If you are modeling a flow that includes more than one chemical species (multicomponent flow), you have the option to define a composition-dependent viscosity. (Note that you can also define the viscosity of the mixture as a constant value or a function of temperature.)
To define a composition-dependent viscosity for a mixture, follow these steps:
For the mixture material, choose
mass-weighted-mixing-law or, if you are using the ideal gas law for density,
ideal-gas-mixing-law in the drop-down list to the right of
Viscosity. If you have a user-defined function that you want to use to model the viscosity, you can choose either the
user-defined method or the
user-defined-mixing-law method for the mixture material in the drop-down list.
Define the viscosity for each of the fluid materials that comprise the mixture. You may define constant or (if applicable) temperature-dependent viscosities for the individual species. You may also use kinetic theory for the individual viscosities, or specify a non-Newtonian viscosity, if applicable.
If you selected
user-defined-mixing-law, define the viscosity for each of the fluid materials that comprise the mixture. You may define constant, or (if applicable) temperature-dependent viscosities, or user-defined viscosities for the individual species. For more information on defining properties with user-defined functions, see the separate
UDF Manual .
The only difference between the
user-defined-mixing-law and the
user-defined option for specifying density, viscosity and thermal conductivity of mixture materials, is that with the
user-defined-mixing-law option, the individual properties of the species materials can also be specified. (Note that only the constant, the polynomial methods and the user-defined methods are available.)
If you are using the ideal gas law, the solver will compute the mixture viscosity based on kinetic theory as
is the mole fraction of species
For non-ideal gas mixtures, the mixture viscosity is computed based on a simple mass fraction average of the pure species viscosities: