Advective transport time scale

Given characteristic length $L>0$ and speed $U>0$ , the transport time scale is

t_{\mathrm{adv}}=\frac{L}{U}.

It estimates the time needed for advection to traverse the chosen length. A nonuniform or time-dependent trajectory need not take exactly this time.

Comparison with diffusion

At the same length and speed scales,

\frac{t_{\mathrm{diff}}}{t_{\mathrm{adv}}} =\frac{UL}{\nu}=\mathrm{Re}.

Thus the Reynolds number also compares the characteristic diffusion time to the transport time. A flow with several geometric directions may have different transport times in each direction.