The general form of the transport equation for an Additional Variable is:
 | (1–174) |
where:
is the fluid velocity in the case
of a fluid or porous domain, or 
in the case of a solid domain with a specified solid
motion velocity, where 
is the solid velocity.
is the mixture density, mass per
unit volume
is the conserved quantity per
unit volume, or concentration
is the conserved quantity per
unit mass
is a volumetric
source term, with units of conserved quantity per unit volume per
unit time
is the kinematic
diffusivity for the scalar
For turbulent flow, this equation is Reynolds-averaged (see Turbulence Models) and becomes:
 | (1–175) |
where:
