Help on specifying the value of viscosity
 

Hello everyone,
From the tutorial of Fluent, it is recommended to set the viscosity in fluid material as three coefficient method Sutherland Law for high speed compressible flows. Since my problem setup is at an altitude and based on that altitude I have a viscosity value, where do I place this value in ansys fluent? Is it on the reference viscosity field? or does the default settings are self calculating the value of viscosity using the relevant static temperature?

The reference viscosity in the reference values pane is purely for post-processing, for calculating Reynolds number. You can put any number and it won't affect the solution. Don't confuse this reference viscosity with the reference viscosity in the Sutherland model.

You need to specify a method or equation of state for viscosity in material properties. If you choose constant viscosity, then you need to provide the actual value of viscosity. If you use Sutherland law, then you only provide the Sutherland coefficients, the actual viscosity will be locally calculated using the local temperature. There is a reference temperature and reference viscosity in the Sutherland model. You normally do not give the viscosity at your altitude. The reference viscosity and reference temperature should be viewed as model constants for your Sutherland law. The reason is....


The idea behind Sutherland's law is to take real viscosity values and then curve-fit them using the Sutherland power law. When you do this curve fit, the reference temperature and viscosity will pop out. Hence they are constants. If you change one of them, you will get the wrong viscosity. You need to change the reference temperature and viscosity together, you cannot and should not change them arbitrarily/independently.

Thankyou, Sir!