[Friendly]

Hi,

I would like to know what exactly is meant by fluctuation scale in OpenFOAM. My goal is to create a turbulent inlet with fluctuation velocity.

For this I need to define a fluctuation sclale, but I dont know whicht value would be appropriate, because I dont know which is meant by that.

This is from turbulentInletFvPatchField.H:

Class
     Foam::turbulentInletFvPatchField

 

 Group

     grpInletBoundaryConditions

 

 Description

     This boundary condition generates a fluctuating inlet condition by adding

     a random component to a reference (mean) field.

 

     \f[

         x_p = (1 - \alpha) x_p^{n-1} + \alpha (x_{ref} + s C_{RMS} x_{ref})

     \f]

 

     where

 

     \vartable

         x_p     | patch values

         x_{ref} | reference patch values

         n       | time level

         \alpha  | fraction of new random component added to previous time value

         C_{RMS} | RMS coefficient

         s       | fluctuation scale

     \endvartable

 

 Usage

     \table

         Property     | Description             | Required    | Default value

         fluctuationScale | RMS fluctuation scale (fraction of mean) | yes |

         referenceField | reference (mean) field | yes        |

         alpha | fraction of new random component added to previous| no| 0.1

     \endtable

 

     Example of the boundary condition specification:

     \verbatim

     <patchName>

     {

         type            turbulentInlet;

         fluctuationScale 0.1;

         referenceField  uniform 10;

         alpha           0.1;

     }

     \endverbatim

[fisichel]

Hi,

You did the right thing by pulling up the header file. In that equation they typed, they are multiplying the RMS coefficient by the scaling value. I think 1 would be a good start. I see they example later on they set the scaling value to 0.1. I don't have an answer for that except to say they are using that parameter to scale their turbulence in a convenient manner.

Best Regards,
Chris

Originally Posted by Friendly

Hi,

I would like to know what exactly is meant by fluctuation scale in OpenFOAM. My goal is to create a turbulent inlet with fluctuation velocity.

For this I need to define a fluctuation sclale, but I dont know whicht value would be appropriate, because I dont know which is meant by that.

This is from turbulentInletFvPatchField.H:

Class
     Foam::turbulentInletFvPatchField

 

 Group

     grpInletBoundaryConditions

 

 Description

     This boundary condition generates a fluctuating inlet condition by adding

     a random component to a reference (mean) field.

 

     \f[

         x_p = (1 - \alpha) x_p^{n-1} + \alpha (x_{ref} + s C_{RMS} x_{ref})

     \f]

 

     where

 

     \vartable

         x_p     | patch values

         x_{ref} | reference patch values

         n       | time level

         \alpha  | fraction of new random component added to previous time value

         C_{RMS} | RMS coefficient

         s       | fluctuation scale

     \endvartable

 

 Usage

     \table

         Property     | Description             | Required    | Default value

         fluctuationScale | RMS fluctuation scale (fraction of mean) | yes |

         referenceField | reference (mean) field | yes        |

         alpha | fraction of new random component added to previous| no| 0.1

     \endtable

 

     Example of the boundary condition specification:

     \verbatim

     <patchName>

     {

         type            turbulentInlet;

         fluctuationScale 0.1;

         referenceField  uniform 10;

         alpha           0.1;

     }

     \endverbatim

[Friendly]

Hi,

thanks for your reply.

1 seems big for fluctuations, but I may be wrong, it depends on what is meant by that. I have the assumption that s (fluctuation scale) is u', u'u' or sqrt(1/3ux'ux'+uy'uy'+uz'uz'). But I couldnt find any hints in the web.

Does nobody use the turbulentInlet boundary condition?

[ht2017]

Originally Posted by Friendly

Hi,

thanks for your reply.

1 seems big for fluctuations, but I may be wrong, it depends on what is meant by that. I have the assumption that s (fluctuation scale) is u', u'u' or sqrt(1/3ux'ux'+uy'uy'+uz'uz'). But I couldnt find any hints in the web.

Does nobody use the turbulentInlet boundary condition?

Did You have resolved this problem? Now I am facing a similar your problem. Please give me some advice if you have a solution.

[cryabroad]

It's a percentage, the actual fluctuating value is whatever you have for the scale times the mean value.

From my experience 0.05, or 5% is a good value to start.

[ht2017]

Originally Posted by cryabroad

It's a percentage, the actual fluctuating value is whatever you have for the scale times the mean value.

From my experience 0.05, or 5% is a good value to start.

could you explain detail that? I still do not understand your advice. sorry for my rookie question.