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Warnings on Fluent 15

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Old   January 16, 2015, 08:28
Default Warnings on Fluent 15
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Hello,

I'm receiving these two warnings on Fluent 15.0.0:

1- udf_names.c(56) : warning C4113: 'void (*)()' differs in parameter lists from 'void (*)(void)'
2- warning: mass diffusivity: invalid (zero) diffusivity

The first occurs during compilation and started just after I changed from version 14.5.7 to 15.0.0. In the process I had to change from VS2013 to VS2012 because VS2013 was not working with version 15.0.0.
The second appeared when I replaced the mass diffusivity (before was a constant value) by this formula:

D_eff = D*(porosity*(1-water_volume_fraction))^1.5

This is an expression for effective diffusivity on porous media that accounts for the effect of liquid water (typically used in PEM fuel cells modeling).

It seems both warnings do not affect my results, but I might be missing something...

Does anyone know what this warnings really mean and if they have implications on the results?

Regards,
Rui
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Old   January 16, 2015, 10:27
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Hi

I have no idea about the first question, but I can try to say something to 2.

For numerical codes diffusivity of 0 is a problem. The problem you have is that the matrix can become singular. This is not a problem for transient simulations, but in steady state ones in regions with very small velocities the coefficients in the matrix become 0 (or close to 0), which can make your simulation crash.
Plus for a poisson equation a zero or negative diffusion crashes the simulation immediately.

You can easily fix the problem by changing the D_eff to something like
D_eff = max(D_eff, 1e-20)

Regards
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Old   January 16, 2015, 17:10
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Thanks Marcus.

You are right, for transient simulations there is no problem, but when I try steady-state cases, the simualtion diverges unexpectedly, without an increase in the residuals.

I will follow your advice and set a minimum value for D_eff.

Regards
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Old   January 19, 2015, 11:35
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For the first warning, I'll just say that I always get these in my UDFs, and it's never caused any problems. I use Visual Studio 2010, and got these warnings both in 14.5.0 and 15.0.0.
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Old   January 19, 2015, 16:29
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Thanks Andrew,

I also couldn't find any problems so far.

Good to know that you got this error also with VS2010. I was planning to try it instead of VS2012 to check if this warning would disappear. I see it wouldn't work.
So thanks, you save some time

Regards
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