[Kaazem_RA]

Dear all
I am trying to simulate natural convection in a square cavity filled with nanofluid using ansys fluent 16. I use mixture model and have written a udf for slip velocity which is defined by a very simple equation. Also i should say that when using mixture model in fluent viscosity and thermal conductivity of mixture (nanofluid in my case) are given by a linear function of volume fraction which is not the case in this simulation. So i wrote two other udfs for theraml conductivity and viscosity of nano particles. my udfs are as follows.

#include "udf.h"

#define pi 4.*atan(1.)

#define kB 1.38066e-23


DEFINE_VECTOR_EXCHANGE_PROPERTY(Buongiorno_slip,c, mixture_thread,second_column_phase_index,first_col umn_phase_index,vector_result)

{

real dp = 150.e-09 ;

real ST = 0.006 ;

real DB, phi, rho_p, rho_f, rho_nf, mu_f, T_nf ;

real grad_T[2], grad_phi[2] ;

Thread *thread_p, *thread_f ;

thread_f = THREAD_SUB_THREAD(mixture_thread, first_column_phase_index) ;

thread_p = THREAD_SUB_THREAD(mixture_thread, second_column_phase_index) ;

T_nf = C_T(c,mixture_thread) ;

mu_f = C_MU_L(c,thread_f) ;

DB = (kB*T_nf)/(3.*pi*mu_f*dp) ;

phi = C_VOF(c,thread_p) ;

if ( NULL != THREAD_STORAGE(mixture_thread,SV_T_G) )

{

grad_T[0] = C_T_G(c,mixture_thread)[0] ;

grad_T[1] = C_T_G(c,mixture_thread)[1] ;

}

else

{

grad_T[0] = 0. ;

grad_T[1] = 0. ;

}

if ( NULL != THREAD_STORAGE(mixture_thread,SV_VOF_G) )

{

grad_phi[0] = C_VOF_G(c,thread_p)[0] ;

grad_phi[1] = C_VOF_G(c,thread_p)[1] ;

}

else

{

grad_phi[0] = 0. ;

grad_phi[1] = 0. ;

}


rho_f = C_R(c,thread_f) ;

rho_p = C_R(c,thread_p) ;

rho_nf = C_R(c,mixture_thread) ;

vector_result[0] = (-DB*grad_phi[0]/phi - ST*mu_f*grad_T[0]/(rho_f*T_nf))/(1. - phi*rho_p/rho_nf) ;

vector_result[1] = (-DB*grad_phi[1]/phi - ST*mu_f*grad_T[1]/(rho_f*T_nf))/(1. - phi*rho_p/rho_nf) ;

}



DEFINE_PROPERTY(mu_particle,c,t)

{

real mu_p, phi ;

real mu_f = 855.e-06 ;

phi = C_VOF(c,t) ;

mu_p = (1./(1. - pow(phi,2.5)) - (1. - phi))*mu_f/phi ;

return mu_p ;

}



DEFINE_PROPERTY(k_particle,c,t)

{

real kf = 0.6 ;

real kp = 40. ;

real k_p, phi ;

phi = C_VOF(c,t) ;

k_p = ((kp + 2.*(kf + phi*(kp - kf)))/(kp + 2.*(kf - phi*(kp - kf))) - (1. - phi)*kf)/phi ;

return k_p ;

}

I compile my udf successfully but the results are not physical. There is no volume fraction gradient and slip velocity. They are equal to zero. please help me fix this problem
Thank you

[abhilasht]

Originally Posted by Kaazem_RA

Dear all
I am trying to simulate natural convection in a square cavity filled with nanofluid using ansys fluent 16. I use mixture model and have written a udf for slip velocity which is defined by a very simple equation. Also i should say that when using mixture model in fluent viscosity and thermal conductivity of mixture (nanofluid in my case) are given by a linear function of volume fraction which is not the case in this simulation. So i wrote two other udfs for theraml conductivity and viscosity of nano particles. my udfs are as follows.

#include "udf.h"

#define pi 4.*atan(1.)

#define kB 1.38066e-23


DEFINE_VECTOR_EXCHANGE_PROPERTY(Buongiorno_slip,c, mixture_thread,second_column_phase_index,first_col umn_phase_index,vector_result)

{

real dp = 150.e-09 ;

real ST = 0.006 ;

real DB, phi, rho_p, rho_f, rho_nf, mu_f, T_nf ;

real grad_T[2], grad_phi[2] ;

Thread *thread_p, *thread_f ;

thread_f = THREAD_SUB_THREAD(mixture_thread, first_column_phase_index) ;

thread_p = THREAD_SUB_THREAD(mixture_thread, second_column_phase_index) ;

T_nf = C_T(c,mixture_thread) ;

mu_f = C_MU_L(c,thread_f) ;

DB = (kB*T_nf)/(3.*pi*mu_f*dp) ;

phi = C_VOF(c,thread_p) ;

if ( NULL != THREAD_STORAGE(mixture_thread,SV_T_G) )

{

grad_T[0] = C_T_G(c,mixture_thread)[0] ;

grad_T[1] = C_T_G(c,mixture_thread)[1] ;

}

else

{

grad_T[0] = 0. ;

grad_T[1] = 0. ;

}

if ( NULL != THREAD_STORAGE(mixture_thread,SV_VOF_G) )

{

grad_phi[0] = C_VOF_G(c,thread_p)[0] ;

grad_phi[1] = C_VOF_G(c,thread_p)[1] ;

}

else

{

grad_phi[0] = 0. ;

grad_phi[1] = 0. ;

}


rho_f = C_R(c,thread_f) ;

rho_p = C_R(c,thread_p) ;

rho_nf = C_R(c,mixture_thread) ;

vector_result[0] = (-DB*grad_phi[0]/phi - ST*mu_f*grad_T[0]/(rho_f*T_nf))/(1. - phi*rho_p/rho_nf) ;

vector_result[1] = (-DB*grad_phi[1]/phi - ST*mu_f*grad_T[1]/(rho_f*T_nf))/(1. - phi*rho_p/rho_nf) ;

}



DEFINE_PROPERTY(mu_particle,c,t)

{

real mu_p, phi ;

real mu_f = 855.e-06 ;

phi = C_VOF(c,t) ;

mu_p = (1./(1. - pow(phi,2.5)) - (1. - phi))*mu_f/phi ;

return mu_p ;

}



DEFINE_PROPERTY(k_particle,c,t)

{

real kf = 0.6 ;

real kp = 40. ;

real k_p, phi ;

phi = C_VOF(c,t) ;

k_p = ((kp + 2.*(kf + phi*(kp - kf)))/(kp + 2.*(kf - phi*(kp - kf))) - (1. - phi)*kf)/phi ;

return k_p ;

}

I compile my udf successfully but the results are not physical. There is no volume fraction gradient and slip velocity. They are equal to zero. please help me fix this problem
Thank you

Did you solve your problem? Kindly help me in getting phase velocities from mixture velocity data. If you have a UDF, please share it.

[vinerm]

Fluent does not store gradients being accessed in the UDF to save memory. So, the space needs to be assigned for the gradients before accessing those. Since the code checks for the existence of the gradients, it does not give an error. It simply does not find the value and the variables retain their original values. You can keep the gradients either by assigning memory to those using commands within UDF or by issuing following command in Fluent

sol set exp

And then by answering yes to the question regarding keeping the memory from being freed. This ensure that the memory used to store gradients is not freed up and, hence, the gradients are available within UDF.