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Calculate variable div zero

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Old   January 16, 2008, 13:44
Default Hi, a pretty simple one... I
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Fabian Braennstroem
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Hi,
a pretty simple one... I hope...

I would like to create a new postprocessing variable using:

volScalarField test
(
IOobject
(
"test",
runTime.timeName(),
mesh,
IOobject::NO_READ
),

(fvc::grad(U))/mag(fvc::laplacian(U.component(i)))
);
test.write();

This works fine as long as U.component(i) has some kind of value, but if the denominator is zero I get obviously an error. Can anyone give me an advice, how an 'if' question could look like? I tried somehting like: if (U.component(i)==0.0){...}

but I get:

test.C: In function 'int main(int, char**)':
test.C:128: error: expected primary-expression before 'if'

Fabian
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Old   January 21, 2008, 12:08
Default Hi, me again... does nobody
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Fabian Braennstroem
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Hi,
me again... does nobody has an idea!?

Regards!
Fabian
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Old   January 21, 2008, 12:41
Default General practice is to add a s
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Eugene de Villiers
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General practice is to add a small constant to the denominator:

fvc::grad(U)/(smallU + mag(fvc::laplacian(U.component(i))

where smallU is something like:

dimensionedScalar("smallU", dimensionSet(0,0,-1,0,0), SMALL);
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Old   January 21, 2008, 13:07
Default Your "if" statement would not
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Gavin Tabor
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Your "if" statement would not work because U.component(i) would never be identically equal to 0.0. Eugene is correct; the usual thing is to add a small value to the denominator. However you might also want to think about what the value of the numerator is when the denominator is zero. If the denominator is non-zero, then the expression is diverging at that point; you might want to think whether your modelling is correct at this point.
introducing smallU on the denominator will set this to an artificial (if quite large) number - is this what you want?

On the other hand, if the denominator is also zero, you have 0/0 from the fraction. This is not zero; it is UNDEFINED. The fraction may in fact tend towards a non-zero value; you need to look at 1st and higher derivatives of the numerator/denominator to determine this. Again, using smallU will fix the value to zero, which may not be what you want in practice.

Gavin
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Old   January 21, 2008, 14:10
Default Hi Eugene, thanks, good ide
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Fabian Braennstroem
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Hi Eugene,

thanks, good idea! It works with a small adjustmentdimensionedScalar smallU ("smallU", dimensionSet(0,1,-1,0,0), SMALL);
. Is there any recommendation where to put the smallU declaration? I put it on top of the .C file!?

Regards!
Fabian
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Old   January 21, 2008, 15:14
Default Hi Gavin, I was a bit to sl
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Fabian Braennstroem
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Hi Gavin,

I was a bit to slow submitting the post... Thanks for your help. The SMALL number will actually work for my case, but is there a chance to use some kind of range for the if-condition in this 'template' (this is an template, isn't?)?

Fabian
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Old   January 22, 2008, 09:37
Default I normally put stuff like that
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Eugene de Villiers
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I normally put stuff like that in "createFields.H", but it doesn't really matter.
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Old   January 24, 2008, 15:55
Default Thanks!
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Thanks!
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Old   January 28, 2008, 11:02
Default Thanks!
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Thanks!
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