[why?]

Hi This is a code I found online and trying to understand.

Could someone tell the difference between using pointer p and tp as the tracked particle pointers.

Also what do the lines

void assign_init_mass_to_tp_and_p(Tracked_Particle *tp, real mass_factor)

and

#if RP_NODE
Injection *I = tp->injection;
Particle *p = NULL;
#endif

and

#if RP_NODE
if (dpm_par.unsteady_tracking)

do.

The full code is below. (Not quite, I have still cut out the second part of the code).

#include "udf.h"

#define BREAKUP_SMALL 1e-8

void assign_init_mass_to_tp_and_p(Tracked_Particle *tp, real mass_factor)
{
#if RP_NODE
Injection *I = tp->injection;
Particle *p = NULL;
#endif

P_INIT_MASS(tp) *= mass_factor;

#if RP_NODE
if (dpm_par.unsteady_tracking)
return;

loop(p, I->p)
if (p->part_id == tp->part_id)
P_INIT_MASS(p) *= mass_factor;
#endif
}

[`e`]

You can name variables and pointers however you desire; using *tp, *p, *areallylongpointername has no affect on your code.

void assign_init_mass_to_tp_and_p(Tracked_Particle *tp, real mass_factor)

Defines a function which returns nothing ("void"). Two arguments are passed to this function: *tp and mass_factor with data types of Tracked_Particle and real, respectively.

#if RP_NODE
Injection *I = tp->injection;
Particle *p = NULL;
#endif

For compute nodes, declare and initialise the *I (injection stream) and *p pointers. As this code does not loop through particle injections, I'd expect this function to be called for each injection.

if (dpm_par.unsteady_tracking)
return;

If the unsteady tracking option is enabled within DPM then the function returns immediately without executing the lines below (initialising the particle parcel masses).

Side note: looping through particles with I->p loops over all particles regardless of their state and residence time. Use I->p_init for initialising unsteady particles.

[why?]

Actually this code is posted here and apparently breaks an injected droplet into two droplets after a certain plane is passed.

http://cape-forum.com/index.php/topic,528.0.html

Defines a function which returns nothing ("void").

Why would you define a function which returns nothing?

Also, is this code in steady state initializing the particle data twice?

loop(p, I->p) if (p->part_id == tp->part_id) P_INIT_MASS(p) *= mass_factor; #endif
and also initializing earlier
#if RP_NODE Injection *I = tp->injection; Particle *p = NULL; #endif

From the second part

if (initialize)
{
p->user[0] = 0.;
}

What is user[0] here?
Thanks

[`e`]

The code modifies values in arrays, namely P_INIT_MASS(p), and therefore has no requirement of returning a value.

The particle and injection pointers are only initialised and declared once, regardless of steady/unsteady state, with:

#if RP_NODE
Injection *I = tp->injection;
Particle *p = NULL;
#endif

I'd imagine this code is from an older version of Fluent where the developers used the -> for collecting variables. It's recommended you use the new equivalent user-defined macro P_USER_REAL(p,0).