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-   -   Unstable flow simpleFoam 2nd order (http://www.cfd-online.com/Forums/openfoam/67780-unstable-flow-simplefoam-2nd-order.html)

 Valle August 26, 2009 08:12

Unstable flow simpleFoam 2nd order

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Hello everyone,

I'm a newbie in OpenFoam calculation, so I try to compare CFX with OpenFoan results in order to delve myself into OpenFoam numerics. For that I choosed a simple geometry and "simple" physics (simpleFoam) to calculate a TT-junction.
The turbulence model is k-epsilon.
The time discretization was fixed to deltaT = 0.001;End Time=10.
All schemes are Gauss linear (corrected for laplacian), except for div(phi,k) > Gauss upwind and div(phi,epsilon) > Gauss upwind;
Tetrahedral mesh.

After calculating and comparing the problem (figures CFX/ OpenFoam), I determined velocity spots in the results of the OpenFoam calculation which don't existed in the CFX calculation. In order to understand this difference between OpenFoam and CFX I changed nearly all parameters in the fvSolution and fvSchemes, but the results were always nearly the same. These spots are always present and I don't really understand why this difference between OpenFoam and CFX.

In my opinion, the velocity spots aren't caused by Postprocessor interpretation, because calculating the problem 1st order, (div(phi,U) > Gauss upwind) no velocity spots could be determined during the complete calculation. Another hint for this discretization problem was the residuum developing (figure Screenshot). When switching to 2nd order div(phi,U) > Gauss linear) the residuum increased and started fluctuating which indicates the 1st/ 2nd order problem in simpleFoam, too. Plotting the results of the 2nd order calculation showed a high unstable flow for a "simple" geometry and physics.
So I tried numerically to stabilize the calculation, but no chance whatever I tried... Should I calculate transient???
It can't be possible that a 2nd order calculation of an incompressible and isothermal problem is so hard to get it converge...

Can someone help me to understand this difference in the results?

Regards
Valle

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