CFD Online Discussion Forums - what do global and cumulative terms in continuity refer to exactly?

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what do global and cumulative terms in continuity refer to exactly?

at results there are 3 types of residuals of continuity equation.whats the definition of them and difference?if cumulative and or global be negative whats wrong in the problem?

For the solvers of incompressible fluid, take a look at the
src/finiteVolume/cfdTools/incompressible/continuityErrs.H:

Code:

    volScalarField contErr(fvc::div(phi));



    scalar sumLocalContErr = runTime.deltaTValue()*

        mag(contErr)().weightedAverage(mesh.V()).value();



    scalar globalContErr = runTime.deltaTValue()*

        contErr.weightedAverage(mesh.V()).value();

    cumulativeContErr += globalContErr;



    Info<< "time step continuity errors : sum local = " << sumLocalContErr

        << ", global = " << globalContErr

        << ", cumulative = " << cumulativeContErr

        << endl;

From the above definitions, the difference between the local and global
continuity error is whether the magnitude of divergence is taken or not.
The "local" error is a measure of the continuity of each cell and the
"global" error represents the continuity in the whole computational domain.
If the global error converges to zero, I think the negative value is of no matter.

I have no idea about the importance of considering the cumulative error.

Fumiya

thanks fumiya
I cant grasp how the total continuity is calculated.my case is compressible.
What does mean cumulative continuity error?
Any hint is accepted here!

hey fumiya,

I am also not able to understand what is the meaning of these errors, if sumLocalcontErr is the continuity error for a cell then for what cell it is giving the value. I mean it should give value for all cell. Is it the maximum that is calculated too?:confused:

I am interpolating the velocity field from coarser mesh to finer mesh and then checking the continuity error, this gives me SumLocalErr a relatively high value (inc. from 1e-9 to 1e-4) but globalErr remains of same (i.e. 1e-19 to 1e-20). What this implies? Is the interpolation wrong? Can this effect my solution?