Viscosity

The absolute viscosity (m) is defined as the shearing stress for a unit rate of change of velocity. It has the units of Newton-sec per metre squared in the SI system. The shearing stresses are proportional to the ratio of absolute viscosity to density, called kinematic viscosity.

Viscosity (the ability to flow) is a property of fluids (both liquids and gases) treated under the heading of rheology. The work rheology derives from the Greek “rheos” meaning flow.

Between two layers of fluid flowing at different speeds, a tan­gential resistance, a shear stress, is developed because of mo­lecular effects. We say that the shear stress is caused by the in­ternal friction of the fluid or conversely that the fluid transmits shear forces by reason of its internal friction.

A liquid in motion is continuously deformed by the effects of these shear forces. The magnitude of the stress depends on the rate of shear deformation and the sluggishness of the liquid,

I. e. the viscosity.

Viscosity is defined for flow in layers, laminar flow, by Newton’s law of viscosity and is illustrated diagrammatically in Figure 2.1.

T=h— Equ2.2

Ay

Where:

T = shear stress (N/m2)

H = dynamic viscosity (kg/ms)

Av = change in viscosity (m/s)

Ay = distance between layers (m)

In viscous flow equations the dynamic viscosity divided by the density of the liquid is given the symbol v. This parameter is called kinematic viscosity.

V = — Equ2.3

P

Where:

V = kinematic viscosity (m2/s)

= dynamic viscosity (kg/ms)

P = density (kg/m3)

The SI unit for kinematic viscosity is 1 m2/s.

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