= Near-wall radiative heating layer
{title2=$\delta\sim(\chi L/s)^{1/3}$}
For a wall shear $u\simeq sz$, heating over streamwise length $L$ gives an outer <temperature> rise proportional to $L/(sz)$. Its transverse <thermal conduction> grows as $z^{-3}$ relative to the local source. If $\chi$ is <thermal diffusivity>, comparing conduction with heating gives $\delta\sim(\chi L/s)^{1/3}$, up to a numerical constant depending on the precise comparison. The same scale follows by comparing transverse diffusion time $z^2/\chi$ with residence time $L/(sz)$.
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