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

immortality · January 12, 2013, 03:55

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?

fumiya · January 13, 2013, 05:17

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

immortality · January 16, 2013, 09:15

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!

kk415 · September 6, 2018, 03:05

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?

kk415 · September 6, 2018, 05:06

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?

January 12, 2013, 03:55	what do global and cumulative terms in continuity refer to exactly?	#1
immortality Senior Member Ehsan Join Date: Oct 2012 Location: Iran Posts: 2,208 Rep Power: 26	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? kcavatar and vivek05 like this.

January 13, 2013, 05:17		#2
fumiya Senior Member Fumiya Nozaki Join Date: Jun 2010 Location: Yokohama, Japan Posts: 266 Blog Entries: 1 Rep Power: 18	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 kiddmax, vbnhfylbh and kcavatar like this.

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January 16, 2013, 09:15		#3
immortality Senior Member Ehsan Join Date: Oct 2012 Location: Iran Posts: 2,208 Rep Power: 26	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!

September 6, 2018, 03:05		#4
kk415 Senior Member krishna kant Join Date: Feb 2016 Location: Hyderabad, India Posts: 133 Rep Power: 10	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?

September 6, 2018, 05:06		#5
kk415 Senior Member krishna kant Join Date: Feb 2016 Location: Hyderabad, India Posts: 133 Rep Power: 10	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?