Evolution of patterns of streamwise vorticity in the turbulent near wake of a circular cylinder

High-image-density particle image velocimetry allows characterization of the instantaneous patterns of streamwise vorticity omega(x) over the cross-section of the near wake of a circular cylinder, and the manner in which they evolve with streamwise distance. Emphasis is on the Reynolds number Re = 10 x 10(3), for which the Kelvin-Helmholtz (K-H) mode in the separating shear layers has a streamwise wavelength much smaller than that of the Karman mode. Consequently, the corresponding spanwise wavelength between omega(x) concentrations increases substantially from its value in the separating shear layer to a larger one in the near wake. This streamwise evolution is defined by spatial correlations of patterns of instantaneous omega(x) and interpreted with the aid of the quasi-two-dimensional topology of the wake in the base region of the cylinder. The principal features of the phase-averaged topology are foci of the initially formed Karman vortices and a saddle point between them. Immediately downstream of this saddle, remarkably coherent patterns of omega(x) concentrations are evident; they have a wavelength approximately equal to the cylinder diameter. Moreover, larger-scale spanwise distortion eventually occurs. This distortion exhibits several modes; the most severe is a nearly discontinuous variation of patterns of omega(x).