Characteristic spanwise length scales of streamwise vortical structures in the wake of a circular cylinder at Re=1500 measured via global and local approaches

Wake characteristics of the flow past a circular cylinder are analysed in detail at Reynolds number Re = 1500 via direct numerical simulation. A periodic spanwise domain of length 1.5ND has been found to yield correct first-and second-order wake statistics in remarkable agreement with published results at the same and closeby Re. A Kelvin-Helmholtz instability with a frequency fKH similar or equal to 0.666 is observed to occur intermittently in the shear layers issued from the top and bottom of the cylinder. The three-dimensional patterns in the wake have an estimated spanwise length scale VD similar or equal to 0.70 (D is the cylinder diameter) in the near-wake at (x, y)/D= (3, 0.5), downstream from the cylinder, when quantified by autocorrelation (global approach). When using the Hilbert-transform (local approach) instead, the predicted length scale of streamwise vortical structures is distributed around Az/D similar or equal to 0.33 at the same sampling location. Our results show that the two approaches measure different aspects of three-dimensionality: while the former informs of the typical spanwise spacing of streamwise vortices, the latter quantifies the local spanwise size of these same flow structures.