= BGK model
{c}
{title2=$C[f]=-(f-\mathcal M[f])/\tau$}
Replace the <nonlinear Boltzmann collision operator> by relaxation to the <local Maxwellian> $\mathcal M[f]$ with the same <mass density>, <mean velocity> and random <kinetic energy> as $f$. Matching those <velocity moments> ensures vanishing collision contributions to <mass>, <momentum> and <kinetic energy> balance. The <collision relaxation time> $\tau$ is often treated as a constant, although microscopic collision rates generally depend on the state and particle <velocity>.
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