### 1.6.1. Transport Equations for Additional Variables

The general form of the transport equation for an Additional Variable is:

| (1–172)

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:

| (1–173)

where:

- is the turbulence Schmidt number

- is the turbulence viscosity

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