The Heavy Species Transport interface also defines a number of additional variables for quantities which are typically of interest during results processing. The number density of species k is given by

where p (SI unit: Pa) is the absolute pressure of the fluid. The molar concentration of species k is given by

where R is the universal gas constant (SI unit: J/(mol·K)).

A useful quantity in systems where large thermal gradients exist (that is, chemical vapor deposition processes) is the thermal drift velocity (SI unit: m/s), which is defined as