Ohnesorge number

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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]

It is defined as:


  • μ is the liquid viscosity
  • ρ is the liquid density
  • σ is the surface tension
  • L is the characteristic length scale (typically drop diameter)
  • Re is the Reynolds number
  • We is the Weber number


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.[2][3]

See also


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