Viscosity

Abstract

A Fluid is normally defined as a substance which is incapable of sustaining a shearing stress. However, this definition is only applicable when the fluid is at rest. If relative motion takes place a measurable resistance is encountered and the fluid is said to exhibit viscosity. Consider a fluid flowing without turbulence over a fixed surface AB (Figure 7.1). Experimentally, it is found that a layer of the fluid at D, at a distance x + dx from AB, moves with a velocity u + du greater than that of a layer of the fluid at C, at a distance x from AB, which moves with velocity u. The velocity gradient between the layers C and D is thus du/dx.