Collision relaxation time ($\tau$)

The timescale on which a kinetic relaxation model damps departures from a matching local Maxwellian. In the spatially homogeneous BGK model with constant $\tau$, conserved velocity moments fix $\mathcal{M}$, and $f(t)=\mathcal{M}+[f(0)-\mathcal{M}]e^{-t/\tau }$. The parameter is of the order of an effective mean time between collisions rather than an exact universal microscopic collision time.