[FLASH-USERS] Question about guard cells

[Klaus Weide]

On Fri, 19 May 2017, Jenks, Malia T. wrote:

> I understand the purpose of guard cells in general. I am running a code 
> in cylindrical coordinates and I know I need guard cells. I was 
> wondering what the guard cells on the inner (at r=0) portion of the r 
> dimension are.

FLASH requires you to set the left X boundary condition to REFLECTING in 
this situation. Cell values in guard cells will then be filled as at other 
reflecting boundaries; the Grid unit does not do anything special here.

Instead of REFLECTING, you can set the boundary condition to AXISYMMETRIC,
but this again is handled like REFLECTING for most variables. 
(The differences are related to sign changes for certain variables
that represent components of velocity or magnetic field.)

Klaus