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Dimensional issues with dimensionless scalar

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Old   September 22, 2012, 03:54
Default Dimensional issues with dimensionless scalar
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Cameron
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Hi all
I have a feeling there is a very obvious solution to this, but I can't seem to find it for the life of me, i must not be searching for the right thing. I'm adding a new dimensionless scalar into the buoyantPimpleFoam solver, which uses a generic scalar transport equation. I've added it as a volScalarField just as shown in the examples here and here. In particular, the transport equation is:

{
fvScalarMatrix zfEqn
(
fvm::ddt(zf)
+ fvm::div(phi, zf)
- fvm::laplacian(alphaz, zf)
);
zfEqn.solve();
}

The issue is that every time I try to run the solver, I get an error saying that the dimensions don't agree (in particular the first section where [0 0 -1 0 0 0 0] + [1 -3 -1 0 0 0 0], i can change the dimensions of alphaz to make the second bit work no problems). I can clearly see that they don't, yet I can't work out how to make sure that they do. I can't just change the dimensions of zf as it is used in other equations where it is important that it is dimensionless. I tried adding another scalar variable to make phi dimensionless, but I kept having issues implementing it and thought there had to be a better way.

Last edited by c_dowd; September 22, 2012 at 04:15.
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Old   September 22, 2012, 07:02
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Florian
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You can turn off dimensions checking though it is not recommended. Set in your controlDict:

dimensionSet 0;
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Old   September 23, 2012, 01:03
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Cameron
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Ah thanks a lot. In fact I ended up just putting a new constant in the ddt part to make the dimensions equal but that's good to know.
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Old   September 23, 2012, 13:52
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Bernhard
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Quote:
Originally Posted by c_dowd View Post
Ah thanks a lot. In fact I ended up just putting a new constant in the ddt part to make the dimensions equal but that's good to know.
The dimension of alphaz should be m^2/s, because it is a diffusion parameter.
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