= Lewis number
{c}
{title2=$Le=\kappa/D$}
The ratio of thermal <diffusivity> $\kappa$ to molecular or solutal <diffusivity> $D$. In coupled <heat equation> and solute <diffusion equation> similarity problems, the thermal coordinate $z/(2\sqrt{\kappa t})$ becomes $\sqrt{Le}$ times larger in the solute <complementary error function> profile. This follows by substituting the thermal coordinate into $C_t=DC_{zz}$, giving $C''+2Le\eta C'=0$. The definition must specify the direction of this ratio because reciprocal conventions occasionally occur.
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