The Ohnesorge number (Oh) is a dimensionless number that relates the viscous forces to inertial and surface tension forces.
The number was defined by Wolfgang von Ohnesorge in his 1936 doctoral thesis.
[1][2] It is defined as: Where The Ohnesorge number for a 3 mm diameter rain drop is typically ~0.002.
Larger Ohnesorge numbers indicate a greater influence of the viscosity.
This is often used to relate to free surface fluid dynamics such as dispersion of liquids in gases and in spray technology.