[FLASH-USERS] Continuing to evolve particles that leave the domain

[Norbert Flocke]

Hi James,

I have dealt with this on two ocasions, when writing the ray tracing and 
the proton imaging units. Both units use a different approach that I will 
explain (they use the grid particles unit for moving particles through 
blocks):

  1) Ray tracing
     -----------

     In this case I needed to keep the rays that exit the domain for some
     internal checks (hence no further evolution). I did this by giving
     them a particle attribute of RAY_OUTOFDOMAIN, which I set to a large
     negative number < -2. Then, before! I entered the grid move particles
     routine, I copied these rays to a different array to save them and
     then I reset the rays with attribute RAY_OUTOFDOMAIN to rays with
     NONEXISTENT.

  2) Proton Imaging
     --------------

     Unlike the ray tracing, in this case I needed to keep track of the
     protons after they leave the domain, because they have to be recorded
     on a detector screen. I did this by copying the domain exiting protons
     to a different array (becoming screen protons), which I then evolve
     by a separate routine. Here the protons leaving the domain are simply
     set to NONEXISTENT. The possible memory constraints (for large number
     of protons) are beeing dealt with by intermediate (small) bucket
     arrays that are emptied (recorded on the detector screen) once they
     are full.

As you see, as soon as my particles (ray, proton) left the domain, I took 
full responsibility of evolution. I did not leave this task to the grid 
particles unit.

I hope this helps you a bit,
Norbert


On Fri, 27 Mar 2015, James Guillochon wrote:

> Hi all, I'm using active particles in my calculation, and I would like to
> continue to evolve the particle trajectories even once the particles have
> left the domain. I see that particles that leave the domain can be saved
> after they leave by enabling a flag, but it appears that their evolution at
> this point is frozen. It looks like a few relatively simple changes in
> Particles_advance may be able to continue their evolution, but I wasn't
> sure if evolving these particles would cause issues.
>
> Has anyone tried to do something like this before?
>
> Cheers,
> - James
>
> -- 
> James Guillochon
> Einstein Fellow at the Harvard-Smithsonian CfA
> jguillochon at cfa.harvard.edu
>