The Newtonian fluid reference article from the English Wikipedia on 24-Apr-2004
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Newtonian fluid

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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.

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

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