Schmidt number (Sc) 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 (18921975).
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 masstransfer boundary layer.
It is defined^{[1]} as:

\mathrm{Sc} = \frac{\nu}{D} = \frac {\mu} {\rho D} = \frac{ \mbox{viscous diffusion rate} }{ \mbox{molecular (mass) diffusion rate} }
where:
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:^{[2]}
\mathrm{Sc}_\mathrm{t} = \frac{\nu_\mathrm{t}}{K}
where:
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 is related to the specific power. Gustav Schmidt of the German Polytechnic Institute of Prague published an analysis in 1871 for the nowfamous closedform solution for an idealized isothermal Stirling engine model.^{[3]}^{[4]}

\mathrm{Sc} = \frac{\sum {\left  {Q} \right }}{\bar p V_{sw}}
where,

\mathrm{Sc} is the Schmidt number

Q is the heat transferred into the working fluid

\bar p is the mean pressure of the working fluid

V_{sw} is the volume swept by the piston.
Notes

^ Incropera, Frank P.; DeWitt, David P. (1990), Fundamentals of Heat and Mass Transfer (3rd ed.), 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)

^ http://mac6.ma.psu.edu/stirling/simulations/isothermal/schmidt.html
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