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Old   September 16, 2006, 11:54
Default Need urgent help Regarding Drag coefficient
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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; }
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Old   September 17, 2006, 10:11
Default Re: Need urgent help Regarding Drag coefficient
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What 'error' message are you getting?

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Old   September 18, 2006, 05:06
Default Re: Need urgent help Regarding Drag coefficient
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if drag coefficient UDF function added the simulations goes divergence in few iterations
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Old   September 20, 2006, 10:10
Default Re: Need urgent help Regarding Drag coefficient
Luis Echeverri
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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.
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