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- - **Schemes for scalar in LES**
(*http://www.cfd-online.com/Forums/openfoam-solving/59801-schemes-scalar-les.html*)

Hello,
I am running a LES tHello,
I am running a LES to simulate the dispersion of a passive contamimant. I have tried linearLimited01 schemes for the scalar transport equation but I still have negative values. Could you please inform me about the best schemes available in OpenFoam to chose to ensure a positive scalar? Thanks You Anne |

Use Gamma01 with the coefficieUse Gamma01 with the coefficient of 0.5 or 1. You also need to limit the Laplace scheme for that scalar, e.g.
laplacian(DkEff,k) Gauss linear limited 1; On bad meshes you may need to use a lower coefficient, say 0.5 That should do it, Hrv |

Forgot to say: temporal discreForgot to say: temporal discretisation may also give you unboundedness. A good way forward is Euler implicit (first order); if you really need second order you can try my new boundedBackward scheme.
Hrv |

Dear Hrvoje,
Thank you,
Dear Hrvoje,
Thank you, I will test your suggestions. Anne |

I want to try different ddt-scI want to try different ddt-schemes.
In order to see the choices I wrote "x": ------------------------------------------ Creating field DpDt --> FOAM FATAL IO ERROR : unknown ddt scheme x Valid ddt schemes are : 6 ( CoEuler boundedBackward CrankNicholson steadyState Euler backward ) file: /home/workdisc/OpenFOAM/pen-1.3/run/boundp3/system/fvSchemes::default at line 27. From function ddtScheme<type>::New(const fvMesh&, Istream&) in file /home/dm2/henry/OpenFOAM/OpenFOAM-1.3/src/finiteVolume/lnInclude/ddtScheme.C at line 80. FOAM exiting ------------------------------------------ When I try boundedBackward I get: ------------------------------------------ Creating field DpDt Starting time loop Time = 2e-06 Mean and max Courant Numbers = 0 0 diagonal: Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0 --> FOAM FATAL IO ERROR : unknown ddt scheme boundedBackward Valid ddt schemes are : 5 ( CoEuler CrankNicholson steadyState Euler backward ) file: /home/workdisc/OpenFOAM/pen-1.3/run/boundp3/system/fvSchemes::default at line 27. From function ddtScheme<type>::New(const fvMesh&, Istream&) in file /home/dm2/henry/OpenFOAM/OpenFOAM-1.3/src/finiteVolume/lnInclude/ddtScheme.C at line 80. FOAM exiting ------------------------------------------ Why is boundedBackward no longer on the list? |

Easy: a bounded backward schemEasy: a bounded backward scheme I wrote is there for scalars only: you cannot define what "bounded" means in terms of a vector or a tensor. Therefore, I cannot define a bounded second-order scheme for a vector.
In order to do this well, you will need to specify a scheme on a per-ddt-term basis. Set default to none and the code will ask you for a scheme one ddt term at a time. Hrv |

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