Extrapolating Boundary Conditions

• Extrapolating boundary conditions are in general unphysical - and should only technically be used when it can be guaranteed that the flow will always be leaving the grid supersonically. However it is often ok as long as the flow is not entering the grid. If material enters the grid at an interpolating boundary condition - then information is traveling both upstream and downstream and a type of feedback is possible which can result in run-away growth… Problems with gravitational sources are particularly susceptible to extrapolating boundary conditions and if extrapolating boundary conditions are used - there should be checks to ensure that there is never any inflow at an extrapolated boundary condition.

User Specified Boundaries

• When there are jets or winds - it is in general better to explicitly set the ghost zones rather than relying on extrapolating boundaries to maintain an initial wind or jet profile. With magnetic fields you cannot just set aux fields without introducing divergence - however as long as you only step on the aux fields in the ghost regions and don't modify any of the aux zones that are internal to the grid ie.

  aux(1:mx+1, 1:my, 1:mz, 1), aux(1:mx, 1:my+1,1:mz,2), aux(1:mx, 1:my, 1:mz+1,3)
  

  it should not create problems

• Alternatively you could use reflective boundary conditions for the field to keep the field lines normal (4) or parallel (3) to the boundary and only step on the fluid variables (density, momentum, energy)…