Probability density of vortical structures in the the wake of the smooth (a) and roughened (b–e) surface blades for and , with (b–e) corresponding to successively added roughness at locations 1–4 (Figure 2). The probabilities along the wake coordinate are summed up in the bar plots at the right. The probability densities of counterclockwisely and clockwisely rotating vortices are associated with shades of blue and red, respectively.