Difference Between Kinematic and Dynamic Viscosity

Kinematic vs Dynamic Viscosity
 

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

What is Dynamic Viscosity?

To understand the concept of dynamic viscosity, a general idea on the field of viscosity is required. 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 unit of viscosity is Pascal-seconds (or Nm^-2s). The cgs system uses the unit “Poise”, named after Jean Louis Marie Poiseuille, to measure 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 a 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.

What is Kinematic Viscosity?

In some cases, the inertial force of the fluid is also significant with respect to the viscosity measurement. The inertial force of the fluid depends on the density of the fluid. Therefore, a new term called kinematic viscosity is defined in order 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.