Lattice size


I was wondering if there is a limit on how big a lattice one can use? I am simply trying to simulate poiseuille flow in parallel plates. The simulation gives me the correct analytic solutions when the lattice size is 51x25 but if I increase the lattice size to say 101x41 I can’t recover the analytic solution.

I am using: Reynolds = 6 and N=100 and Umax = 0.01

If someone can help me out that would be great.