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Kinematic viscosity/ momentum diffusivity:

 Kinematic viscosity/ momentum diffusivity:

  • It is defined as the ratio of dynamic viscosity to the density of the fluid.

Kinematic viscosity

ν = μ / ρ                                                               

where

ν = kinematic viscosity (m2/s)

μ = absolute or dynamic viscosity (N s/m2)

ρ = density (kg/m3)

In the SI-system the theoretical unit of kinematic viscosity is m2/s - or the commonly used Stoke (St) where

  • 1 St (Stokes) = 10-4 m2/s = 1 cm2/s

Stoke comes from the CGS (Centimetre Gram Second) unit system.

Since the Stoke is a large unit it is often divided by 100 into the smaller unit centiStoke (cSt) - where

  • 1 St = 100 cSt
  • 1 cSt (centiStoke) = 10-6 m2/s = 1 mm2/s
  • 1 m2/s = 106 centiStokes
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