Third central velocity moment ($Q_{ijk}=\int (v_i-u_i)(v_j-u_j)(v_k-u_k)f{d}^{3}v$)

For unit particle mass, this symmetric third-order tensor transports the pressure tensor in a kinetic fluid description. Its contraction gives the heat flux $q_i=Q_{ijj}/2$. It vanishes for a local Maxwellian, whose odd central velocity moments vanish, but it need not vanish for a general anisotropic or skewed phase-space distribution function.