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-   -   divScheme div(phi,U) Gauss linear behaves unexpected (http://www.cfd-online.com/Forums/openfoam-solving/122300-divscheme-div-phi-u-gauss-linear-behaves-unexpected.html)

docinger August 16, 2013 09:18

divScheme div(phi,U) Gauss linear behaves unexpected
 
1 Attachment(s)
Hi everybody!
After reading in this awesome forum for months now, i'd like to actively post my first question today ;)

I'm using openfoam / pimpleDyMFoam to solve incompressible flow around NACA airfoils (there'll be more details about my project in the future ...)

And i found a unexpected behaviour in the flowfields when i switch divSchemes in fvSchemes for div(phi,U) from Gauss linearUpwind grad(U) to Gauss linear

Attachment 24469

In the attached image you can see the completely "chopped" velocity field.
I use a 2D hexa mesh.

Can somebody explain to me why this happens when i use the Gauss linear scheme?

(And also can somebody tell me what the "grad(XXX)" behind the schemes is for? I seem to be incapable of finding the answer myself)

Thanks and welcome to myself,
Andreas

yhy20081016 October 19, 2013 03:56

"linear" scheme is unbounded, therefore it is usually unstable, unless the local Peclet number is less than 2. You can refer to the section titled "Divergence schemes" in "UserGuide.pdf". I suggest that you try "linearUpwind" scheme.

docinger October 19, 2013 05:15

yes, linearUpwind was what i started with and what I ended up using. The simulations run smooth by now and provide plausible results.

With the Peclet number dependency you mentioned, that would mean that my mesh simply is too coarse to run with linear / central differences, right?

Thanks,
A


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