St
stokes
Diffusivityexact by definition
The stokes is the CGS unit of kinematic viscosity, exactly 10⁻⁴ m²/s, or 1 cm²/s. Water at 20 °C is about 0.01 St, which is why the practical unit everywhere is the centistokes rather than the stokes itself.
Watch out: Kinematic viscosity is dynamic viscosity divided by density, so a stokes is a poise per gram per cubic centimetre. Lubricant grades quoted as "ISO VG 46" refer to a kinematic viscosity of 46 cSt at 40 °C, and comparing that against a dynamic viscosity in centipoise requires the density.
| 1 St | 100 mm²/s |
Where the unit came from
Named for George Gabriel Stokes, whose law for the drag on a sphere is the basis of the falling-ball viscometer and of the settling velocity calculations in water treatment.
Solvers that use diffusivity