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Grid independence study problem

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Old   February 2, 2016, 05:26
Default Grid independence study problem
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Kamal Bisht
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Hi, during the grid independence study I am getting the results closer to the experimental values as expected. But if I refine my mesh too much, the results start varying and shows an error more than the coarsest mesh. What is the reason for this behavior?
I have attached a graph showing different results.
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Old   February 2, 2016, 10:37
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Hard to say without knowing what you are modeling or how you modeled it. My initial guess would be that it's related to turbulence model and the its assumptions, but I can't say for sure.
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Old   February 2, 2016, 11:57
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I am doing multiphase simulation (water and air) in a pipe with VOF method and realizable k-epsilon method
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Old   February 2, 2016, 12:02
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Ok... can you show a plot that has cell count of your meshes for each of your mesh dependency models along X axis and error with respect to experimental data along the Y axis? The plot you show is too busy to interpret properly.

I might also help to see shots of the mesh for each level of the mesh dependency study to make sure there isn't something funky going on elsewhere.
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Old   February 2, 2016, 12:28
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Kamal Bisht
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Sorry but I don't get what do you want me to plot. What do you mean by error wrt to experimental on Y axis? Does it mean that I should plot the difference between the experimental and simulation result for each mesh by a line graph, which mean 5 different lines for each type of mesh?
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Old   February 2, 2016, 12:42
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You tell me what error you meant by:

"But if I refine my mesh too much, the results start varying and shows an error more than the coarsest mesh."

Your plot doesn't show an error, just overlaid curves so it is hard to quantify and visualize the error you are talking about. What I do see is a VERY chaotic experimental curve and then a bunch of CFD curves that are very close to each other. It is very difficult to read.

What I would suggest doing is calculating the error (difference between CFD1 and EXP, then CFD2 and EXP), and so on for each instant t in your time domain. Then calculate the RMS error for the whole time domain and use this single value as your estimate of error for each CFD model. Then you will have a plot or bar graph that has mesh count along the X axis and RMS error along the Y axis consisting of 5 points. That will be much easier to read and interpret.

Another question that comes to mind... since this is transient. Are each of your time steps converging?
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Old   February 2, 2016, 13:19
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Actually lines in my graph are pressure values. The minimum pressure observed during experiments is -92 Pa. As I am refining the mesh, the pressure drop predicted by the simulation getting better but for the finest mesh it's around -46 Pa. however after 0.1 s, pressure is stabilized around 0 pa which is mostly accurate for all simulations.
Yeah my time steps are converging as the residuals fall to 10e-5 and below.
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Old   February 2, 2016, 13:22
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I am not sure that calculating the error based on lowest pressure is a good approach since your experimental values are very erratic. I stand by my suggestion of RMS error.
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Old   February 2, 2016, 13:28
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To be 100% clear, let;

CFD(t) represent the pressure value as a function of time as calculated by your CFD analysis.

EXP(t) represent the pressure value as a function of time for your experimental results.

ERR(t) = (CFD(t)-EXP(t))/EXP(t) x 100 % (this is the instantaneous error of your CFD solution with respect to your experimental results.)

Then the RMS error (assuming you have a constant time step) is sqrt(sum(ERR^2)/n), where n is the number of samples in ERR. This will give you a single value that represents the average error for a given CFD case across the whole time domain.
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