[hwet]

Good day

I want a udf for the discrete phase to be activated only when the particle passes through a cell with a specific property.

For example, the DEFINE_DPM_BC is activated only when the particle hits a wall. But, here I am trying to activate the UDF while the particle is travelling away from the walls and only when it passes through a cell with a certain property.

To be specific the cells representing a different eulerian phase which is moving constantly as well. So the UDF is called only when the particle enters such a cell.

Is this even possible to achieve? Am i looking at a DEFINE_ON_DEMAND macro? Usually the DPM macros are called at everytime step but this wont be very efficient in my case where the cell occupied by the eulerian phase is only a small fraction of the total number of cells.

[`e`]

The DEFINE_ON_DEMAND macro is only executed when you manually enable it (unless you're using scripts). The DEFINE_DPM_SCALAR_UPDATE macro is called each time a particle position is updated. With conditional statements, you can restrict when code is executed and this method is generally quick (I can't think of a more elegant solution). For an arbitrary example, a particle travelling away from a wall (on the XZ plane) within a cell of a high velocity:

DEFINE_DPM_SCALAR_UPDATE(certain_particles,c,t,initialise,p)
{
	if (P_VEL(p)[1] > 0. && C_U(c, t) > 1.)
	{
		// a code block
	}
}

If, however, you wanted to modify the force acting on the particle in certain regions of your domain then you could use the DEFINE_DPM_BODY_FORCE macro.

DEFINE_DPM_BODY_FORCE(certain_particles_force,p,i)
{
	cell_t c;
	Thread *t;

	c = P_CELL(p);
	t = P_CELL_THREAD(p);

	if (P_VEL(p)[1] > 0. && C_U(c, t) > 1.)
	{
		// a code block
	}
}

[hwet]

Aren't these already passed in the macro?

DEFINE_DPM_SCALAR_UPDATE(certain_particles,''c'',''t'',initialise,p)

Why would we need this then?.

cell_t c;
Thread *t;

c = P_CELL(p);
t = P_CELL_THREAD(p);

[`e`]

Woops, that's correct: you won't need to find the cell number and thread for the DEFINE_DPM_SCALAR_UPDATE macro (already provided) but it's required for DEFINE_DPM_BODY_FORCE (only provides the particle and direction index).