[jojo911]

Hey guys,

I have a big problem. I do a CFX calculation for the investigation of a high temperature flow probe for a temperature of 1800°C. I have defined the temperature and pressure for air as ideal gas. The density is calculated correctly. But the dynamic viscosity isn't calculated and is still the same for
T =25°C. I don't have any idea what might be the problem. Can anyone helm me?

I'm from Germany, so my English is not the best. Excuse me!

Best regards
Johannes

[Gert-Jan]

This is not a big problem.

The density comes from the ideal gas law. Viscosity is not influenced by this ideal gas law. Moreover, if you go the transport properties of your gas, there you will find the dynamic viscosity as a constant.

Therefore, in the CEL Expression menu, you need to define your own viscosity as a function of temperature, and use that function in your transport properties tab.

Mostly a higher order polynomial is required. Please pay a lot of attention to the accuracy (enough digits!) of your polynomial coefficients a,b,c, etc. and carefully check if the resulting viscosity is realistic on a wide range of T. For example, if CFX goes to 2500°C during the iteration process, your viscosity should still be positive. Otherwise it will crash definitely.

Regs, Gert-Jan

[jojo911]

Hey Gert-Jan,

thank you very much. Now it works! I thought that CFX calculates the dynamic viscosity automatically but now I am disabused.

Best regards
Johannes