4.4.1. Basic Material Properties

For simple problems with constant or non-complex properties, material properties can be specified directly in the material block. For example, for an isothermal laminar flow the only required properties are density and viscosity. To set constant values for these, one could do

# At the sierra-block level
Begin Property Specification for Fuego Material air
    Density   = 1.2
    Viscosity = 1.8e-5
End

For a non-isothermal problem, thermal conductivity, specific heat, and a function for enthalpy as a function of temperature are required. These can be specified in a similar manner, using either in-line string functions or tabulated user functions.

# At the sierra-block level
Begin Property Specification for Fuego Material air

  Reference Temperature = 300

  # rho = P*W/(R*T) = Ts / T where Ts = P*W/R
  # Cp = { Cp = 1000 }
  Density = "300 / temperature"
  Viscosity = 1.8e-5
  Specific_Heat = {Cp}
  Thermal_Conductivity = 0.03
  Enthalpy = "{Cp}*temperature"
End

Note

When properties have dependencies, like temperature here, a reference value for those dependencies must also be specified (reference temperature here).

To use a tabulated function, the function must be defined in a separate user function block

# At the sierra-block level
Begin Property Specification for Fuego Material air

  Reference Temperature = 300

  # rho = P*W/(R*T) = Ts / T where Ts = P*W/R
  # Cp = { Cp = 1000 }
  Density = "300 / temperature"
  Viscosity = 1.8e-5
  Specific_Heat = {Cp}
  Thermal_Conductivity = 0.03
  Function for Enthalpy = hFuncAir
End

Begin Definition For Function hFuncAir
  TYPE IS PIECEWISE LINEAR
  ABSCISSA = temperature
  BEGIN VALUES
       0.  0.0
    5000. {Cp*5000}
  END VALUES
END

Note

When Fuego calculates temperature from enthalpy, it is assumed that $C_p = \frac{\partial h}{\partial T}$ . When providing properties manually, be sure this relationship is true for the specified enthalpy and specific heat.

4.4.1.1. Units

When defining material properties directly, their units should be in your problem units. For example, if your mesh is in meters you should provide values in MKS units. There is no automatic unit conversion done for directly specified properties.

One exception to this is reference pressure, which should always be specified in atmospheres.

4.4.1.2. Non-Dimensional parameters

Fuego typically uses the Schmidt number ( $Sc = \frac{\mu}{\rho D}$ ) and Prandtl number ( $Pr = \frac{\mu}{\rho \alpha}$ ) to define diffusion of mass and energy as a function of viscosity. When setting up a turbulent flow problem, these parameters should be specified in the material property block.

Begin Property Specification for Fuego Material air

    # Other property definitions

    SCHMIDT_NUMBER = 0.9
    PRANDTL_NUMBER = 0.9
End

Note

These are separate from the turbulent Prandtl and turbulent Schmidt numbers, which are specified in the global constants block.