Hello

I have a quick question,

First, we ran transient successfully with a coarse grid. After using the coarse grid solution to restart the fine grid case using the same time step and the default solution parameters, the code crashed with negative densities. Is this a symptom of having too big a time step or relaxation parameters? Can anyone recommend some values?

The Courant number is inversely proportional to the cell size, so if you have the same velocity and time-step, Co will increase. The higher Co, the more likely instability. If you refine the mesh, it's good practice to reduce the time-step accordingly.