I'm trying to solve a simple 2D problem (fluid passing round pillars, see attached figure 1) in micro scale (drew in mm, scaling in "Setup") with the boundary conditions of pressure inlet & outlet, symmetry up & down, and no-slip wall for pillars (arcs in the figure 1).
The problem is when I change the mesh size, the result came out very different (factor of 2 to even 10) under the same convergence criteria. And when I change the convergence criteria (from 1e-3 to 1e-5, several trials in between) with the same mesh size, the results still vary a lot. The velocity profiles physically make sense (see attached figure 2), but the values just do not match.
I don't see this to be a simple problem of mesh size (i.e. not enough cells) or convergence criteria (i.e. not converged yet), because we had the exactly same case for 3D, which has finer mesh and much more cells, and that never gave us a problem like this. For example, under the convergence criteria of 1e-3, the results from different mesh sizes are within 1% error. And the error is also within 1% when the convergence criteria is 1e-4. The results match pretty well under all conditions.
Any kinds of help will be appreciated!
Figure 1 - Geometry & Mesh
Figure 2 - Results - Velocity Profile