Newtonian fluid

A Newtonian fluid is a fluid in which shear stress is proportional to the velocity gradient, perpendicular to the plane of shear. The constant of proportionality is known as the viscosity. If the fluid is incompressible and viscosity is constant across the fluid,

$\tau =\mu {\frac {\partial u}{\partial y}}.$

The motion of Newtonian fluids is described by the Navier-Stokes equations.

See also: Non-Newtonian fluid | Poiseuille's law | Viscosity