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November 18, 2011, 07:24 
OF ignores relaxation factor

#1 
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Hi foamers,
I'm calculation temperature fields (steadystate) using laplacianFoam. Now I need to force underrelaxation in order to obtain convergence  but OpenFoam does not consider the relexation factor I define in fvSolution by relaxationFactors { T 0.7; } What could be wrong? Regards Hugo 

November 18, 2011, 12:39 

#2 
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Steven van Haren
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there is no call to the relaxation function in laplacianFoam
This function needs to be called and will then read the relaxationfactor from the dictionary. But why do you need relaxation for this simple diffusion equation? 

November 18, 2011, 15:15 

#3  
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Bernhard Gschaider
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Quote:
In general: if you're unsure whether your relaxationFactor is being read replace it with an x (or your name). If the solver fails, then you're sure that it is being read 

November 21, 2011, 03:23 
Relaxation in laplacianFoam

#4 
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Thank you very much for your remarks!
I have got a groovyBC which calculates the gradient depending on the wall temperature (simulating steadystate heat flow through an insulation which is beyond the OpenFOAM domain). When this "virtual" insulation is relatively thin, huge temperature gradients occure at the boundary  and convergence is not reached. Thus, I think that underrelaxation should allow convergence. Is it difficult to add relaxation to the laplacianFoam solver? Which solver is the simplest one with relaxation which I can use as an example? Regards Hugo 

November 21, 2011, 18:38 

#5  
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Bernhard Gschaider
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Quote:
solve( <the eqn >); write fvScalarMatrix TEqn( < the eqn >); TEqn.relax(); TEqn.solve(); 

November 22, 2011, 10:35 

#6 
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Thank you so much, Bernhard. I created own_laplacianFoam including the following algorithm:
for (int nonOrth=0; nonOrth<=simple.nNonOrthCorr(); nonOrth++) { fvScalarMatrix TEqn ( fvm::ddt(T)  fvm::laplacian(DT, T) ); TEqn.relax(); TEqn.solve(); } Unfortunately, the relaxation factor is still not read. Do you have any idea? Regards Hugo 

November 22, 2011, 15:17 

#7  
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Bernhard Gschaider
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Quote:
Or do you mean "is rad but has no effect"? 

November 23, 2011, 03:28 

#8 
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Your are right  actually, it does not have any effect, but it's read.
I checked if I'm using the right binary by adding a message output to own_laplacianFoam  it is displayed when I run the solver. My entry in fvSolution is relaxationFactors { T 0; } and if I change "0" to "b", I get an error  hence, it is read. But what could be the reason that the relaxationFactor is not taken into account by the solver? 

November 23, 2011, 18:15 

#9  
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Bernhard Gschaider
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Quote:
Or, in other words: your relaxation parameter would give a superstable solution: nothing changes at all (as the new solution would not be considered) 

November 24, 2011, 03:37 

#10 
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1. After 10 iteration steps, the results are identical both with relaxation factor 0 and 1  but different from the results after one step (but with the factor 0 considered, the results after all iteration steps would be the same if I understand it in the right way)
2. During calculation, the output is "Initial residual = 1" for time=1, "Initial residual = 0.75..." for time =2 and so on both with relaxation factor 0 and 1  but I expect the initial residual remaining 1 for every time step when the relaxation factor 0 is considered? 

January 13, 2012, 12:52 

#11 
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Illya Shevchuk
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Hi,
I have the same problem. In my PIMPLEbased solver I try to relax the TEqn. In the outer iteration cycle I have: Code:
{ fvScalarMatrix TEqn (...); TEqn.relax(); eqnResidual = TEqn.solve().initialResidual(); } Code:
relaxationFactors { T x; } Best regards, Illya 

May 10, 2012, 13:25 

#12 
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Alton Luder III
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I had similar problems in 2.0.x.
It seems that for incompressible flow the relxation factors are specified as: relaxationFactors { p 0.3; U 0.7; } while in multiphase they must be specified: relaxationFactors { "p.*" 0.3; "U.*" 0.7; } You can add some info statements before and after UEqn.relax() to check that your code is indeed finding the relaxation factors. (Or just hard code using UEqn.relax(0.7) ) It looks like 2.1 has changed the format to relaxationFactors { fields { "p.*" 0.3; } equations { "U.*" 0.7; } } So good luck finding your answer. 

May 10, 2012, 14:11 

#13 
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Bernhard Gschaider
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No need to change the solver. Just replace the relaxation factor with "x" (or your name). If the solver fails ("not a scalar" or something like this) then the factor is read


May 10, 2012, 16:18 

#14 
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Alton Luder III
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That trick only checks that the value is getting read in right, not that its actually getting used properly inside the solver.
For instance, it won't tell you if you put a relaxation factor on "p" when only the "p_rgh" field can be relaxed. Speaking of this problem, do any of you know a way to have to the solver output what field names can use an underrelaxation factor? 

May 10, 2012, 19:26 

#15  
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Bernhard Gschaider
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Quote:
Quote:
Maybe (but I havn't tried it) a variation of the above might word. Set a "catch all" regular expression in the relaxation factors: ".+" x; and remove all other entries. Then the first relaxation should fail. Add a "correct" (==numeric) factor for that field and retry. As soon as the solver doesn't fail you found all relaxations for that solver 

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