Newtonian fluid

A Newtonian fluid (named for Isaac Newton) is a fluid that flows like water—its  stress versus rate of strain curve is linear and passes through the origin. The constant of proportionality is known as the viscosity. A simple equation to describe Newtonian fluid behaviour is where is the shear stress exerted by the fluid ("drag") [Pa] is the fluid viscosity - a constant of proportionality [Pa·s] is the velocity gradient perpendicular to the direction of shear [s−1]
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Compare with non-Newtonian fluid .A fluid whose viscosity doesn't depend on gradients in flow speed. Gases and low-molecular weight liquids are usually Newtonian fluids.