= Shock cooling length
{title2=$\ell_{\rm cool}\sim u_2t_{\rm cool}$}
The distance a downstream fluid element travels while losing a substantial fraction of its thermal <energy>. Multiplying the immediate downstream <velocity> by a <radiative gas cooling time> gives an order-of-magnitude length. A resolved layer instead uses $\ell=\int u(t)\,dt$, because <mass density>, <temperature> and speed change during cooling. It is distinct from the microscopic dissipative thickness of the <shock wave>.
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