In this example it is assumed that some of the fluid properties, including the dynamic viscosity, are not known. However the Reynolds number, inlet velocity and a length scale are known. The flow is compressible and therefore the density is variable.

Given this information it is possible to calculate the fluid dynamic viscosity based on the Reynolds number. The Reynolds number is given by:

where is density, U a velocity scale, L a length scale and the dynamic viscosity. The velocity scale is taken as the inlet velocity, the length scale as the inlet width and the density is calculated as the average density over the inlet area.

The LIBRARY section of the CCL (CFX Command Language) file appears as follows:

LIBRARY :
   CEL :
      EXPRESSIONS :
         Re = 4.29E6 [  ]
         Vel = 60 [m s^-1]
         L=1.044[m]
         Visc=areaAve(density)@in*Vel*L/Re
      END
   END
   MATERIAL : Air Ideal Gas
      Option = Pure Substance
      PROPERTIES :
         Option = Ideal Gas
         Molar Mass = 2.896E1 [kg kmol^-1]
         Dynamic Viscosity = Visc
         Specific Heat Capacity = 1.E3 [J kg^-1 K^-1]
         Thermal Conductivity = 2.52E-2 [W m^-1 K^-1]
      END
   END
END

This shows that four CEL expressions have been created. The first three expressions define constant values that are used in the Visc expression. The Visc expression calculates the dynamic viscosity based on the equation for Reynolds number given above. Within the expression the function areaAve(density)@in is used to evaluate the average density at the inlet.

The Visc expression can now be used to replace the value of Dynamic Viscosity in the MATERIAL > PROPERTIES section.