Additionally, the simulation may displace a particle such that it is outside of the X–Y–Z coordinate boundaries of the block
holding its data structures, since recall that the Eulerian grid is spatially fixed whereas particles are not. In this case, the particle
data structures must also be moved to a different leaf block. In general, we use the term displacement to refer to the
change in a particle’s position in the physical domain, while the terms movement and migration interchangeably refer to
the movement of a particle’s data between blocks. FLASH’s solvers, being explicit, limit the size of the timestep based upon
stability conditions. The timestep limit in turn constrains a particle to move either within the block, or at most to one of the
nearest neighbor blocks during displacement
of the old leaf blocks become the new leaf blocks. We refer to the mesh being refined or derefined as regridding. Every
time the mesh regrids each particle is checked to make sure it is on a leaf block. If it is not, it is moved.