[zahrae]

Hello!
I want to insert a drag force by a UDF.
Fluent use this equation to specify the drag coefficient between particles and the fluid.
F_D=(18μC_D R_e)/(〖24ρ〗_p d_p^2 )

In the description of the DEFINE_DPM_Drag macro, It was noticed that : ‘’your UDF will need to return the real value of the drag force on a particle. The value returned to the solver must be dimensionless and represent 18 * Cd * Re / 24.’’
However, I want to model drag force by this equation :
F_D=(π.d_p^2.ρ_p.〖(U_p-U_p)〗^2.C_d)/〖8C〗_c

My questions are :
1-Is it necessary to return exactly this value 18 * Cd * Re / 24 ?

In the udf bellow I have computed a dimonsionless value correspond to cd/8Cc.
For the drag force I multiply this dimensionless value by π.d_p^2.ρ_p.〖(U_p-U_p)〗^2, and finally I returned this value at the end of the macro.

2-Is fluent will consider the expression of my drag force or it will multiply the value I define by μ/(ρ_p d_p^2 ) ?

#include "udf.h"
#include "dpm.h"
#include "mem.h"
#define Boltzmann 1.3806505e-23
#define pi 3.14159265358979323846
#define nu_p 0.12
#define nu_f 0.001075
#define b1 0.186
#define b2 0.653
#define b3 0.437
#define b4 7178.74
DEFINE_DPM_DRAG(particle_drag_force,Re,p)
{
Thread *t=RP_THREAD(&(p->cCell));
cell_t c=RP_CELL(&(p->cCell));
real value;
real lamda ,knudsen,Cc,cd,drag_force,p_dia,p_den,p_tem;
lamda=(Boltzmann*p_tem)/(sqrt(2*pi)* p_dia*p_dia*C_P(c,t));
knudsen=2.*lamda/p_dia;
Cc=1+knudsen*(1.257+0.4*exp(-1.1/knudsen));
cd=(24.0/Re)*(1+b1*pow(Re,b2))+(b3*Re)/(b4+Re);
value=cd/(8*Cc);
drag_force=pi*p_dia*p_dia*C_R(c,t)* pow((uf-up),2)*value;
return(value) ;
}

Can you please give me some guidelines it is urgent !

[AlexanderZ]

in description of DEFINE_DPM_DRAG macro it was shown that it is just a part of equation for calculating force

best regards

[zahrae]

Thanks for your help!
My equation for drag force is : Fd=[pi * dp² * (uf-up)² * Cd]/8Cc. So the part I should return will be (pi * Cd)/8Cc ?

Is fluent will consider my equation to compute drag force (Fd=[pi * dp² * (uf-up)² * Cd]/8Cc. ) ?
Can you take a look to my UDF?

[AlexanderZ]

I think you should divide your equation on equation from manual, but without DEFINE_DPM_DRAG part

best regards

[zahrae]

I don't understand you very well! Can you give me more details please?
Do you mean I have to use the Define dpm force macro for coding my drag force? Because in the manual they mentioned that the define dpm force macro is used to specify a body force other than a gravitational or drag force on the particles.

[AlexanderZ]

my vision

Fluent equation:

F_D=(18μC_D R_e)/(〖24ρ〗_p d_p^2 )(u-u_p)
here (18C_D R_e)/(24) is DEFINE_DPM_DRAG

Your equation:

F_D=(π.d_p^2.ρ_p.〖(U_p-U_p)〗^2.C_d)/〖8C〗_c

your DEFINE_DPM_DRAG(particle_drag_force,Re,p):

return (π.d_p^2.ρ_p.〖(U_p-U_p)〗^2.C_d)/〖8C〗_c / (μ)/(〖ρ〗_p d_p^2 )(u-u_p)

best regards

[zahrae]

Thank you I will try to this !