{"id":308742,"date":"2023-09-26T14:34:47","date_gmt":"2023-09-26T14:34:47","guid":{"rendered":"\/forum\/forums\/topic\/fluent-drag-modification-udf\/"},"modified":"2023-09-26T14:34:47","modified_gmt":"2023-09-26T14:34:47","slug":"fluent-drag-modification-udf","status":"closed","type":"topic","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/fluent-drag-modification-udf\/","title":{"rendered":"FLUENT- Drag modification UDF"},"content":{"rendered":"

Hello everyone,
I am trying to simulate a gas-liquid stirred tank. I am using a Grace drag model, for the modification, I dont want to use Brucato model, but I want to define a UDF based on another model. I know that I need to use the DEFINE_EXCHANGE_PROPERTY macro. The formula I would like to use can be defined as :<\/p>\n

C_D_modified= modification_coefficient* C_D<\/span>
* C_D being the drag coefficient that is already being calculated by the Grace model.<\/p>\n

My issue is that I don’t know how to call the drag coefficient that FLUENT is already calculating through the Grace model. I searched the UDF manual and was not able to find any macro for this. I found a code online which redefines the drag coefficient within the modification. So, I ended up writing the UDF below:<\/p>\n

# include “udf.h”<\/span><\/p>\n

#define sigma= 0.072;<\/span>
#define mu_ref=0.0009;<\/span>
#define C_exp=0;<\/span><\/p>\n

DEFINE_EXCHANGE_PROPERTY(lane_drag_mod, cell, mixture_thread, scnd_clmn, frst_clmn)<\/span><\/p>\n

{<\/span>
Thread *thread_f, *thread_g;<\/span>
real x-vel-f, x-vel-g, y-vel-f, y-vel-g, z-vel-f, z-vel-g; <\/span>
real rho_f, rho_g, mu_f,g;<\/span>
real eps_f, k_f;<\/span>
real tau_b, int_t;<\/span>
real stk, m_coeff;<\/span>
real Eo, Mo, Re, Hfac, Jfac, C_Dsph, C_Dcap, C_Delip:<\/span>
real C_D, C_Dmod;<\/span><\/p>\n

thread_f = THREAD_SUB_THREAD(mix_thread, scnd_clmn);     \/* liquid phase *\/<\/span>
thread_g = THREAD_SUB_THREAD(mix_thread, f_col);    \/* gas phase*\/<\/span><\/p>\n

\/* ===================find phase velocities in all spatial directions================*\/<\/span>
x_vel_f = C_U(cell, thread_f);<\/span>
y_vel_f = C_V(cell, thread_f);<\/span>
z_vel_f = C_W(cell, thread_f);<\/span>
x_vel_g = C_U(cell, thread_g);<\/span>
y_vel_g = C_V(cell, thread_g);<\/span>
z_vel_g = C_W(cell, thread_g);<\/span><\/p>\n

\/* ====================compute velocitiy differences=================================*\/<\/span>
slip_x = x_vel_g – x_vel_f;<\/span>
slip_y = y_vel_g – y_vel_f;<\/span>
slip_z = z_vel_g – z_vel_f;<\/span>
abs_v = sqrt(slip_x*slip_x + slip_y*slip_y + slip_z*slip_z);<\/span><\/p>\n

\/*=====================finding the properties of gas and liquid======================*\/<\/span><\/p>\n

rho_f = C_R(cell, thread_f);<\/span>
rho_g = C_R(cell, thread_g);<\/span>
mu_f = C_MU_L(cell, thread_f);<\/span>
k_f= C_K(cell, thread_f);<\/span>
eps_f= C_D(cell, threaf_f);<\/span>
g= NV_MAG(M-gravity);<\/span>
diam = C_PHASE_DIAMETER(cell, thread_g);<\/span>
void_g = C_VOF(cell, thread_g);<\/span><\/p>\n

\/*================calculating bubble relaxation time and the integral time===========*\/<\/span>
tau_b=rho_g*diam*diam\/18.\/mu_g;<\/span>
int_t=0.135*k_f\/eps_f;<\/span><\/p>\n

\/*===================calculating modification coefficient============================*\/<\/span>
stk=tau_b\/int_t;<\/span>
m_coeff= powf((1 – 1.4 * powf(stk,0.7) * exp(-0.6 * stk)),-2.0)<\/span><\/p>\n

\/*=======================Grace drag model defining==================================*\/<\/span>
Eo= (g * (rho_f – rho_g) * diam * diam)\/ sigma;<\/span>
Mo= (powf(mu_f,4) * g * (rho_f – rho_g))\/(powf(rho_f,2) * powf(sigma,3);<\/span>
Re= (rho_f * abs_v * diam)\/ mu_f;<\/span>
Hfac= 4\/3 * Eo * powf (Mo,-0.149) * powf (mu_f\/mu_ref, -0.14);<\/span><\/p>\n

if (2<Hfac<=59.3)<\/span>
{<\/span>
Jfac= 0.94 * powf(Hfac,0.757);<\/span>
}<\/span>
else if (Hfac>59.3)<\/span>
{<\/span>
Jfac= 3.42 * powf(Hfac,0.441);<\/span>
}<\/span><\/p>\n

U_ter=(mu_f\/(rho_f * diam)) * powf(Mo,-0.149) * (Jfac-0.857);<\/span><\/p>\n

if (Re<0.01)<\/span>
{<\/span>
C_Dsph=24\/Re;<\/span>
}<\/span>
else if (Re=>0.01)<\/span>
{<\/span>
C_Dsph=24*(1+0.15*powf(Re,0.687))\/Re;<\/span>
}<\/span><\/p>\n

C_Dcap=8\/3.;<\/span><\/p>\n

C_Delip=(4*g*diam*(rho_f-rho_g))\/(3*powf(U_ter,2)*rho_f);<\/span><\/p>\n

C_D=powf(void_g,C_exp)*max(min(C_Delip,C_Dcap),C_Dsph);<\/span>
\/*==========================Calculating modified drag===============================*\/<\/span>
C_Dmod=m_coeff*C_D;<\/span><\/p>\n

return C_Dmod<\/span>
}<\/span><\/p>\n

Now, can anyone tell me if this is corect?
Also, looking at the Theory guide, I can see that the interphase Exchange coefficient (K_pq) is dependent on the drag coefficient. Should I also embed that within the code? 
I appreciate any help you can give me. I am really stuck with this and no one can help me.<\/p>\n

Many thanks,Roya<\/p>\n","protected":false},"template":"","class_list":["post-308742","topic","type-topic","status-closed","hentry","topic-tag-drag-coefficient","topic-tag-multiphase-eulerian","topic-tag-udf"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t