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August 14, 2013, 13:45 
OF 2.2.0: “fixedMean” bug

#1 
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don lenardo
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Hello,
there seems to be a bug in the "fixedMean" condition in OF2.2.0. Small values for the pressure lead to wrong results. So using "0" isn't possible any more compared to OF1.6ext. Here is the description of my case, a sketch is attached: Differences in total pressure between In and Outlet (deltaPtot) for a static (nonrotating) Pipe without any GGI/AMI after 500 iterations using simpleFoam and different OFversions. deltaPtot (OF1.6ext): meanValue = 0 > deltaPtot = 136.18; meanValue = 1e30 > deltaPtot = 136.18; meanValue = 1e10 > deltaPtot = 136.178 deltaPtot (OF 2.2.0): meanValue = 0 > deltaPtot = 386.64; meanValue = 1e30 > deltaPtot = 386.64; meanValue = 1e10 > deltaPtot = 118.662 Has anyone a shared experience with this BC combined with Of 2.2.0??? //Markus 

August 15, 2013, 03:04 

#2 
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Bernhard
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Did you use the same settings for both cases?
Did you check the difference in source code between the simpleFoam solvers of the different distributions? And of the boundary conditions? 

August 15, 2013, 07:14 

#3 
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don lenardo
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settings are the same!
I have not checked the code yet. I am not well versed in this //Markus 

August 17, 2013, 15:56 

#4 
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Niels Gjoel Jacobsen
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Hi Markus,
I might be a bit to late into the discussion, but could you please share with us the correct answer. I suppose that you have an analytical solution, which you are trying to compare with the numerical result? Secondly, could you try to undertake the same exercise with a square cross section of the pipe, such that you are not having any nonorthogonalities in the mesh, i.e. a simple blockMesh and perhaps with a bit of stretching away from the boundaries if you fancy. Thirdly. Are you using any turbulence models in your setup? If the answer is no, could you please try to run the circular cross section with any turbulence model of your liking and report your findings. Fourthly, are the models converged after 500 iterations, or is this number arbitrarily chosen based on prior experience? This was a lot of questions, once we have an answer to these, we might help you forward, but I would emphasise the importance of point 2 and 3. Kind regards Niels
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August 18, 2013, 09:44 

#5 
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don lenardo
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The mesh was created with ICEM, the result of checkMesh can be seen below:
Create time Create polyMesh for time = 0 Time = 0 Mesh stats points: 100542 faces: 294476 internal faces: 287644 cells: 97020 boundary patches: 4 point zones: 0 face zones: 3 cell zones: 1 Overall number of cells of each type: hexahedra: 97020 prisms: 0 wedges: 0 pyramids: 0 tet wedges: 0 tetrahedra: 0 polyhedra: 0 Checking topology... Boundary definition OK. Point usage OK. Upper triangular ordering OK. Face vertices OK. Number of regions: 1 (OK). Checking patch topology for multiply connected surfaces ... Patch Faces Points Surface topology INLET 1260 1289 ok (nonclosed singly connected) OUTLET 1260 1289 ok (nonclosed singly connected) WALL_STAT 2128 2240 ok (nonclosed singly connected) WALL_ROT 2184 2240 ok (nonclosed singly connected) Checking geometry... This is a 3D mesh Overall domain bounding box (0 1 1) (15 1 1) Mesh (nonempty, nonwedge) directions (1 1 1) Mesh (nonempty) directions (1 1 1) Mesh (nonempty, nonwedge) dimensions 3 Boundary openness (2.60553e17 2.62945e16 2.45788e17) Threshold = 1e06 OK. Max cell openness = 2.57603e15 OK. Max aspect ratio = 237.872 OK. Minumum face area = 9.99999e05. Maximum face area = 0.0346467. Face area magnitudes OK. Min volume = 1.14835e05. Max volume = 0.00160775. Total volume = 46.9561. Cell volumes OK. Mesh nonorthogonality Max: 26.6077 average: 4.2815 Threshold = 70 Nonorthogonality check OK. Face pyramids OK. Max skewness = 0.618008 OK. Mesh OK. End So there are no problems with nonorthogonal cells, the quality of the whole mesh is good. The cells are also expanded with approaching the In and Outlet. The turbulence model is the kOmegaSST. I also did some calculations with >2000 iterations but there is no difference in totalpressure from about 200 iterations > 500 iterations are enough. The analytical calculation leads to a result of deltaPtot = 104Pa, so the CFDresults of deltaPtot ~120130Pa are quite proper but a deltaPtot of ~386Pa isn't adequate!! > Problems with the fixedMean condition (OF 2.2.0) in combination with a very small pressure value (tested: 1e10, 1e30 and 0)!! Unfortunately, I can't upload the case because of its size of 1.8MB! //Markus 

