Schmidt number is a dimensionless number defined as the ratio of momentum diffusivity (viscosity) and mass diffusivity, and is used to characterize fluid flows in which there are simultaneous momentum and mass diffusion convection processes. It was named after the German engineer Ernst Heinrich Wilhelm Schmidt (1892-1975).
Schmidt number is the ratio of the shear component for diffusivity viscosity/density to the diffusivity for mass transfer D. It physically relates the relative thickness of the hydrodynamic layer and mass-transfer boundary layer.
It is defined as:
- is the kinematic viscosity or (/) in units of (m2/s)
- is the mass diffusivity (m2/s).
- is the dynamic viscosity of the fluid (Pa·s or N·s/m² or kg/m·s)
- is the density of the fluid (kg/m³).
The heat transfer analog of the Schmidt number is the Prandtl number.
Turbulent Schmidt Number 
The turbulent Schmidt number is commonly used in turbulence research and is defined as:
The turbulent Schmidt number describes the ratio between the rates of turbulent transport of momentum and the turbulent transport of mass (or any passive scalar). It is related to the turbulent Prandtl number which is concerned with turbulent heat transfer rather than turbulent mass transfer.
Stirling engines 
For Stirling engines, the Schmidt number represents dimensionless specific power[disambiguation needed]. Gustav Schmidt of the German Polytechnic Institute of Prague published an analysis in 1871 for the now-famous closed-form solution for an idealized isothermal Stirling engine model.
- is the Schmidt number
- is the heat transferred into the working fluid
- is the mean pressure of the working fluid
- is the volume swept by the piston.
- Incropera, Frank P.; DeWitt, David P. (1990), Fundamentals of Heat and Mass Transfer (3rd ed.), John Wiley & Sons, p. 345, ISBN 0-471-51729-1 Eq. 6.71.
- Brethouwer, G. (2005). "The eﬀect of rotation on rapidly sheared homogeneous turbulence and passive scalar transport. Linear theory and direct numerical simulation". J. Fluid Mech. 542: 305–342.
- Schmidt Analysis (updated 12/05/07)