- **OpenFOAM**
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- - **how can use the LES filter? (laplace filter and anistropic filter)**
(*http://www.cfd-online.com/Forums/openfoam/95796-how-can-use-les-filter-laplace-filter-anistropic-filter.html*)

how can use the LES filter? (laplace filter and anistropic filter)Our thease is about filtration in LES.
We study LESfilters codes (LESfilter,NewFilter,Simple, Laplace,anistropic) but we have following problem about them: 1-Algorithm and Formulation of filtration that used in these codes. 2-According to the discription part of laplacefilter.H and anisotropicfilter.H : Description Kernel as filter as Test filter with ratio 2 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~ Box filter: g = delta2/24 -> g = delta2/6 Spherical box filter: g = delta2/64 -> g = delta2/16 Gaussian filter: g = delta2/24 -> g = delta2/6 what is "g" parameter and where is the effect of "g" in these codes. 3-What is "widthcoeff_" that used in these codes? We REALLY need the answer of these question.thanks for your help. |

The implementation of the filter can be found for example in de laplacaFilter.C file. It is defined in the operator() for different field types. This is smart, since if you somewhere define
Code:
`laplaceFilter filter_;` Now if you follow the coeff_ in this piece of code, you will find how widthCoeff_ is used. With respect to your second question, I am not so familiar with the Laplace filter, so I cannot help you there. |

Hello,
Could I kindly ask you which filter type is used in LES of incompressible flows OpenFoam? In fact, we don't set any filter in LESproprties for interPhaseChangeFoam. Regards |

Hi Ehsan,
I think the available filters in Foam (simple, laplace, anisotropic,...) are in fact the filters for the dynamic models (synSmagorinsky, dynOneEqEddy, maybe dynLagrangian,...). In fact, these are the filter for the test level, not the grid level. With oneEqEddy or Smagorinsky, there is only one level, so only one filtering, chosen by delta (PrandtlDelta, cubeRootVolDelta, smoothDelta, maxDeltaxyz).Please, correct me if I am wrong! ;) Sincerely, Djub |

Filter for Smagorinsky modelHi Djub
I am trying to simulate a roundjet. I am using Smagorisnsky model and wish to use a Gaussian filter. According to you the filter is dictated by delta. So how can i implement a Gaussian filter for my problem? Which 'delta' will give a fairly Gaussian filter? Thank you |

Trying to helphi
arunsmec,OK, it seems I was not exact. Not wrong neither, but not exact... delta is the SIZE of the filter, not his SHAPE. The LESfilter can act on the both levels (subgrid and dynamic):- simpleFilter acts only on the dynamic level, with a simple top-hat filter - laplaceFilter and anisotropicFilter can affect both levels: Kernel as filter as Test filter with ratio 2 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~ Box filter: g = delta2/24 -> g = delta2/6 Spherical box filter: g = delta2/64 -> g = delta2/16 Gaussian filter: g = delta2/24 -> g = delta2/6 Thus, you should be able to use a Gaussian filter, changing the kernel. but I don't know how... |

For LES, OpenFOAM adopts the so-called
implicit filtering technique, i.e., the equations are implicitly filtered by the discretization schemes and computational grid: no explicit filtering is applied. In such technique, the filter width (which appears in the SGS models) is typically related to a certain measure of the local grid cell size, which can be chosen via the 'delta' parameter; a pretty usual choice is to take the cube root volume of the local cell.However, some SGS models, for instance those belonging to the dynamic family, require two levels of filtering. When using those models, the first level of filtering is implicit (as usual), whilst the second one (the so-called test filter) has to be performed explicitly by means of one of the discrete forms of filters provided. |

Hello
Could any one says what is the purpose of: pow(mesh.V(), 2.0/3.0)/widthCoeff_ in laplcaFilter.C. Regards |

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