Influid dynamics throughporous media, theDarcy number (Da) represents the relative effect of the permeability of the medium versus its cross-sectional area—commonly thediameter squared. The number is named afterHenry Darcy and is found from nondimensionalizing the differential form ofDarcy's law. This number should not be confused with theDarcy friction factor which applies topressure drop in a pipe. It is defined as

${\displaystyle \mathrm{D}\mathrm{a}=\frac{K}{d^2}}$

where

• K is thepermeability of the medium (SI units:m²);
• d is the characteristic length, e.g. thediameter of the particle (SI units:m).^[1]

Alternative forms of this number do exist depending on the approach by which Darcy's Law is made dimensionless and the geometry of the system.^[2] The Darcy number is commonly used inheat transfer through porous media.^[3]

• Capillary pressure
• Dimensionless quantity

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