In many non-Newtonian fluids, the shear stress due to viscosity, , can be modeled by
where
* k is the consistency index
* n is the flow behavior index
* du/dy is the shear rate, with velocity u and position y
These fluids are called power-law fluids.
To ensure that  has the same sign as du/dy, this is often written as
where the term
gives the apparent viscosity.