Kinematic & Dynamic Viscosity

Kinematic Viscosity

Kinematic viscosity is the ratio of - absolute (or dynamic) viscosity to density - a quantity in which no force is involved. Kinematic viscosity can be obtained by dividing the absolute viscosity of a fluid with the fluid mass density.

ν = μ / ρ        (2)

where

ν = kinematic viscosity (m²/s)

μ = absolute or dynamic viscosity (N s/m²)

ρ = density (kg/m³)

In the SI-system the theoretical unit of kinematic viscosity is m²/s - or Stoke (St) where

• 1 St (Stokes) = 10^-4 m²/s = 1 cm²/s

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

The dynamic viscosity can be expressed

τ = μ dc / dy         (1)

where

τ = shearing stress (N/m²)

μ = dynamic viscosity (N s/m²)

dc = unit velocity (m/s)

dy = unit distance between layers (m)

Equation (1) is known as the Newtons Law of Friction.

In the SI system the dynamic viscosity units are N s/m², Pa s or kg/(m s) - where

• 1 Pa s = 1 N s/m² = 1 kg/(m s)

Dynamic viscosity may also be expressed in the metric CGS (centimeter-gram-second) system as g/(cm s), dyne s/cm² or poise (p) where

• 1 poise = 1 dyne s/cm² = 1 g/(cm s) = 1/10 Pa s = 1/10 N s/m²

For practical use the Poise is normally too large and the unit is often divided by 100 - into the smaller unit centipoise (cP) - where

• 1 P = 100 cP
• 1 cP = 0.01 poise = 0.01 gram per cm second = 0.001 Pascal second = 1 milliPascal second = 0.001 N s/m²

Water at 20.2^oC (68.4^oF) has the absolute viscosity of one - 1 - centiPoise.

Liquid	Absolute Viscosity*) (Pa s)
Air	1.983 x 10-5
Water	1 x 10-3
Olive Oil	1 x 10-1
Glycerol	1 x 100
Liquid Honey	1 x 101
Golden Syrup	1 x 102
Glass	1 x 1040