How to... choose additional options in turbulence models Ansys CFX

 Often, when choosing turbulence models in Ansys CFX, we do not focus on additional functions that we can use for our problem. Proper definition of the turbulent model in the analysis can have a decisive impact on the gas flow in the domains and on the overall fluid mechanics.

Additional options of turbulence in Ansys CFX

 Proper selection of this model may also determine the speed of convergence of the residual equations in the solver. So it is worth getting to know the additional functions of turbulent models in Ansys CFX to make our work more efficient in terms of optimization of computer calculations and correct definition of the problem.. In this post, we will try to explain functions of the additional options of the turbulence models. 

RED FRAME 

High speed (compressible) Wall Heat Transfer Model  - this function is very usefull in supersonic flow problem, specially with complex geometry (for example convulted channels). When U need to analyze, or U know that flow speed near wall have impact on Ur problem - turn on Wall Heat Transfer Model with High Speed. 

Turbulent Flux Closure for Heat Transfer  - this otpion is necessary for Closure Models / Problems

When the heat transfer model is set to either Thermal Energy or Total Energy, the Turbulent Flux Closure for Heat Transfer may be optionally selected in the Turbulence settings. When Turbulent Flux Closure for Heat Transfer is not selected, the default is the Eddy Diffusivity model with the turbulent Prandtl number of 0.9. The turbulent Prandtl number may be non-constant using by expressions (CEL), for example, as a function of physical space or solution variables.

Source:

https://ansyshelp.ansys.com/

How to define additional turbulence options in Ansys CFX ?

Very hard to find a detailed description (explanation) of advanced turbulence control parameters from Ansys CFX. In this post we will try to explain each of these parameters. In the first part, we will deal with the first four parameters (in orange frame). 

Additional Turbulence in prepost 

In this chapter I would like to explain further additional functions in turbulent models in Ansys CFX. As I mentioned in the previous post, it is extremely important to learn about these additional functions and their influence on the shape of our analysis. It should be noted that the functions described below are rarely used by CFX users. There is also little information about them. I have searched many sources and here is what I could find.

What is EDDY VISCOSITY in Advanced Turbulence Control (Ansys CFX) ? - probably this parameter fix the average value of eddy viscosity in fluid domain. But if U define this parameter U have reckon with the fact that U make turbulence linear. You can use this if the initial conditions with the desired turbulence model (startup) is not possible.  

What is BC TKI Factor in advanced turbulence control (Ansys CFX) ? - this is dimensionless factor that influences into turbulence intensity. This parameter is assigned to SST turbulence model. 

What is CMU Coefficient in advanced turbulence control (Ansys CFX) ? - this parameter is very usefull if U want (for example) make addiction for density to altitiude. To do this U must to make user defined equation like  - Turbulent viscosity (function of height)  = Rho * Cmu**k^2/e.

What is compressible production in advanced turbulence control (Ansys CFX) ? - this is coefficient factor to define turbulent viscosity manually. If U want to define global viscosity in Ur model for simulation U need to turn on this parameter. 

What is curvature correction in Ansys CFX ?? - this factor increase or decrease value of turbulence based on curvature of the flow. If U define this parameter then results of turbulence will be closer to Reynolds Stress. 

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