Hello Ying,

giving the ratio between momentum diffusivity and thermal diffusivity (conductivity) the Prandtl number is defined not only at the wall but throughout your flow field, and can be used in CFD for computing the thermal conductivity from the values of viscosity and specific heat.

You can create a user defined field function for the thermal conductivity, which is defined as

* ${DynamicViscosity} * ${SpecificHeat}

and set it as an input for your thermal conductivity at the physics continua level.

Regards,

Christian

http://en.wikipedia.org/wiki/Prandtl_number

 Thank Christian,

I' m a green hand of Star CCM+. I spend some time to understand what you said and  the user guide about user code and field funciton, but i still have some queations.

1. Now i know what's the meaning of the prt, but if i use standard k-e model and set like this"region-fluid- boundary-my wall boundary" in Star CCM+, i will find   E and kappa, the coefficients of  dimensionless temperature function T+. I dont know the relation between E, kappa and Prt in Star CCM+. I just know a funtion of T+ in fluent. It is  about Molecular prandtl numbe, turbulent prandtl number, E and kappa. That is to say what is the function of Prt  in Star CCM+?

2. I just know Prt=DynamicViscosity * SpecificHeat, how can i set the defined field function as you said? just in field fucntion is enougn or i have to compile the user code? I think the latter is more likely, but i think i should  make sure from you.

Appreciates,

Ying

Hello Ying,

Let me appologize for missing that you are seeking to set a turbulent prandtl number.

When you select the Segregated Fluid Temperature model In addition to the K-Epsilon model, a node for Turbulent Prandtl Number will appear in the Physics Continuum's Gas/Liquid Material Properties folder. You can set the value of the Turbulent Prandtl Number there, if you prefer not to use a constant one you can set the value to be determined by a user defined field function there (need to create first).

There is no need to compile anything when you define user defined field functions - you can just go to the bottom of the tree and right click on the tools > field functions subnode to create a new one. If the syntax of the definition is corrrect you will see some assembly code in the properties window, that's all - you can set the field function as a material property of the physics continuum then.

Newer versions of STAR-CCM+ will provide you with a list of field functions that you can use when defining your user defined field function from the field function editor (to open field function editor click the small (...) icon on the far right of the definition line). You should find all functions you need for making up your own definition there (like Turbulent Kinetic Energy, ...)

Regards,

Christian