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how can use the LES filter? (laplace filter and anistropic filter) 

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January 3, 2012, 10:20 
how can use the LES filter? (laplace filter and anistropic filter)

#1 
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ethan oh
Join Date: Feb 2011
Posts: 6
Rep Power: 6 
Our thease is about filtration in LES.
We study LESfilters codes (LESfilter,NewFilter,Simple,Laplace,anistropic) but we have following problem about them: 1Algorithm and Formulation of filtration that used in these codes. 2According 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. 3What is "widthcoeff_" that used in these codes? We REALLY need the answer of these question.thanks for your help. 

January 3, 2012, 11:14 

#2 
Senior Member
Bernhard
Join Date: Sep 2009
Location: Delft
Posts: 790
Rep Power: 12 
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. 

February 26, 2012, 06:25 

#3 
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ehsan
Join Date: Mar 2009
Posts: 92
Rep Power: 8 
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 

July 18, 2012, 11:16 

#4 
Senior Member
Julien
Join Date: Jun 2012
Location: France
Posts: 153
Rep Power: 5 
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 

November 7, 2012, 01:09 
Filter for Smagorinsky model

#5 
New Member

Hi 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 

November 7, 2012, 05:11 
Trying to help

#6 
Senior Member
Julien
Join Date: Jun 2012
Location: France
Posts: 153
Rep Power: 5 
hi 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 tophat 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... 

November 28, 2012, 17:04 

#7 
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Francesco Capuano
Join Date: May 2010
Posts: 78
Rep Power: 7 
For LES, OpenFOAM adopts the socalled 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 socalled test filter) has to be performed explicitly by means of one of the discrete forms of filters provided. 

February 7, 2014, 17:13 

#8 
Senior Member
Join Date: Nov 2012
Location: Bavaria
Posts: 144
Rep Power: 4 
Hello francesco_capuano,, Hello Djub!
In case I would like to simulate transition from laminar to turbulent flow I guess I have to choose a dynamical sgs model like dynSmagorinsky. That way I ensure that in different flow regimes the appropriate value of the coefficient (SmagorinskyCoefficient C) is used, instead of some damping function (van Driest). condition: fine grid (in the direction I want to do explicit filtering). Does this mean in case the grid spacing is fine enough I don't have to care neither for wall functions nor for damping functions? And in case I have a coarse grid I don't have to care for damping functions but I have to care for wall functions? Do I have to set one of the availabe filters ( laplaceFilter, simpleFilter or anisotropicFilter) only in the case I use a dynamic LESmodel (dynSmagorinsky)? @Djub You've written that is the size of the filter. I've seen in an example case ( that the LESModel and delta are defined in the LESProperties file with help of appropriate keywords and in the same tutorial case the defined solver is pisoFoam. How does OpenFoam account for in the NavierStokes equations which results from analytical procedure of filtering? How is the filtered nonlinear convective term of momentum equation incorporated in the solution procedure? pisoFoam is a solver for transient incompressible flow, initially without containing an additional shear stress term that accounts for turbulence effects? Are the stress components of the tensor computed with help of Smagorinsky's closure idea: ? ...and somehow add (?) to the discretized equations? In case of high Re: = constant  no dynamik model  close to wall: damping function necessary  if mesh is coarse > wall functions necessary In case of low Re + wall treatment + high Re: = computed tensor  dynamik model necessary (to account for all situations)  therefore no damping function necessary  wall function > depending on mesh resolution (yes if mesh=coarse, no if mesh=fine) ???? To what extent do the discretization schemes in fvSchemes influence the choosen LESmodel??? How exactly is the filter technique defined? I thought the transfer function of the filter function is determined by the energy spectrum of the flow. So I thought depending on the spectrum you receive a transfer function and this one leads to the Coefficient C respectively to the filter width ??? But in OpenFoam the user is prompted to decide the filter size . How is the user defined filter size processed in OpenFoam? Is the spectrum determined by the choice of the LESmodel? I really hope for your expert support! Thanks. Aylalisa 

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filter, laplace, les 
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