Definitions of transport properties
Transport properties of gases at moderate pressure

We say that each transport property is defined by a gradient law of proportionality between a flow and a gradient.

If the gas is subjected to a velocity gradient (du /dy ), this results in a flow of momentum which translates into a shear stressτ :$\tau =\eta \frac{\text{d}u}{\text{d}y}$

The dynamic viscosityη (in Pl or Pa · s) is the coefficient of proportionality between τ (in N · m^{– 2}) and du /dy (in s^{– 1}).

The thermal conductivityλ (W · m^–1· K ^–1) is the coefficient relating the heat flux density ψ (W · m^{– 2}) in the x direction to the temperature gradient dT / dx (K · m ^–1) :$\phi =-\text{λ}\frac{\text{d}T}{\text{ d}x}$