[physics1]

I am solving a transport equation ddt(n)+div(n,U)=0 for a ciruclar density area. As it transport the density begins to break up. We have fixed this by inputing a diffusion coefficient into the equation making it ddt(n)+div(n,u)-k*laplacian(n)=0, where k is defined as a dimensioned scalar. The issue is that at k=10,000 the diffusion works and the density does not break up but if i increase it to k=100,000 the density structures again. Ive compared it to the case where k=0 and they look identical. Is there a maximum value for a dimensioned scalar that i am hitting when i increase k from 10,000 to 100,000? If so how can i make that value larger?