff1,C_W_G(cell,c_thread))) ;

/* evaluate the du/dy (normal to surface ) term in the local coord. System */
normal_slip=-1 * NVD_DOT(Coeff1, NV_DOT(Coeff2,C_U_G(cell,c_thread)), NV_DOT(Coeff2,C_V_G(cell,c_thread)), NV_DOT(Coeff2,C_W_G(cell,c_thread))) ;

/* add theses values and multiply with MFP and TMAC */
slip = ((2-TMAC)/TMAC) * 5e-4 * (tangential_slip+normal_slip) ;





dveloc = Coeff1[0]* (slip) ;

dveloc = (1-UDRLXCOEFF) *u[0] + UDRLXCOEFF * dveloc;

/* boundary condition value is returned */
F_PROFILE(face,f_thread,index) = dveloc;
}
end_f_loop(face,f_thread)

}