[MachZero]

I am using Fluent, and as many users do, I often monitor convergence (at least partially) by monitoring the residuals. I think that many CFD users can get confused about exactly what the residuals mean. I am occasionally one of those users.

In theory, residuals are just the change from one iteration to the next. As a solution converges one would expect the residuals to drop such that there is no change from iteration n to n+1. But that can be misleading. For example, as you drop the under-relaxation factors, residuals drop. At first glance one could say "it is converging better!" without that being the case. Taking this to the extreme... If UR factors are dropped to 0.001, the solution simply isnt progressing, almost at all. Therefore the residuals will drop to approx. machine precision.

My ultimate point is that, residuals can mean many things, from "the solution is converged" to "the solution is not progressing". Or, when the residuals are constant at a higher value, that "the solution is changing consistently or oscillating".

So my ultimate question is, are there any good ways to weed out some of these issues? Yes, I like most people set surface monitors and judge those for convergence, but I want this conversation to remain about residuals. Are there any experts out there with some insight into this matter? I would be interested in having some discussion.