[FLASH-USERS] Particle defaults

[John ZuHone]

Hi Sean,

> I'd like to know how Flash handles jumps in refinement when calculating the acceleration between two particles?

Unfortunately, at this time the public version of FLASH really doesn't "handle" it. I am guessing that you are referring to the violation of Newton's Third Law that occurs when particles interacting across a refinement boundary have their forces interpolated and their masses mapped according to a kernel with a width equal to the cell size on each particle's refinement level. Right now the public version of FLASH doesn't account for this, and so you will get some violations of the 3rd Law at refinement boundaries. In practice, these are typically very small when compared to the mean field when you have lots of particles, but this is not true for all applications. 

Paul Ricker has written some code to handle this, but I don't know if it's public. You can reach him at pmricker at illinois.edu. Paul, I wonder if it's time to get this into the main code?

> Also, how does the runtime parameter 'refine_on_particle_count' work and how does it differ from setting 'refine_var_n' equal to "pden".

Since the "refine_var_n" runtime parameters turn on the second derivative refinement criteria for the given variable, setting it to "pden" will mean that whenever "pden" is fairly discontinuous (which is usually just about everywhere) you will refine. This is probably not the behavior you are looking for. 

refine_on_particle_count is probably your best bet, since it will turn on refining and derefining for certain numbers of particles per block that you can specify, the relevant parameters being max_particles_per_blk and min_particles_per_blk. 

Best,

John ZuHone