In fluid mechanics and heat transfer, the ratio of dynamic viscosity to density appears frequently. For convenience, this ratio is given the name kinematic viscosity ν and is expressed as ν = μ/ρ. Two common units of kinematic viscosity are m2/s and stoke (1 stoke = 1 cm2/s = 0.0001 m2/s).
In general, the viscosity of a fluid depends on both temperature and pressure, although the dependence on pressure is rather weak. For liquids, both the dynamic and kinematic viscosities are practically independent of pressure, and any small variation with pressure is usually disregarded, except at extremely high pressures. For gases, this is also the case for dynamic viscosity (at low to moderate pressures), but not for kinematic viscosity since the density of a gas is proportional to its pressure.
The second form of Re illustrates that the ratio of μ to ρ has its own name, the kinematic viscosity:
It is called kinematic because the mass units cancel, leaving only the dimensions of length squared over time.