Ohnesorge number
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The Ohnesorge number, Z , is a dimensionless number that relates the viscous and surface tension force.
It is defined as: 
Where
μ is the liquid viscosity
ρ is the liquid density
σ is the surface tension
D is the characteristic length scale
This is often used to relate to free surface fluid dynamics such as dispersion of liquids is gases and in spray technology. The Ohnesorge number for a 3 mm diameter rain drop is 0.002, noting this depends only on liquid properties and drop size. Larger Ohnesorge numbers indicate a more influence of the viscosity . There is an inverse relationship (
) between the Laplace number and the Ohnesorge number.
The Ohnesorge number is used in spray technology[1] [2]area in combination with either the Reynolds number or Weber number to map regimes of drop formation. Other uses of this dimensionless number include the prediction of the result of drop impact on a surface.
Lefebvre, Arthur, Atomization and Sprays, Hemisphere, 1989, ISBN 0-89116-603-3
Lui, Huimin, Science and Engineering of Droplets, p171-182, 2000, Noyes Publications, ISBN 0–8155-1436-0
