The Ohnesorge number, Oh, is a dimensionless number that relates the viscous forces to inertial and surface tension forces.

It is defined as:


Where

• 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
  Weber number
  The Weber number is a dimensionless number in fluid mechanics that is often useful in analysing fluid flows where there is an interface between two different fluids, especially for multiphase flows with strongly curved surfaces. It can be thought of as a measure of the relative importance of the...
  
  

Applications

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.

See also

• Laplace number
  Laplace number
  The Laplace number , also known as the Suratman number , is a dimensionless number used in the characterization of free surface fluid dynamics...
  
   - There is an inverse relationship, , between the Laplace number
  Laplace number
  The Laplace number , also known as the Suratman number , is a dimensionless number used in the characterization of free surface fluid dynamics...
  
   and the Ohnesorge number. It is more historically correct to use the Ohnesorge number, but often mathematically neater to use the Laplace number
  Laplace number
  The Laplace number , also known as the Suratman number , is a dimensionless number used in the characterization of free surface fluid dynamics...
  
  .