Why do we care about how much residual drops from the initial condition?
 

Hi,

I know that the residual value is the way to quantify and judge whether the simulation has converged or not. So I always use this value and look it as an important criteria to judge the convergence of my simulation.

But I also heard about that some CFD engineers also care about how much residual drops from the initial condition.

I'm wondering is that value useful? Why? What does it imply?

some times residual will be converged but solution wont converge, in that case you have to monitor some parameters in critical region, we have to check wheather it is stablized

Usually we will look convegent trends from intial stage

Yes. Stabilization with respect to time step of the parameter we care about is also an important criteria for convergence. That's another thing.

But still that question, why some engineers care about the drop of the residual from the initial condition but not the residual value itself?

(Source-ESI)Only upon looking at the drop in residuals can we get an indication of the overall level of convergence. The rule of thumb is to look at the first residual or the maximum residual calculated and then look for 3-5 orders of magnitude drop from there.

http://support.esi-cfd.com/esi-users/convergence/

You can more idea from this link

I like that link. We look at the drop of the residual but not its value when the solver doesn't normalize the residual by itself. So it depends on the solver. Thanks.