Stokes Number


The Stokes number (Stk), named after George Gabriel Stokes, is a dimensionless number corresponding to the behavior of particles suspended in a fluid flow. Stokes number is defined as the ratio of the characteristic time of a particle (or droplet) to a characteristic time of the flow or of an obstacle.
Particles with low Stokes number follow fluid streamlines (perfect advection) whereas for large Stokes number, the particle’s inertia dominates so that the particle will continue along its initial trajectory.

Related formulas


StkStokes number (dimensionless)
τrelaxation time of the particle (the time constant in the exponential decay of the particle velocity due to drag) (sec)
Uofluid velocity of the flow well away from the obstacle (m/s)
dCcharacteristic dimension of the obstacle (typically its diameter) (m)