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Unable to plot residuals (Interfoam)

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Old   February 9, 2018, 13:27
Default Unable to plot residuals (Interfoam) [Solved]
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Shafik Walakaka
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Hi everyone,

I am currently working on a two-phase laminar flow through a pipe. To check my mesh, I ran a simulation with only air through the pipe (alpha = 0) using the interFoam solver and the results matches the hagen-poiseuille equation.

However, I am currently trying to plot the residuals using help from the forum in the following link:
plot residuals

The residual plot was successful in the past when I was using the simpleFoam solver. However, using the interFoam solver, I am getting an error message stating the following
  • warning: skipping data with no valid points
  • unable to open display
  • x11 aborted

Does anyone have any idea of what I am doing wrong?

p/s: I also notice that in my log file, they are only solving for the p_rgh term and no ux,uy or uz term

Kind regards
Shaq

Last edited by walakaka; February 28, 2018 at 15:04. Reason: solved
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Old   February 10, 2018, 13:54
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Hi,

you donīt solve for p, thus you have to grep for p_rgh if you want to get the residuals for p_rgh. In most cases there is no need to solve the momentum prediction. In your fvSolutions you momentumPredictor is false. Thats why you donīt see Ux, Uy and Uz residuals.
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Old   February 10, 2018, 18:59
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Originally Posted by Tobi View Post
Hi,

you donīt solve for p, thus you have to grep for p_rgh if you want to get the residuals for p_rgh. In most cases there is no need to solve the momentum prediction. In your fvSolutions you momentumPredictor is false. Thats why you donīt see Ux, Uy and Uz residuals.
Hi Tobi,

Thanks for getting back to me. I had a look in the fvSolution folder and it is true that my momentumPredictor is set to "no".

Could you kindly explain the following please:
  • Why is there a momentumPredictor option in the interFoam solver compared to the simpleFoam solver (which has no momentum Predictor option and plots out ux, uy and uz residuals)
  • Would my residual plot be fully legitimate in showing that my simulation has converged if it only contains the plot for the residual of p_rgh?

Kind regards
Shaq
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Old   February 10, 2018, 20:33
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Hi,

  • in steady-state solvers the calculation (estimation) of the velocity field helps to go to the solution. Transient normally have the momentumPredictor flag. Its nothing than estimating the U field.
  • Yes, if p_rgh is converged, the fluxes are converged as well and thus the U field. Normally the momentumPredictior is needed if the viscosity ratio is large (e.g. in two phase flows). If one does not activate the momentum predictor, the tolerance for the pressure has to be set very strictly
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Old   February 18, 2018, 20:49
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Quote:
Originally Posted by Tobi View Post
Hi,

  • in steady-state solvers the calculation (estimation) of the velocity field helps to go to the solution. Transient normally have the momentumPredictor flag. Its nothing than estimating the U field.
  • Yes, if p_rgh is converged, the fluxes are converged as well and thus the U field. Normally the momentumPredictior is needed if the viscosity ratio is large (e.g. in two phase flows). If one does not activate the momentum predictor, the tolerance for the pressure has to be set very strictly
Thank you Tobi!

I have another question:

When not activating the momentum predictor, the solver only solves the residual for p_rgh. However, every time iteration has 3 p_rgh residuals, Which one should I take from each iteration?

Also, if one does not activate the momentum predicture, what are the recommended tolerance for pressure residuals? Will it have to be below 1e-7 or will a residual of magnitude below 1e-5 be reasonable?

Regards
Shafik

Last edited by walakaka; February 26, 2018 at 19:42.
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Old   February 26, 2018, 19:42
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Originally Posted by walakaka View Post
Thank you Tobi!

I have another question:

When not activating the momentum predictor, the solver only solves the residual for p_rgh. However, every time iteration has 3 p_rgh residuals, Which one should I take from each iteration?

Also, if one does not activate the momentum predicture, what are the recommended tolerance for pressure residuals? Will it have to be below 1e-7 or will a residual of magnitude below 1e-5 be reasonable?

Regards
Shafik
Has anyone got any info on this matter please?
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Old   February 26, 2018, 19:59
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Has anyone got any info on this matter please?
For example, the logs below, Which residual do I grep for? The first, second or final one? Or do I grep for each of them in succession (even though they are the resultant value from 1 particular iteration)



PIMPLE: iteration 1
smoothSolver: Solving for alpha.water, Initial residual = 0.000529543, Final residual = 2.66809e-11, No Iterations 2
Phase-1 volume fraction = 0.517887 Min(alpha.water) = 0 Max(alpha.water) = 1.06837
MULES: Correcting alpha.water
MULES: Correcting alpha.water
Phase-1 volume fraction = 0.517887 Min(alpha.water) = -8.72144e-07 Max(alpha.water) = 1.06794
DICPCG: Solving for p_rgh, Initial residual = 1, Final residual = 0.880426, No Iterations 1001
time step continuity errors : sum local = 0.00204219, global = -2.39327e-05, cumulative = -4.39327e-05
DICPCG: Solving for p_rgh, Initial residual = 0.270516, Final residual = 0.753417, No Iterations 1001
time step continuity errors : sum local = 0.00579181, global = -2.39253e-05, cumulative = -6.7858e-05
DICPCG: Solving for p_rgh, Initial residual = 0.510678, Final residual = 0.1488, No Iterations 1001
time step continuity errors : sum local = 0.00170815, global = -1.19552e-05, cumulative = -7.98132e-05
ExecutionTime = 12.26 s ClockTime = 12 s

Regards
Shaq
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Old   March 7, 2018, 17:27
Default Pimplefoam residual
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Quote:
Originally Posted by Tobi View Post
Hi,

  • in steady-state solvers the calculation (estimation) of the velocity field helps to go to the solution. Transient normally have the momentumPredictor flag. Its nothing than estimating the U field.
  • Yes, if p_rgh is converged, the fluxes are converged as well and thus the U field. Normally the momentumPredictior is needed if the viscosity ratio is large (e.g. in two phase flows). If one does not activate the momentum predictor, the tolerance for the pressure has to be set very strictly
Hi Tobi,

I have used your advice and greped for p_rgh successfully, is there a command to only grep the final residual in every outer loop iteration?
Because at the moment, my residual plot is plotting every single iteration (inner loop and outer loop) as shown in the attached file.

Yaxis: residuals
Xaxis: Iterations

Kind regards
Shafik
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File Type: jpg residuals_re300.jpg (51.8 KB, 83 views)
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