Collision relaxation time
= Collision relaxation time
{title2=$\tau$}
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.