Viscosity is an important fluid property when analyzing liquid behavior and fluid motion near solid boundaries. The viscosity of a fluid is a measure of its resistance to gradual deformation by shear stress or tensile stress. The shear resistance in a fluid is caused by inter-molecular friction exerted when layers of fluid attempt to slide by one another.
- viscosity is the measure of a fluid’s resistance to flow
- molasses is highly viscous
- water is medium viscous
- gas is low viscous
There are two related measures of fluid viscosity
- dynamic (or absolute)
- kinematic
Dynamic (absolute) Viscosity
Absolute viscosity – coefficient of absolute viscosity – is a measure of internal resistance. Dynamic (absolute) viscosity is the tangential force per unit area required to move one horizontal plane with respect to an other plane – at an unit velocity – when maintaining an unit distance apart in the fluid.
The shearing stress between the layers of a non turbulent fluid moving in straight parallel lines can be defined for a Newtonian fluid as
Shear stress can be expressed
τ = μ dc / dy
= μ γ (1)
where
τ = shearing stress in fluid (N/m2)
μ = dynamic viscosity of fluid (N s/m2)
dc = unit velocity (m/s)
dy = unit distance between layers (m)
γ = dc / dy = shear rate (s-1)
Equation (1) is known as the Newtons Law of Friction.
(1) can be rearranged to express Dynamic viscosity as
μ = τ dy / dc
= τ / γ
Kinematic Dynamic Viscosity 