Difference Between Viscosity and Kinematic Viscosity

Viscosity vs Kinematic Viscosity | Dynamic Viscosity, Absolute Viscosity
 

Viscosity is a very important parameter discussed in fluid mechanics. Viscosity and kinematic viscosity have a variety of applications in fields such as fluid dynamics, fluid mechanics, aerodynamics, chemistry and even medical science. A good understanding in the concepts of viscosity and kinematic viscosity is required to excel in the above mentioned fields. In this article, we are going to discuss what viscosity and kinematic viscosity are, their definitions, the applications of viscosity and kinematic viscosity, the similarities and finally the differences between kinematic viscosity and viscosity.

Viscosity

Viscosity is defined as a measure of the resistance of a fluid, which is being deformed by either shear stress or tensile stress. In more common words, viscosity is the “internal friction” of a fluid. It’s also referred as the thickness of a fluid. Viscosity is simply the friction between two layers of a fluid when the two layers move relative to each other. Sir Isaac Newton was a pioneer in fluid mechanics. He postulated that, for a Newtonian fluid, the shear stress between layers is proportional to the velocity gradient in the direction perpendicular to the layers. The proportional constant (proportionality factor) used here is the viscosity of the fluid. The viscosity is usually denoted by the Greek letter “µ”. Viscosity of a fluid can be measured using Viscometers and Rheometers. The units of viscosity are Pascal-seconds or Nm^-2s. The cgs system uses the unit “poise”, named after Jean Louis Marie Poiseuille, to measure viscosity. Viscosity of a fluid can also be measured by several experiments. The viscosity of a fluid depends on the temperature. The viscosity decreases as the temperature is increased.

τ=μ (∂u/∂y)

Viscosity equations and models are very complex for non-Newtonian fluids. There are two main forms of viscosity. They are namely dynamic viscosity and kinematic viscosity. Dynamic viscosity is also known as the absolute viscosity. Dynamic viscosity is the general viscosity measurement used in most of the calculations. This is denoted by either µ or ɳ. The SI unit of dynamic viscosity is Pascal seconds. If a fluid with viscosity of 1 Pascal seconds is placed between two plates and one plate is pushed sideways with a shear stress of 1 Pascal, it moves a distance equal to the thickness of the layer between the plates in 1 second.

Kinematic Viscosity

In some cases, the inertial force of the fluid is also important with respect to the viscosity measurement. The inertial force of the fluid depends on the density of the fluid. Therefore, a new term called the kinematic viscosity is defined, to help such calculations. The kinematic viscosity is defined as the ratio of the dynamic viscosity to the density of the fluid. The kinematic viscosity is referred by the term ν (Greek letter nu). Kinematic viscosity has units of meters squared divided by seconds. The unit stoke is also used to measure the kinematic viscosity.