August 18, 2013, 10:00 

#6 
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Niels Gjoel Jacobsen
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Hi Markus,
The reason I am asking about the nonorthogonalities is that the treatment of the nonorthogonal corrections have changed between the two versions [1], which could be the reason, why your see the large difference in pressure drop between 1.6ext and 2.2.0. As you will see in the checkMesh results, your mesh is nonorthogonal (not severely), but nonorthogonal corrections will be finite. Still, I agree with you that such a large impact from a small change in a boundary condition is worrying. Have you tried looking at the pressure at the inlet and outlets to see, whether the pressures are reasonably uniformly distributed? Kind regards Niels [1]: I have been told that the change in delta coefficients used in the discretisation of the Laplacian operators has had an impact on the nonorthogonal correction as well.
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August 18, 2013, 18:34 

#7 
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don lenardo
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Hy Niels,
there is no nonorthogonal correction in use for these calculations! The differences in static pressure between the two OFversions can be located both at the In and Outlet! //Markus 

August 19, 2013, 14:04 

#8 
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Daniel P. Combest
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I had a quick read of the discussion thus far....have you tried to use the fixedMeanValue BC in 2.2.x directly? There was one provided
Pressure outlet boundary condition Post 40 for a 2.2.x compatible version. that you can compile. I have not tested that version extensively in 2.2.x, but it will compile. Then you may be able to compare and eliminate some of the reasons or causes for the difference. 

August 19, 2013, 15:29 

#9 
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Niels Gjoel Jacobsen
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Good evening,
I went into the code, as it was a bit puzzling, and my previous thought are obviously redundant, so here it goes: Code:
File: OpenFOAM1.6ext/src/finiteVolume/fields/fvPatchFields/derived/fixedMeanValue/fixedMeanValueFvPatchField.C // Update the coefficients associated with the patch field template<class Type> void fixedMeanValueFvPatchField<Type>::updateCoeffs() { if (this>updated()) { return; } if (curTimeIndex_ != this>db().time().timeIndex()) { Field<Type>& patchField = *this; Field<Type> pif = this>patchInternalField(); patchField = meanValue_  gAverage(pif) + pif; curTimeIndex_ = this>db().time().timeIndex(); } fixedValueFvPatchField<Type>::updateCoeffs(); } Code:
File: OpenFOAM2.2.0/src/finiteVolume/fields/fvPatchFields/derived/fixedMean/fixedMeanFvPatchField.C template<class Type> void fixedMeanFvPatchField<Type>::updateCoeffs() { if (this>updated()) { return; } Field<Type> newValues(this>patchInternalField()); Type meanValuePsi = gSum(this>patch().magSf()*newValues) /gSum(this>patch().magSf()); if (mag(meanValue_) > SMALL && mag(meanValuePsi)/mag(meanValue_) > 0.5) { newValues *= mag(meanValue_)/mag(meanValuePsi); } else { newValues += (meanValue_  meanValuePsi); } this>operator==(newValues); fixedValueFvPatchField<Type>::updateCoeffs(); } Code:
gAverage(pif) I suppose that you use fixedMean in order to robustly handle such things as vortices approaching the outlet? This probably will not happen in your case, so could you try to use fixedValue on the pressure instead to check, whether it is that particular boundary condition or something else, where the answer is clouded by the behaviour of the boundary condition. Kind regards Niels
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August 21, 2013, 03:51 

#10 
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don lenardo
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Ok, with fixedValue p = 0 I got deltaPtot = 119Pa  the same value as with fixedMean p = 1e10 which leads also to the correct value of deltaPtot!
So how can I adptat the fixedMean condition in order to get a proper result with p = 0?? Because I'll need the fixedMean condition for my futher investigations. //Markus Last edited by lentschi; August 21, 2013 at 08:28. 

August 21, 2013, 08:20 

#11  
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don lenardo
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Quote:
//Markus 

August 21, 2013, 08:44 

#12 
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don lenardo
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Ok, with this adapted "fixedMeanValue" the result is deltaPtot = 120Pa...so this is similar to the 119Pa I have reached with p=1e10 in combination with fixedMean!!
Looks quite good! Has anyone shared experiences with this adpated "fixedMeanValue" for OF 2.2.x??? //Markus 

August 21, 2013, 12:23 

#13 
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Niels Gjoel Jacobsen
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OK, so as I read it, two values of the flow resistance are suggested as the true answer. Namely original 1.6ext computations and the recent fixedValue results. As neither compare well with the analytical result, I would suggest that you try the directMapped value on the inlet for U. This removes the development of the boundary layer through the pipe, so comparison with uniform pipeflow becomes easier.
After that you will have to decide on a formulation, which is consistent across an average value of 0. The implementation part will probably be the easy part. Kind regards, Niels
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