[marialhm]

Hi dear all,

I have a model in which the inlet velocity is in x direction, while random perturbations in Y direction is needed. What kind of boundary condition or functions should I use, and how?

Thanks in advance!

Maria

[tomf]

Hi Maria,

I think you should look at either groovyBC (from swak4Foam) or turbulentInlet. I guess the latter would do just fine. Please note that there may be some pressure fluctuations introduced and when the fluctuations are too large you may get problems with convergence..

Regards,
Tom

[Tobi]

Hi Maria and Tom,

I think Tom's idea is one of the easiest and simplest. For competence, you also could use a codedFixedBC, in which you define your x-value as a constant and the y-value could be build randomly using the Random class of OpenFOAM. However, using groovyBC should be more easier (depend on the point of view and your programming knowledge). I just wanted to mention this way too.

Cheers.

[marialhm]

Thanks, Tom and Tobias,

I tried to use groovyBC, but I dont know which random function I should use. I tried random(), but it seems Openfoam does not recognize it.

Also, for turbulentInlet or codedFixedBC, should I install more subroutines?

Best,

Maria

[Tobi]

Hi,

the random functionality within groovy is a function of groovy. So it should do the work. Right now I cannot test it - so I am sorry. The codedFixedBC is named codedFixedValue in OpenFOAM and is an boundary condition which already exists. In the header file (finiteVolume/fields/fvPatchFields/derived/codedFixedValue/) you will find an example of how to use it.

Out of the box an example which I did not tried but should give you some hints:

myPatch
{
    type codedFixedValue;
    value uniform (0 0 0);
    name myOwnBC;

    code
    #{
        const Random randObj(time(NULL));      
        const scalar Uy = randObj.scalar01()*10;
        const vector U = vector(1,Uy,0);

        operator==(U);
    #};
}

Here we build a Random function which is everytime different (after calling) based on time(NULL). Then we make the Uy value and after the U vector. Keep in mind that this will not make a perputation because Uy is on each face similar. So you have to make a scalarField which corresponds to the faces. Sourceflux made some nice example how to do that (http://www.sourceflux.de/blog/the-co...ary-condition/).

However, based on the fact that I am using the Random function too, I guess it is not the best idea using that code above because the initialization of the RandomObj is slow because we build some X vector etc. (http://pubs.opengroup.org/onlinepubs...h/drand48.html).

I think groovy is much faster here but I wanted to show you how to make it with the codedFixedValue boundary condition.

GroovyBC

type            groovyBC;
variables     "Uy=random(); Ux=10;";
valueExpression "vector(Ux,Uy,0)";
value           uniform (Ux 0 0);

But again, here I guess the Uy distribution is similar to each face. I guess there are possible ways to get rid of that and to make each face randomly. Sorry that I am limited in that kind of question.

[marialhm]

Originally Posted by Tobi

Hi,

the random functionality within groovy is a function of groovy. So it should do the work. Right now I cannot test it - so I am sorry. The codedFixedBC is named codedFixedValue in OpenFOAM and is an boundary condition which already exists. In the header file (finiteVolume/fields/fvPatchFields/derived/codedFixedValue/) you will find an example of how to use it.

Out of the box an example which I did not tried but should give you some hints:

myPatch
{
    type codedFixedValue;
    value uniform (0 0 0);
    name myOwnBC;

    code
    #{
        const Random randObj(time(NULL));      
        const scalar Uy = randObj.scalar01()*10;
        const vector U = vector(1,Uy,0);

        operator==(U);
    #};
}

Here we build a Random function which is everytime different (after calling) based on time(NULL). Then we make the Uy value and after the U vector. Keep in mind that this will not make a perputation because Uy is on each face similar. So you have to make a scalarField which corresponds to the faces. Sourceflux made some nice example how to do that (http://www.sourceflux.de/blog/the-co...ary-condition/).

However, based on the fact that I am using the Random function too, I guess it is not the best idea using that code above because the initialization of the RandomObj is slow because we build some X vector etc. (http://pubs.opengroup.org/onlinepubs...h/drand48.html).

I think groovy is much faster here but I wanted to show you how to make it with the codedFixedValue boundary condition.

GroovyBC

type            groovyBC;
variables     "Uy=random(); Ux=10;";
valueExpression "vector(Ux,Uy,0)";
value           uniform (Ux 0 0);

But again, here I guess the Uy distribution is similar to each face. I guess there are possible ways to get rid of that and to make each face randomly. Sorry that I am limited in that kind of question.

I used "Uy=random(); ", but it seems Openfoam doesnt recognize "random()", did I miss something?
Also, the codedFixedValue BC in your reply doesn not work in my case. Should I add some libs?

[tomf]

Hi,

Just to complete the options, the turbulentInlet option would be like this (version 4.x). No additional libraries are needed.

    
    inlet
    {
        type            turbulentInlet;
        referenceField  uniform (10 0 0);
        fluctuationScale (0 0.01 0);
        value           uniform (10 0 0);
    }

The source code tells us that

fluctuationScale is RMS fluctuation scale (fraction of mean)

Regards,
Tom

[marialhm]

Originally Posted by tomf

Hi,

Just to complete the options, the turbulentInlet option would be like this (version 4.x). No additional libraries are needed.

    
    inlet
    {
        type            turbulentInlet;
        referenceField  uniform (10 0 0);
        fluctuationScale (0 0.01 0);
        value           uniform (10 0 0);
    }

The source code tells us that

fluctuationScale is RMS fluctuation scale (fraction of mean)

Regards,
Tom

Thanks, It seems good. I will try this.

[jorkolino]

Originally Posted by tomf

Hi,

Just to complete the options, the turbulentInlet option would be like this (version 4.x). No additional libraries are needed.

    
    inlet
    {
        type            turbulentInlet;
        referenceField  uniform (10 0 0);
        fluctuationScale (0 0.01 0);
        value           uniform (10 0 0);
    }

The source code tells us that

fluctuationScale is RMS fluctuation scale (fraction of mean)

Regards,
Tom

However this usage will use the same seed so the results will replicate if you make another run.

[Zitzeronion]

Originally Posted by Tobi

Hi,

the random functionality within groovy is a function of groovy. So it should do the work. Right now I cannot test it - so I am sorry. The codedFixedBC is named codedFixedValue in OpenFOAM and is an boundary condition which already exists. In the header file (finiteVolume/fields/fvPatchFields/derived/codedFixedValue/) you will find an example of how to use it.

Out of the box an example which I did not tried but should give you some hints:

myPatch
{
    type codedFixedValue;
    value uniform (0 0 0);
    name myOwnBC;

    code
    #{
        const Random randObj(time(NULL));      
        const scalar Uy = randObj.scalar01()*10;
        const vector U = vector(1,Uy,0);

        operator==(U);
    #};
}

This solution works for me with minor modification, see below

myPatch
{
    type codedFixedValue;
    value uniform (0 0 0);
    name myOwnBC;

    code
    #{
        const Random randObj(1234);      
        const scalar Uy = randObj.sample01<scalar>()*10;
        const vector U = vector(1,Uy,0);

        operator==(U);
    #};


     codeInclude
     #{
         #include "Random.H"
     #};

 }

One thing is that scalar01 is now sample01 and it needs the <type>. The other thing is the codeInclude, which ensures that Random.H is linked properly (without it I couldn't make it work). The 1234 in randObj is just to set a fixed seed.