Need urgent help Regarding Drag coefficient
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*VgVl/viscosity; Dia = Diameter of bubble VgVl> 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_velabs_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; } 
Re: Need urgent help Regarding Drag coefficient
What 'error' message are you getting?
Mark 
Re: Need urgent help Regarding Drag coefficient
if drag coefficient UDF function added the simulations goes divergence in few iterations

Re: Need urgent help Regarding Drag coefficient
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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