Kinematic Viscosity

Definition - What does Kinematic Viscosity mean?

The kinematic viscosity of a fluid is the ratio of the viscosity of the fluid to the fluid's density. Mathematically, it is expressed as:

ν = μ / ρ,

where ν is the kinematic viscosity, μ is the dynamic viscosity, and ρ is the density.

By removing the factor of density, the viscosity of a fluid can be understood from the standpoint of the frictional forces between the molecules of a fluid rather than the gravitational effects. Kinematic viscosity is a temperature dependent property.

In industrial applications, kinematic viscosity becomes important in fluid transport and lubrication. The latter relationship relates to corrosion protection when choosing a lubricant for its protective qualities.

Corrosionpedia explains Kinematic Viscosity

Viscosity results from the intermolecular forces between molecules in a liquid. Strongly polarized interactions like hydrogen bonding in water or interactions between large non-polar surfaces in viscous oils produce stronger intermolecular forces. The stronger the intermolecular force, the more the molecules will stick together as they collide, manifesting a frictional interaction. This intermolecular friction is what makes the fluid resistant to change in shape and explains the thick and slow moving properties of highly viscous fluids.

Kinematic viscosity is not the same as dynamic viscosity, although they are related mathematically by the density of the fluid. Understanding the distinction comes into play in industrial settings as record data is normally reported as kinematic viscosity, but onsite measurements are often measured as dynamic viscosity. Care must be taken when interpreting values and the temperature at which the values were measured. Viscosity is measured by viscometers. Kinematic viscometers determine the kinematic viscosity by measuring the time it takes for the fluid to pass through a portion of a capillary.

The viscosity of oil and lubricants are often measured in industrial settings. These films are used to protect equipment from mechanical wear and may also have extra benefits such as creating a barrier to corrosion. Viscosity will affect the thickness of the oil film as well as the effectiveness of its protective qualities. Oils with too high viscosity don't protect well against mechanical friction, resulting in heat generation. On the contrary, low viscosity fluids are more volatile, creating environmental problems. When choosing oil for its viscous properties, be aware of the factors that may alter its viscosity:

  • Additives that improve the viscosity index (VI)
  • Contamination from water
  • Effect of byproducts from thermal or oxidative degradation
  • Possible contamination from soot
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