I'm working on the problem of a steady 2-dimensional incompressible viscous flow through a straight delimited fluid pipe, in which a rectangular object is found,
This problem is aimed to be solved numerically by the stream-vorticity method. However I have a problem to understand how to choose the characteristic length $L$ for the system. In a document its proposed that the characteristic length of the system is $2W$(the length of the barrier perpendicular to the flow), however they don't explain why did they choose it. I haven't found any reference for this problem, thus some references are appreciated. In this document the Reynolds number for this problem is :
$$Re = \frac{2Wv_o}{\nu}$$
where $v_0$ is the velocity of the flow very far from the rectangular object and $ \nu$ is the dynamic viscosity. Why they choose this characteristic length for this system ?, Why not $T$ ?