Protection of Variables

With some numerical schemes it is possible for variables to achieve unphysical values (negative pressure or density, for instance). AstroBEAR can protect against these unphysical values by replacing the bad value with the minimum value from the surrounding cells. This is not ideal as a broad fix, since protections can violate conservation laws; it's okay if a few protections happen in isolated cells, but frequent protections in a large number of cells indicate serious problems.

It's also important to note that most protection algorithms rely on the integrity of neighboring cells. If there are no nearby cells with valid values, then protection algorithms will not work.