text: 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.