Theory

For an ideal gas, the specific gas constant is the universal gas constant divided by the molar mass, equation (1); the density follows from the ideal-gas law, equation (2). For a mixture of fixed composition, the molar mass is the mole-fraction-weighted sum of the species' molar masses, equation (3). The value is written as molWeight into the thermophysical properties of the compressible solver (OpenFOAM user guide, 2026).

The heat capacity must exceed the gas constant (otherwise $c_v = c_p - R$ would not be positive); the material library rejects a material that violates this.

Equations

R = \frac{\mathcal{R}}{M}

(1)

\rho = \frac{p}{R\,T}

(2)

M = \sum_i x_i\,M_i

(3)

where	meaning	unit
$R$	Specific gas constant	J/(kg K)
$\mathcal{R}$	Universal gas constant, 8314.47	J/(kmol K)
$M$	Molar mass of the gas	kg/kmol
$\rho$	Density	kg/m³
$p$	Absolute pressure	Pa
$T$	Temperature	K
$x_i$	Mole fraction of species $i$	-
$M_i$	Molar mass of species $i$	kg/kmol

Default and rule

Default: 28.9644 (dry air) in a new material
Rule: A new material starts at 28.9644 kg/kmol (dry air).
Source: Ideal-gas relations; the program uses a universal gas constant of 8314.47 J/(kmol K).

Valid range 0 < M <= 1000 kg/kmol

Values

Above 0 and up to 1000 kg/kmol. Helium 4.003, methane 16.04, air 28.96, CO₂ 44.01.

References 1

OpenCFD Ltd. (2026). OpenFOAM user guide. openfoam.com. Web page

Theory

Equations

Default and rule

Valid range 0 < M <= 1000 kg/kmol

Related 3

References 1