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Residuals

cfd enthusiast

Hello Everyone,

I've been having some trouble getting my Simulation (using Open Solver) to converge to the Residual I want (-4). I'm using coarse grid initialization and 3 grids for the Simulation. on the first two grids it looks fine, but as soon as the finest grid is reached the residuals don't drop anymore and seem stuck on one value (above -4).

I'm trying to understand what could be causing that. What might help me further is understanding what type of Residuals I'm dealing with; hence my main question would be: How are the representative residuals calculated in fidelity (what metric is being used? L1,L2...). Maybe a convergence criterion of -4 is too high depending on the metric that's being used to get the representative residulas.

Thank you!

  • +1

    Hi,

    For the Turbo solver and the Open density-based solver the residuals are normalized by the residual in the first iteration. This may explain why it is not always possible to decrease the same orders of magnitude. It can be understood by taking the extreme case of a simulation where the initial solution is already very close to the final converged solution. In that case the residuals in the first iteration will already be very small and it may not be possible to decrease even more than 1 order of magnitude. Still it can be a well converged solution. 

    I also recommend to look at the residuals of the other equations (momentum and energy conservation) and to look at physical quantities at various relevant probe locations. It may happen that the density does not vary much and you don't get below -4 for mass conservation, but you may still see an evolution for other residuals or physical quantities at probe locations. 


    For more detail on the definition of the residuals see also the section "Monitor simulation" in the Fidelity 2023.2 documentation. Look under "Customize the input data >> Residuals.

    I hope this helps. If a question is remaining, it could be helpful if you show a few plots of the various residuals and probe evolutions (with their location). 

    Best regards,
    Colinda

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