Learning Forum

[trippleD]

Hello,

 

i"m using the k-epsilon turbulence model. There are different ways to calculate the dissipation rate. It is possible to estimate the turbulent dissipation rate from a length scale or from the turbulent viscosity ration etc. The documentation says that the calculation depneds on the method (for ecample: Intensity and Hydraulic Diameter, Intensity and Length Scale, or Intensity and Viscosity Ratio method) which way of calculation is used. 

Where can i choose the method and where can i find in which way the turbulent dissipation rate is calculated?

 

Thanks a lot for your help,

 

bumper

[Rob]

The intensity, hydraulic diameter are set on the flow boundaries: you set these values. Fluent then uses this to calculate k & epsilon. It should be covered in the Documentation: check in boundary conditions and turbulence modelling. 

[trippleD]

I ddin't find something useful in the documentation. And i didn"t find the input for the hydraulic diameter or intensity in fluent...

And if i want to calculate k and epsilon with intensity and viscosity ratio, where can i change the method?

[Rob]

The boundary options will be in the Inlet - look for the pull down option in the turbulence part of the panel. This should also give the option to set k, e, w etc as a value, profile or via UDF.   Section 7.3.2.1 in the User's Guide has most of the equations. 

[trippleD]

If i go to boundary conditions, inlet i don't have a pull down option. Can you describe excactly where i can find it.

I don"t have the Section 7.3.2.1 in the User's Guide of Fluent, there is only the section 7.3.2 with the title "Input of Constant Density"

[Rob]

Open the velocity inlet panel & you should see a section for turbulent boundary conditions: to the right of where it says intensity & length scale there is an arrow - press that. 

[trippleD]

Ah okay, now i know why i couldn't find it. I don't have a velocity inlet. I have a closed system with two concentric cylinders, both are defined as walls and the inner one is rotating. The left and right side are stationary walls. I need these types of boundary conditions. Is there another way to do the input or change the method of calculation or do i have to change my boundary conditions?

[Rob]

If the system is closed the turbulence model will generate k & e (w) based on the flow field: there is no additional input. If you need an initial condition for k & e (w) use 0.01 and expect the solver to complain a bit for the first some iterations/time steps. 

[trippleD]

Ok thank you for your help.

For the input, i go to Solution Initialization-Standard Initialization-Initial Values right?