S = 2 * eta * D

where S is the extra-stress tensor, D is the rate of deformation tensor, and eta is the viscosity.

The shear-rate dependence of the viscosity is denoted by a function F(g), where g is the shear-rate.

For the Carreau-Yasuda law, the function F(g) is written as follows :

F(g) = facinf + (fac-facinf) * [ 1 + (tnat *g)^expoa ]^{((expo-1)/expoa)}

where fac and facinf are the shear viscosities at zero and high shear-rates, respectively; tnat is a time scale;
and expo and expoa are power indexes.

By default, fac = 1, expo = expoa = 1, facinf = 0 and tnat = 0.

The units for g and the parameters fac, tnat, expo, expoa and facinf are :
 
 

parameter	mass	length	time	temperature
g	-	-	-1	-
tnat	-	-	1	-
fac	1	-1	-1	-
facinf	1	-1	-1	-
expo	-	-	-	-
expoa	-	-	-	-