What does Newtonian fluid mean in Fluid Mechanics ?
Newtonian fluid, fluid with a strain rate that is directly proportional to the shear stress that causes flow. In other words, if the shear stress on the surface parallel to the flow direction and the velocity gradient on that surface (the change of velocity perpendicular to the surface) is directly proportional at each point, the fluid is Newtonian type.
Since Isaac Newton first expressed the relationship between strain rate and shear stress using a differential equation, fluids that fit this linear relationship are called Newtonian types.
