= Boundary-layer renewal model
{title2=$\overline F\sim k\Delta T/\sqrt{\kappa\tau}$}
In a renewal model, a diffusive <thermal boundary layer> grows until a convective instability overturns it, after which growth restarts. A layer reaching thickness $\delta\sim\sqrt{\kappa\tau}$ has mean <heat flux> of order $k\Delta T/\delta$. A diffusive $t^{-1/2}$ flux averaged over one cycle differs only by an order-one coefficient. Distinct thermal and material layers need not share a renewal time.
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