[Users] evolving flat boundary conditions

Hello,

I am trying to do a cosmology simulation project using Cactus. For this project, I'm evolving an expanding homogeneous fluid in a box (FLRW spacetime), and I would like to use flat boundary conditions. I've attached the par file I am currently using. However, when I run this, the boundary cells do not seem to update at each time step to match the current value of the grid cells on the edge, as seen in this plot of the second time step output: [image: image.png]

Is there a way to make the the boundary cells be updated at each time step according to the evolution of the edge of the grid?

Thank you, Ryn