how to calculate recirculation length after square cylinder
 

i want to know how to calculate the re circulation length from streamlines drawing

i attached pic for streamlines

When I was undergraduate, I researched about characteristics of square cylinder. And this is the reference paper what I referred.

Flow past a square cylinder with an angle of incidence, 2010, physics of fluid

They set recirculation region as the distance between center of cylinder and near-wake saddle point which is referred by Perry et al. ( It is mentioned in paper)