CFD Online Discussion Forums - a mark for filter size (delta) in LES

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Dear guys
Hi

I want to write the sub-grid scale mixture fraction variance expression in Open FOAM. In this expression, we have delta that means filter size. Would somebody PLZ hint me that How do we write filter size in Open FOAM?

(merged from another post)

I want to write the expression in LES for mixture fraction variance. In this expression we have filter size (delta). would somebody hint me how can I write filter size in OpenFOAM format?

Regards
Bobi

Greetings Bobi,

Please provide more details on what exactly you want to do.

Best regards,
Bruno

Hi Bruno

As you might Know, libOpenSmoke and its modified versions does not support LES solver, I am trying to write a solver that involves LES solution and 80% I was successful. There was a failure in my thermochemical data (not velocity or mixture fraction fields). I have found that it is because I have used a transport equation for mixture fraction variance. Instead I should have exerted a modeling formulation. Fortunately fireFoam (which has a kind of flamelet solver: FSD) has implemented this modelling formulation in a way. I am trying to implement it into modified OpenSmoke to finally create the LES solver . This is what I found in FSD.H file:

Code:

// Calculation of the mixture fraction variance (ftVar)

    const compressible::LESModel& lesModel =

        YO2.db().lookupObject<compressible::LESModel>("LESProperties");



    const volScalarField& delta = lesModel.delta();

    const volScalarField ftVar(Cv_*sqr(delta)*sqr(mgft));

As it can be seen lesModel.delta() identifies what I mean i.e. filter size (delta)

Regards
Bobi

Hi Bobi,

My guess is that this delta size is actually defined in the dictionary "constant/LESProperties".

You can find several examples on OpenFOAM, by running:

Code:

find $FOAM_TUTORIALS -name "LESProperties"

And if I understand things correctly, "delta()" is actually a field of the same dimensions of the mesh, namely the same size as the pressure field; therefore, I think the "delta size" in OpenFOAM is actually the values of "delta" at each cell centre. And if this interpretation is correct, then it's as I wrote above: this is configurable in the "constant/LESProperties" dictionary, without any need to hack directly into the code :)

By the way, the OpenFOAM User Guide is a bit vague on how these are used:

basically they also tell us to look at the tutorials :D

Best regards,
Bruno

Hi Bruno

I am going to take a deep look at your hints. Many thanks for your kind reply.

Regards
Bobi

Hi all.
Greeting!

I am also trying to access LESDelta.

Actually in the transport equation of variance of mixture fraction (varZ) it is required for calculating scalar dissipation of mixture fraction.

When I type delta or LESDelta an error appear saying : "delta/LESDelta is not declared in this scope".

Can anyone please suggest how to access it.

Thanks and regards
Sandeep Lamba

Hi Sandeep,

I am also trying to access LESDelta to solve scalar dissipation rate. Did you solve this problem?

Regards,

Shang

Hi Wang Shang,
I could not actually access the LESDelta directly but I solve the problem by locally calculating Delta from local cell volume by accessing mesh properties.

regards