hi, can any body help me to write UDF for drag coefficient. i have written UDF but its not working... i am attaching my UDF here. can anyone find where is the mistake in UDF. Its very urgent.

My drag coefficient function is:

Cd = max{ 24(1+0.15*pow(Re,0.687))/24, 8Em/(3(Em+4))}

Re = density(l)*Dia*|Vg-Vl|/viscosity;

Dia = Diameter of bubble

|Vg-Vl|---> Relative slip velocity

Em = g*density diff*pow(Dia,2)/surface tension;

#include <udf.h> #define diam2 0.005

DEFINE_EXCHANGE_PROPERTY(custom_drag,cell,mix_thre ad,s_col,f_col) { Thread *thread_g, *thread_l; real x_vel_g, x_vel_l, y_vel_g, y_vel_l, z_vel_g, z_vel_l, U_vel, abs_v,

rho_g, rho_l, mu_l, reyp, k_g_s,sre,psv,sv,Em,f,vasu;

/* find the threads for the gas (secondary) */ /* and liquids (primary phases) */

thread_g = THREAD_SUB_THREAD(mix_thread, f_col);/* gas phase */ thread_l = THREAD_SUB_THREAD(mix_thread, s_col);/* liquid phase*/

/* find phase velocities and properties*/

x_vel_g = C_U(cell, thread_g); y_vel_g = C_V(cell, thread_g); z_vel_g = C_W(cell, thread_g);

x_vel_l = C_U(cell, thread_l); y_vel_l = C_V(cell, thread_l); z_vel_l = C_W(cell, thread_l); U_vel = (x_vel_l + y_vel_l + z_vel_l)/3;

rho_g = 1.225; rho_l = 998;

mu_l = 0.0001;

/*compute slip*/ abs_v = sqrt(x_vel_g*x_vel_g + y_vel_g*y_vel_g + z_vel_g*z_vel_g);

/*compute Reynold's number*/ f=(U_vel-abs_v); if (f>0) f=f; else f=-f;

reyp = rho_l*f*diam2/mu_l;/*Reynold number*/

sre = pow(reyp,-0.313); psv = pow(reyp,-1); sv = 24*psv + 3.6*sre; Em = 133946.8*diam2*diam2; vasu = 8*Em/(3*Em + 12);

if(sv < vasu)

k_g_s = vasu; else

k_g_s = sv; return k_g_s; }

What 'error' message are you getting?

Mark

if drag coefficient UDF function added the simulations goes divergence in few iterations

Try starting the solution with the symmetric model for example. After some steeps change to your drag UDF. That could work, probably Re~0 gives a hard time to the solver.