Understanding Viscosity

Dynamic Viscosity

The resistance between two consecutive layers of fluid is viscosity. This resistance is generated because of the fact that different layers flow at different velocity.  Or, in other words, there exists some amount of relative velocity which causes this friction.


Fig.1: Dynamic Viscosity


The shear stress between consecutive fluid layers can be written as:

T = μ k……………………………………eqn.1


T – Shear stress between consecutive fluid layers

μ – Dynamic viscosity k – Velocity gradient, i.e. how much change in velocity occurs with depth of the fluid

SI (or MKS) Units of μ: N-s/m2 or Pa-s or kg/m-s. They all are equal, i.e., 1 N-s/m2 = 1 Pa-s = 1 kg/m-s.

CGS Units of μ: Poise or Dyne-s/cm2 or g/cm-s. The relation is: 1 Poise=1 Dyne-s/cm2 = 1 g/cm-s=100 centiPoise = 0.1Pa-s.

FPS unit of μ: Lb/ft-s.  1 lb/ft-s = 14.88 Poise


Kinematic viscosity It is the ratio between the dynamic viscosity and density.  Use consistent units system while calculating the ratio. The units here are: m2/s , ft2/s and Stokes (St). 1 St = 10000 m2/s



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