Every type of fluid possesses differing amounts of resistances against deformation. The measure of that resistance is called viscosity. Viscosity expresses the fluid’s resistance against either tensional stress, or shear stress.
In common terms, viscosity is the thinness or thickness of the fluid. A great example of this, is the difference in the viscosities of water and honey. Water is considered as ‘thin’, thus, the viscosity is lower. On the other hand, honey is significantly ‘thick’, and is a fluid with a higher viscosity.
Viscosity can also be considered as a measure of the friction of fluids, as it also describes the internal resistance of the flow of fluid. There are two ways to report or measure a fluid’s viscosity. It can either be expressed as dynamic viscosity, or kinematic viscosity. Many are confused between these two types of viscosity expression, and some even consider them to be one and the same. In reality, they are two significantly different expressions.
Dynamic viscosity, which is also referred to as absolute viscosity, or just viscosity, is the quantitative expression of a fluid’s resistance to flow (shear). Fluid dynamicists, chemical engineers and mechanical engineers commonly consider the use of the Greek letter mu (µ) as the symbol to denote dynamic viscosity. Chemists and physicists, on the other hand, normally use ‘n’ as a symbol.
Its SI unit is in pascal-second (Pa.s) or N.m^-2.s. For cgs, dynamic viscosity is in a unit called ‘poise’, which is taken from the name of Jean Louis Marie Poiseuille. However, the most common expression is centipoise (cP), which is mainly used in ASTM standards.
Kinematic viscosity, on the other hand, is a ratio of the viscous force to the inertial force. Inertial force is characterized by fluid density (p). Kinematic viscosity is symbolized by the Greek letter nu (v).
Kinematic viscosity is mathematically defined as:
v = µ / p
For SI units, it is expressed as m^2/s. Kinematic viscosity is also expressed in stokes (St) or centistokes (ctsk or cSt), for cgs units. It is named after George Gabriel Stokes. It should be noted that water (H2O) at 20 degrees centigrade is about 1 cSt.
Kinematic viscosity is sometimes called the diffusivity of momentum, due to the fact that it has the same unit when compared to the diffusivity of mass and diffusivity of heat. Therefore, it is used in dimensionless numbers, which compares the ratio of the diffusivities.
Difference Between Kinematic and Dynamic Viscosity:
1. Dynamic viscosity is the quantitative expression of fluid’s resistance to flow, while Kinematic viscosity is the ratio of the fluid’s viscous force to the inertial force.
2. Dynamic viscosity is symbolized by either ‘µ’ or ‘n’, while Kinematic viscosity is mathematically symbolized by ‘v’.
3. In a cgs unit system, dynamic viscosity is in a unit called ‘poise’, which is taken from the name of Jean Louis Marie Poiseuille, while Kinematic viscosity is expressed in ‘stokes’ (St) or centistokes (ctsk or cSt), which are named after George Gabriel Stokes.
4. Dynamic viscosity is sometimes referred to as absolute viscosity, or just viscosity, while Kinematic viscosity is sometimes called the diffusivity of momentum.
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