CFD Online Discussion Forums

Hi

First at all, Fluent user's guide define DPM concentration as the total concentration of discrete phase, and its unit quantity is density.

As I understood DPM concentration gives you the mass of discrete phase per control volume.

As a results you can get both the P_MASS(p) at the current position and the C_VOLUME(c,t)

For instance if you want to get the concentration at the end of a time step or iteration you can write a UDF simmilar to this example in fluent

/************************************************** *********

Example UDF that demonstrates DEFINE_DPM_SPRAY_COLLIDE ************************************************** **********/ #include "udf.h" #include "dpm.h" #include "surf.h"

DEFINE_DPM_SPRAY_COLLIDE(udf_man_spray_collide, tp, p) { /* non-physical collision UDF that relaxes the particle */ /* velocity and diameter in a cell to the mean over the */ /* specified time scale t_relax */

const real t_relax = 0.001; /* seconds */

/* get the cell and Thread that the particle is currently in */ cell_t c = RP_CELL(&(tp->cCell)); Thread *t = RP_THREAD(&(tp->cCell));

/* Particle index for looping over all particles in the cell */ Particle *pi;

/* loop over all particles in the cell to find their mass */ /* weighted mean velocity and diameter */ int i; real u_mean[3]={0.}, mass_mean=0.; real d_orig = tp->state.diam; real decay = 1. - exp(-t_relax); begin_particle_cell_loop(pi,c,t)

{

mass_mean += pi->state.mass;

for(i=0;i<3;i++)

u_mean[i] += pi->state.V[i]*pi->state.mass;

} end_particle_cell_loop(pi,c,t)

/* relax particle velocity to the mean and diameter to the */ /* initial diameter over the relaxation time scale t_relax */ if( mass_mean > 0. )

{

for(i=0;i<3;i++)

u_mean[i] /= mass_mean;

for(i=0;i<3;i++)

tp->state.V[i] += decay*( u_mean[i] - tp->state.V[i] );

tp->state.diam += decay*( P_INIT_DIAM(tp) - tp->state.diam );

/* adjust the number in the droplet parcel to conserve mass */

tp->number_in_parcel *= CUB( d_orig/tp->state.diam );

} }

================================================= see in this example how they get the cell index throgh the macro cell_t c = RP_CELL(&(tp->cCell)); Thread *t = RP_THREAD(&(tp->cCell));

and then how they loop over all the particles in the cell to find their mass, you can define a "real conc"to calculate the concentation on that cell as follow

begin_particle_cell_loop(pi,c,t)

{

mass_mean += pi->state.mass;

conc=mass_mean/C_volume(c,t)

C_UDMI(c,t,UDM_RH)=conc; } end_particle_cell_loop(pi,c,t)

do not forget a pointer to the particle Particle *pi; ================================================== = The only doubt that I have is if this DPM macro will allow to print the concentration.

I guess you can define a UDM to store the value (donot forget to define a UDM_RH (i.e.#define UDM_RH 0) ================================================== == I cannot help you more, you have to work on your own to implement the UDF

Best regards

Alex Munoz